Low Voltage Grid Connection of Photovoltaic Power Systems

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LOW VOLTAGE GRID CONNECTION OF PHOTOVOLTAIC POWER SYSTEMS

ETSU Report No. S/P2/00215/REP EATL Report No: 5010

Contractor: EA Technology Ltd

Prepared by: A Collinson, EA Technology J Thornycroft, Halcrow Gilbert

The work described in this report was carried out under contract as part of the New and Renewable Energy Programme, managed by the Energy Technology Support Unit (ETSU) on behalf of the Department of Trade and Industry (DTI). The views and judgements expressed in this report are those of the contractor and do not necessarily reflect those of ETSU or the DTI.

First Published September 1999 Executive Summary

This report forms the final report of the project entitled “Technical Criteria for the Electrical Integration of Photovoltaic Systems into Electricity Supply Networks”, supported by ETSU and the DTI (contract number S/P2/00215/00/00). The project was jointly supported by EA Technology’s Core Research Programme, carried out on behalf of the UK Electricity Supply Industry (ESI). Note, the Core Research Programme subsequently developed into EA Technology’s Strategic Technology Programme in 1997.

Introduction & Background

The use of photovoltaic (PV) systems is well established for stand-alone applications, but in recent years there has been growing interest in grid-connected PV systems using DC to AC inverters. Building-integrated PV systems for the domestic market have been identified as offering a large potential power source in the UK. However, the integration of photovoltaic power systems into local electricity supply networks in the UK is currently in the early stages of development. The current high costs of PV systems mean that relatively few requests have been received by UK electricity companies to date for the connection of such systems. However, the increasing international activity related to cost-reduction of PV systems, especially in the area of building integration, means that significant reductions in basic system costs will continue to be made. If the current trend in cost reduction continues it will not be long before PV systems are a true commercial proposition. Consequently, any increased use of PV systems is likely to have a significant impact on the design, operation and management of electricity supply networks. The electricity supply industry has identified several technical and commercial issues which have caused them some concern. These issues needed resolution before grid connected systems could become more widespread.

Consequently in 1994, the DTI established a programme of work to investigate and overcome the barriers to the introduction of grid connected PV and release this potential, as well as supporting other projects designed to stimulate the new technology.

With this in mind, and taking into account the greater practical experience of grid- connected PV systems of countries overseas, the UK Government made the decision to become involved in the international collaborative research work which was being established to investigate the technical issues related to grid-connected PV, with the aim of tackling these issues to ensure that no unnecessary technical barriers existed to inhibit the future expansion of grid connected PV systems in the UK. Involvement in this work would have the added benefit of encouraging UK research and manufacturing companies to develop PV power systems for the UK and overseas markets. Objectives of the Work

The project was designed to:-

• explore the concerns of the UK ESI and address the concerns where possible • raise awareness of the potential for application of PV in the UK • provide the UK input to Task V of the IEA PV Power Systems Implementing Agreement and benefit from the results of that programme.

In detail, the objectives set down were:-

• To develop specific research and demonstration activities which will resolve concerns over technical problems • To assist the ESI to participate in the development of international standards and norms which will eventually apply in the UK via participation in their IEA work on pre-standards • To add to technical understanding of PV and grid connection in the ESI engineering centres • To assist the existing UK PV industry and new entrants to learn from the Task V technical activities and to channel the industry’s input to Task V discussion and decisions • To encourage co-operation between the PV industry and the ESI in the development and acceptance of new products • To develop the guidelines and reference materials for application of grid connected PV systems in the UK • To improve awareness in the UK PV industry and ESI of export opportunities • To develop recommendations for improved procedural practices

Summary of Work

It is generally accepted that grid interconnection of photovoltaic (PV) power generation systems enables effective utilisation of the generated power. It enables constant and automatic adjustment of electric power from the utility grid side and the PV system side. However, technical requirements of both the utility power system grid and the PV system must be satisfied to ensure the safety of the PV operator and the reliability of the utility grid. Clarifying the technical requirements for grid interconnection and solving the problems are therefore very important issues for the widespread application of grid-connected PV systems.

In order to address the technical issues and develop a consensus view, the project established a consultative group of representatives from the ESI. This group has been instrumental in both guiding the technical work and raising awareness in the industry.

This report also summarises the important findings from the first stage of activities of the International Energy Agency Implementing Agreement on Photovoltaic Power Systems working group examining “Grid Interconnection of Building Integrated and Other Dispersed Photovoltaic Power Systems” (commonly known as Task V) and critically examines these issues from a UK perspective. The Task V work took place between 1993 and 1998 and covered a review of existing techniques and rules related to grid interconnection of photovoltaic power generation systems and included both theoretical and experimental investigations.

The main objective of this work was to provide technical input into the development of technical guidelines for the grid-connection of photovoltaic (PV) inverter generators. These technical guidelines have subsequently been taken on board by the electricity supply industry, who are about to produce Engineering Recommendation G77, entitled , “UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA” to provide an industry-endorsed guide to grid-connected PV in the UK. The technical guidelines produced as part of this work formed the original basis for G77.

Summary of Results

A major output from the project has been the Draft G77 “UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA” which is currently being prepared for publication as formal guidance to the ESI. The provisional draft guidelines were launched to an audience of ESI representatives and PV industry representatives at a workshop 1999 hosted by EA Technology in February 1999. As well as this, the sharing of experiences between the members of this group and the PV specialists has helped to smooth the path for new PV installations during the life of the project.

Conclusions and Recommendations

The UK Consultative Group in conjunction with the IEA PVPS Task V working group has successfully completed the first stage of work by identifying grid interconnection issues of PV systems and drafting possible recommendations for improvement. During the period of this work it has been observed that some countries have developed new guidelines for grid interconnection and some countries have revised their guidelines due to the increased knowledge of requirements for grid interconnection of PV systems. The introduction of small-scale, grid-connected PV systems have also progressed in the UK and on a larger scale abroad. AC modules, a new concept of a PV module with a tiny embedded inverter (typically up to 100Wp output power), have been developed and are now sold in some countries, including the UK.

There are still many issues still to be resolved such as islanding issues, interconnection of many PV systems in a concentrated area, cost evaluation of grid interconnected PV systems and so on. These issues will be studied in future work, both within the UK PV Experimental Programme (Phase 2) and the newly established UK Strategic PV Network Study, in collaboration with the continuing work of the Task V Working Group. In general there is a need to look at the effects of a range of PV penetration scenarios on system operation and performance, with concentrations of multiple PV systems operating in UK climatic conditions. The development of the technical guidelines document highlighted the main areas of concern for electricity companies. These areas of concern are:

• voltage level control • power quality, including harmonics, power factor and DC injection • the real risks associated with islanding • safety, including earthing and generator isolation • rate of change conditions (e.g. fast moving clouds) and flicker • capacity of the existing LV network to accept PV systems • future design of LV distribution networks containing PV systems • the impact on infrastructure costs • the impact on operating and maintenance costs • effects on spinning reserve and unloading of conventional plant • effects of renewable energy systems with unsecure, variable generation characteristics • the value of the energy from PV to the integrated system • implications on distribution price control (i.e. DuoS) • connection agreements with PV generators.

There is a need to increase the awareness of photovoltaics within the UK and a need for further research focused on the wider impacts of large-scale embedded PV generation. Also, many of the issues discussed within the context of PV are also considered by electricity companies to be very relevant to the future development of other network-connected generator systems, such as fuel cells and small-scale micro- CHP.

It is also clear from the work carried out to date that the development of international standards is essential to allow the grid-connected PV market to develop in a sustainable fashion for the commercialisation of grid-connected PV systems. It has proven difficult to establish complete unanimity amongst the international community on grid-connected PV standards because of the differences in electrical network topologies in different countries. These differences have been magnified by some PV manufacturers who naturally endeavour to develop standards which are advantageous to their own products. However, a major step forward from the UK perspective has been the recent development of the UK's "Action Plan for PV Standards". The overall action plan covers PV cells and modules, stand-alone systems and building integration issues as well as grid connection issues. The main issues which were identified include the development of a type approval testing methodology (this is actually being carried out as part of the UK PV Experimental Programme), including type approval accreditation and installation testing and commissioning procedure specification.

Many of the issues raised during the course of this study will be tackled by a new project supported by the DTI, entitled "Strategic PV Network Study". This study will concentrate on the main areas of technical uncertainty which will need to be resolved before grid-connected PV systems become widespread. Consideration in this study will be given to possible roll-out scenarios for grid-connected PV inverter systems as a pre-cursor to developing models for PV/grid interaction (primarily looking at voltage levels and profiles on the LV distribution network). Another important element of the study will be a risk analysis of islanding. The results of the modelling work and the risk analysis will then be used to produce an impact study, examining the likely effects multiple grid-connected PV inverters will have on conventional electricity supplies.

Part of the UK activity will involve continued liaison with the activities of the IEA “Task V” working group, who will be continuing their successful work carried out to date within a new subtask, Subtask 50. The objective of the new subtask, called “Study on Highly Concentrated Penetration of Grid-Interconnected PV Systems”, is to assess the net impact of highly concentrated PV systems on electricity distribution systems and to establish recommendations for both distribution and PV inverter systems to enable widespread deployment of solar energy. GLOSSARY OF TERMS

AC Alternating Current DNO Distribution Network Operator DC Direct Current ESI Electricity Supply Industry IA Implementing Agreement IEA International Energy Agency LV Low Voltage MV Medium Voltage MWp MegaWatt (peak) OECD Organisation for Economic Co-operation and Development PES Public Electricity Supplier PV Photovoltaic PVPS Photovoltaic Power Systems RCD Residual Current Device REC Regional Electricity Company STP Strategic Technology Programme Contents Page

1 Photovoltaics in the UK 1 1.1 Background 1 1.2 Installed Systems 1 1.3 The UK Initiative 3 1.3.1 Early Grid-Connected PV Studies 3 1.3.2 Current Grid-Connected PV Studies 4

2 Technical Criteria for Electrical Integration of PV Systems 5 2.1 Origins of Project 5 2.2 Project Objectives 5 2.3 Project Participants 6 2.4 Project Activities 6 2.5 Task V from a UK perspective 8

3 UK Guidelines for Grid Connected PV Systems 9 3.1 The Development of PV Grid Connection Guidelines 10 3.2 Protection 12 3.3 Operation and safety 13 3.4 Power quality 14 3.4.1 Harmonics 15 3.4.2 Power factor 15 3.4.4 Flicker 16 3.4.5 DC injection 16 3.5 Commissioning and acceptance testing 16 3.5.1 Metering 17 3.5.2 Labelling 17

4 Summary of Task V Activities 18 4.1 Task V Activities 18 4.1.1 Subtask 10 19 4.1.2 Subtask 20 19 4.1.3 Subtask 30 20

5 Results 20

6 Conclusions 22

7 Future Work 23

8 Acknowledgements 26

References 27 Appendices

Appendix A: "Draft UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA"

Appendix B: "Summary of IEA Task V Activities"

Appendix C: "Review of Systems and Guidelines (Subtask 10)"

Appendix D: "Utility Aspects of Grid-Connected PV Systems (Subtask 20)"

Appendix E: "Demonstration Tests of Grid-Connected PV (Subtask 30)"

Appendix F: "List of Task V Reports"

Appendix G: "List of PV Systems Installed in the UK" 1 Photovoltaics in the UK

1.1 Background

The technology of solar photovoltaic (PV) cells and systems has developed rapidly over the last two decades in parallel with the development of the markets. Frequently a first choice power source in remote locations as ‘stand alone’ battery systems, photovoltaics are now being developed for connection to the local electricity distribution network as ‘embedded generators’. These systems are described as ‘network connected’, ‘mains connected’ or ‘grid connected’ in international parlance.

Many thousand Photovoltaic ‘embedded generators’ have now been connected worldwide, and their use is rapidly gaining ground in the leading industrialised countries. There have been a significant number of high profile projects and demonstration programmes leading to increased interest, whilst at the same time there have been rapid strides in the development of the inverters and other components in the power conditioner units, as well as improvements in cost.

Although they are not yet fully competitive with conventional power generation, or even most other renewable generators on purely economic grounds they attract interest because:-

• they are a clean power source • they are quiet and have no moving parts • they can be installed near the point of power demand • they offer high reliability, long life and low maintenance • they offer ownership of a power source to individuals and companies • they are modular and therefore are easy to adapt to a location and to extend • there are good prospects for continued reduction in costs as the markets and the technology develop

In addition, they offer the following advantages over stand alone systems:-

• they do not require expensive battery storage, as the grid provides the back-up source • they can be integrated into buildings and other structures, thus saving on cladding costs and land requirements

1.2 Installed Systems

In the UK the potential is seen for building-integrated grid-connected systems. However, it is interesting to note that although economic analysis suggests that the biggest potential in the market should be for large systems integrated into industrial type buildings, much of the current take-up is for domestic buildings, funded by ‘pioneer’ enthusiast owners who are members of the public.

1 The table in Appendix G lists some of the systems installed to date in the UK (derived from ETSU report ‘Update of the Database of Photovoltaic Installations in the UK, ETSU S/P2/00301/REP’ by Altechnica). Statistics from the table are that over 100 buildings utilised some form of photovoltaic array, with a total generating capacity approaching 1MWp. The buildings include some 60 houses, 9 offices, 5 university buildings, 10 school buildings and a factory. It is assumed that the majority of the systems are grid-connected although this data is not supplied in the report.

Ten of the schools are part of the Scolar Programme which is now installing 100 small systems in schools and colleges. Two of the projects are petrol service stations in the south of England which utilise various types of PV modules incorporated into the facia and roof canopies of the fuel retail areas. These are both part of roll out programmes, and will be the first contact with photovoltaic embedded generators for many Electricity Companies.

The scene internationally is very different. Environmental benefits and other positive factors are providing significant motivation to accelerate the technological and market development of grid connected PV systems. In the USA, Japan, Germany and the Netherlands there are major projects to introduce the systems on a large scale. There are demonstration projects in many countries and under the European Union RD&D programmes.

The USA initiated the ‘Million Solar Roofs’ programme in 1997 to install PV systems and/or solar hot water on one million roofs by the year 2010, Japan has planned to increase its installed capacity from 10,000 residential systems in 1997 to a capacity of 4,600MWp by 2010, and the German 1,000 roofs project in fact led to the installation of more than 2,250 systems.

This increasing international activity is resulting in cost-reduction of PV systems, especially in the area of building integration, and this means that significant reductions in basic system costs will continue to be made. If the current trend in cost reduction continues it will not be long before PV systems are a true commercial proposition for the UK, with a consequent benefit to the export market.

With this in mind, and taking into account the greater practical experience of grid- connected PV systems of countries overseas, the UK Government made the decision to become involved in international collaborative research work which was being established to investigate the technical issues related to grid-connected PV. This as part of a UK programme would aim to tackle these issues to ensure that no unnecessary technical barriers existed to inhibit the future expansion of grid connected PV systems in the UK. Involvement in this work would have the added benefit of encouraging UK research and manufacturing companies to develop PV power systems for the UK and overseas markets.

2 1.3 The UK Initiative

1.3.1 Early Grid-Connected PV Studies

In the UK the potential for the application of grid connected and building integrated photovoltaic systems for the home market has been investigated in a previous series of reports prepared for DTI between 1992 and 1993:

The first by the Newcastle Photovoltaic Applications Centre (1992), “The Potential Generating Capacity of PV-Clad Buildings in the UK”, ETSU Report S1365-P1, confirmed the large potential to integrate PV power generators into many of the building types found in the UK.

A second by Halcrow Gilbert (1993), “Grid Connection of Photovoltaic Systems”, ETSU Report S1394-P1, established that there are no fundamental reasons, technical or non-technical, why PV systems cannot be grid connected. However, it identified several issues of concern, both technical and commercial, to the supply industry which would need resolution before grid connected systems could become more widespread.

This research has been both on the UK level and on an international level, in collaboration with countries where more systems have been installed. It has involved a diverse range of interested parties all of whom are crucial to the take-up of PV in the UK, bringing together the expertise from the Electricity Industry, the PV industry and University research as well as interaction with the IEA Task V working group. This is shown in the figure below:

RECs CONSULTATIVE GROUP

UNIVERSITY COLLABORATIVE INTERNATIONAL RESEARCH IEATASKV PROJECTS

PV INDUSTRY STANDARDS BODIES

Figure 1: Collaboration in the UK for Grid Connected PV

3 1.3.2 Current Grid-Connected PV Studies

As part of the work programme developed by the DTI, a project was initiated in 1995, entitled “Technical Criteria for Electrical Integration of PV Systems”. This project was developed by EA Technology and Halcrow Gilbert with funding from both the DTI and the Electricity Supply Industry. This report forms the final report from this study.

The project was designed to:

• explore the concerns of the UK ESI and address the concerns where possible • raise awareness of the potential for application of PV in the UK • provide the UK input to Task V and benefit from the results of that programme.

The project established a consultative group of representatives from the ESI who have been instrumental in both guiding the technical work and raising awareness in the industry. A major output from the project has been the Draft G77 “UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA” which is currently being prepared for publication as formal guidance to the supply industry. As well as this, the sharing of experiences between the members of this group and the PV specialists has helped to smooth the path for new PV installations during the life of the project. It also resulted in the identification of the experimental work described below.

The identification of additional, experimental work required to support the above project led to the initiation of an experimentally-focused research programme. This programme started in 1996 and was titled ‘Co-ordinated Experimental Research into PV Power Interaction with the Supply Network’. This project represented a novel collaboration between EPSRC and ETSU for the DTI in which conventional EPSRC- funded university research projects were closely co-ordinated with a DTI research programme with co-ordination from the DTI/ETSU contractor.

The objectives for Phase I which were developed in conjunction with the electricity supply industry (ESI) were:

• to research the design and performance of small PV power conditioners for network connection • to investigate the potential for small power conditioners to operate in “islanded” mode • to investigate the effects of multiple installations and possible interactions between inverters • to resolve key technical issues relating to the integration and operation of PV systems connected to the UK electricity supply • to disseminate information and experience from the project work • to help raise the understanding of PV technology within the electricity supply industry

4 • to provide the UK R&D input for Task V of the IEA Photovoltaic Power Systems Programme

The key issues chosen were the performance of small power conditioners in terms of power quality and safety of the network and connected equipment, the interaction between power conditioners, and the cumulative effect of multiple installations on a section of the distribution network.

2 Technical Criteria for Electrical Integration of PV Systems

2.1 Origins of Project

This project was set up in the UK with the overall objective to resolve or assist the resolution of the important technical criteria and procedural issues which constitute barriers to the development of the application of grid connected PV systems, and in so doing disseminate technical understanding of the technology. The UK is achieving this within the framework of Task V activities, bringing the considerable benefits of extensive international experience and knowledge.

2.2 Project Objectives

In detail, the objectives originally set down are:

1. To develop specific research and demonstration activities which will resolve concerns over technical problems 2. To assist the ESI to participate in the development of international standards and norms which will eventually apply in the UK via participation in their IEA work on pre-standards 3. To add to technical understanding of PV and grid connection in the ESI engineering centres 4. To assist the existing UK PV industry and new entrants to learn from the Task V technical activities and to channel the industry’s input to Task V discussion and decisions 5. To encourage co-operation between the PV industry and the ESI in the development and acceptance of new products 6. To develop the guidelines and reference materials for application of grid connected PV systems in the UK 7. To improve awareness in the UK PV industry and ESI of export opportunities 8. To develop recommendations for improved procedural practices

The contract was extended to ensure that the activities in the first phase of the programme were brought to a satisfactory conclusion and to achieve full benefit from participation in Task V after that programme was extended. The plan for the extension emphasised the dissemination activities and the preparatory work for the

5 development of international standards. A final seminar in the UK was also added to the programme.

2.3 Project Participants

The project was operated by a team of representatives taken from both the ESI and the UK PV Industry. EA Technology took the lead role in co-ordination and management of the activities and was assisted by Halcrow Gilbert with representation at IEA meetings, the production of technical documents, and the organisation of dissemination or consultation exercises. The Electricity Association, in partnership with Halcrow Gilbert, act to promote the transfer of information and to co-ordinate the Consultation Group from the ESI, the members of which are named in the next section. The consultation group contribute their knowledge and experience by acting as a primary source of information on ESI requirements and interests. This is at their own cost, and this has been an invaluable input to the programme. EA Technology and Halcrow Gilbert act under the auspices of the UK PV Association to co-ordinate the PV industry interests.

Additional effort and interest from the academic community and interested manufacturers is encouraged and has resulted in useful contributions as well as wider dissemination of the results.

2.4 Project Activities

The project activities are summarised below.

Activity 1 Standards and Regulations

1. This is the core activity for the programme. A draft document on Guidelines for grid connection of small PV systems has been drafted with the ESI to supplement current Engineering Recommendations. Available standards and recommendations have been reviewed with the REC consultative group and the routes for introduction of change have been explored.

2. A review of BS 7671 (16th of IEE Wiring Regulations) for areas that are unclear about PV systems or may need revision and stimulation of action for the next revision of the standard was proposed, but has not been progressed into a documented form because it is not of direct interest to the electricity utilities.

Activity 2 Experience and Guidelines

1. Experience of both UK and overseas projects has been exchanged within the REC consultative group to increase awareness, and has been a resource for the draft Guidelines.

2. Guidance for the REC system planners is contained in the draft Guidelines.

Activity 3 Technical Development

6 1. Metering: A study of acceptable two way power metering systems has been difficult to develop as the electricity industry has been engaged in the greater task of dealing with metering issues for the open market. OFFER have been approached and a dialogue established on the concerns of the PV industry regarding the impact of new metering arrangements.

2. The work in this project identified the research themes for the co-ordinated experimental programme which is discussed in section 1.3.3. A study of the effects of multiple inverter connections to typical UK distribution lines has been undertaken. A research programme on power conditioner performance and interaction has also been undertaken.

Activity 4 Dissemination

1. Dissemination activities have continued throughout the project, including presentations at conferences and workshops, the relations with the consultative group of RECs and other supply industry contacts, and dissemination through the British Photovoltaic Association events and newsletter. This effort has resulted in an improved awareness of photovoltaics in the electricity supply.

2. The small group of UK manufacturers with activity in grid connected PV equipment has shown interest in the project. New companies have entered the scene in the last three years.

3. An interface and improved understanding between the RECs and the PV industry has been established which will assist the connection of new systems on to networks and the development of mutually acceptable standards.

Activity 5 Programme Management

1. Overall project management has been provided by EA Technology while Halcrow Gilbert have provided support, particularly in the development of the REC consultative group and liaison with PV-UK.

Activity 6 Integration in IEA Task V

1. EA Technology and Halcrow Gilbert have provided representatives to the Task V six-monthly meetings and disseminated the information from Task V in the UK. In addition, they have provided major contributions to selected parts of the Subtasks as follows:

Sub Task 10 Review of Systems and Guidelines

• Submit UK responses to questionnaire survey; • Collect ESI views on problems and technical issues to be addressed in Task V; • Comment on questionnaire form and results; • Prepare a technical briefing paper for the ESI and for manufacturers/other industries on the programme and objectives of Task V.

7 Sub Task 20 Definition of Guidelines

• Liaise with ESI over their technical concerns and presentation of their requirements for inclusion in the guidelines; • Arrange consultation with the ESI through a technical workshop; • Assist with drafting the guidelines and identifying key issues, representing the UK views; • Disseminate the guidelines in the UK and collate comments for feedback to Sub Task. Workshop on new standards and new products for grid connection; • Endeavour to obtain commitment from the ESI and industry to develop practical projects for inclusion in the remaining stages of the programme.

Sub Task 30 Demonstration Test Sites

• Participation in testing and demonstration of the new guidelines. Expand UK participation in Task V by introducing new partners who will either develop testing/ demonstration in the UK or will work with other countries on their test sites.

Sub Task 40 Summary of Results

• Report on UK activities and contribution to sub task reports; • Dissemination through publications and seminars for ESI and industry; • Participation in preparation of new UK standards/specifications and guidelines.

A new Subtask 50 has now been established, with the UK input to be provided under the follow-on ‘Network Strategy’ project.

2.5 Task V from a UK perspective

The UK’s participation in the international activities of the Task V working group has allowed UK electricity companies and the UK PV industry an opportunity to consider the technical issues related to grid-connection of PV systems before such systems become widely used. It is important that any PV system which is likely to be connected to the UK’s electricity network combines safety, reliability and cost- effectiveness.

There are no fundamental principles that prevent the connection of PV systems into local electricity supply . However, electricity companies have a legal requirement to maintain safety standards and at the same time achieve a high quality of supply to customers, both of which could be compromised by the characteristics of PV systems incorporated into the network. Safety considerations include the fact that in the event of a network power supply interruption, the PV generator could still maintain power in some parts of the network - referred to as 'islanding'.

The Task V working group has provided useful background information and validation for the UK’s technical guidelines document (see Appendix A), which was

8 officially launched in conjunction with the 2nd UK PV Conference which took place at the Renaissance Hotel in Manchester on the 17th/18th February 1999. The guidelines have been taken on board by the Electricity Association and (following on from an official electricity industry review) are shortly to be issued as an official industry standard document in the form of an Engineering Recommendation, G77.

3 UK Guidelines for Grid Connected PV Systems

A key feature of the studies outlined in the previous chapter was the involvement of a Consultative Group from the Electricity Supply Industry (ESI). This was originally convened in 1994 under the project reported on here, but has also proved invaluable to subsequent projects to date. It is important to acknowledge that this support in time and cost has been given as an in-kind contribution by the electricity companies to the various projects, and that without this group it would have been impossible to develop the forerunner to the Draft G77 Recommendations to the point where they are now near publication.

The Consultative Group currently consists of:

• the Electricity Association as the overall body responsible for publishing ESI regulations in the UK

• a group of electricity generators and distributors including:

London Electricity South Western Electricity Eastern Electricity East Midlands Electricity South Wales Electricity Northern Electric Manweb PowerGen Midlands Electricity Seeboard

• co-ordinators Halcrow Gilbert and EA Technology

Its terms of reference are to meet approximately twice a year to consolidate work with other working groups on topics such as the guidelines for grid connection. Information was exchanged between the PV and Electricity Industry, including experience on other forms of distributed generation and research on photovoltaic systems. It has also provided valuable input into the Task V work through the supply of survey data and feedback on international results.

It was formed by drawing on two existing links; the Electricity Association’s continued representation of the ESI in some areas including standards development, and EA Technology’s co-ordination or management of a number of common programmes which are supported by some or all of the RECs. These links were drawn

9 together by Halcrow Gilbert in 1994, to set up a consultative group of industry representatives which grew to involve the companies or their subsidiaries listed above.

The requirement for the work was that, despite the large potential, the integration of Photovoltaic Power Systems into local electricity supply networks in the UK was currently in the early stages of development compared with countries abroad. The current high costs of PV systems and lack of a major government installation initiative, meant that relatively few requests had been received by UK electricity companies to date to stimulate development for the connection of such systems. In fact, despite the table in the previous chapter, the UK has one of the lowest quotients of installed PV capacity per head of population (i.e. kWp/capita) of any of the countries involved in the IEA’s PV agreement.

The Consultative Group has been instrumental in both guiding the technical work and raising awareness in the industry. The sharing of experiences between the members of this group and the PV specialists has helped to smooth the path for new PV installations during the life of the projects so far.

3.1 The Development of PV Grid Connection Guidelines

The rationale behind the development of the PV Grid Connection Guidelines (contained in Appendix A) can be traced back many years when the growing potential for the connection of single-phase low voltage (LV) generators to the mains supply at the domestic household level was first recognised. The connection of small-scale generators dispersed within the LV distribution network is often referred to as “dispersed or “embedded” generation. This type of generation effectively changes the electricity distribution system from a passive to an active network and therefore has implications for the design, management and operation of the distribution network. Grid connected generators operate in parallel with the existing mains supply and therefore the supply network operates in a manner for which it was not originally designed.

The UK electricity transmission and distribution networks were designed to transport electrical energy from the few very large generating power stations to the many domestic, commercial and industrial users of electricity. There is a predominance of generation in the north of England, with major load centres in the south. Thus, bulk electricity transmission from north to south is achieved by the National Grid transmission network, operating at 275kV and 400kV, whilst local electricity distribution is achieved on the medium voltage (MV) distribution network, mostly at 33kV and 11kV. Distribution to domestic premises is then achieved using the LV distribution network, operating at 230 volts AC.

Considered in its simplest form, the design of the distribution network is based on the assumption that power flows from the higher voltage levels to the lower levels via transformers and the direction of current flow is from the primary and distribution transformers to the customer loads. This assumption is used to predict the voltage profiles along the cables and overhead lines which supply the customers In this way,

10 it can be ensured that the electricity supply that customers receive is within the statutory voltage limits (i.e. 230V+10%, -6%). The size and ratings for the network plant (such as the distribution transformer, associated switchgear, cables/overhead lines and protection) are determined, based on predicted consumer load profiles and known local physical topology. However, the addition of dispersed generation makes these original design calculations no longer valid, since the embedded generation will effect the actual voltage profiles on the network, as well as associated fault levels and protection settings.

Many technical aspects related to the connection of embedded generators are covered by the electricity supply industry (ESI) Engineering Recommendation G59/1 (amendment 1). Whilst this document does consider single-phase LV connected generators, it was written mainly to cater for larger generators (typically >1MW power rating), which would typically be connected to the MV network, (11kV is the typical MV distribution voltage level in the UK). Also, G59/1 tends to treat the connection of single-phase LV generators as a special case derived from the general case, but if the numbers of single-phase embedded generators increases significantly in the near future, a different approach will need to be taken towards such generation.

The procedures laid down in G59/1 place a significant burden in terms of time on the technical engineering departments of electricity companies to ensure safe, competent installation of embedded generators. This is acceptable when dealing with a handful of larger (i.e. >1MW) schemes. However, such a high level of direct involvement by technical personnel would not be appropriate for a larger number of smaller schemes. Thus, the aim of the technical guidelines (and associated type approvals document) is to provide a cost effective solution to the safe and reliable connection of single phase PV generators to the LV network.

The technical guidelines document covers the four main areas considered to be important in the context of achieving safe and practical interconnection of single phase PV inverter generators. These areas are:-

• protection • operation and safety • power quality • commissioning and acceptance testing

These issues are discussed in more detail later in this chapter.

The technical guidelines document gives recommendations for best practice and highlights any existing standards that need to be adhered to in the grid connection process, including European standards, British standards and ESI standards and regulations. In the absence of formal standards, the guidelines recommend tests which can be carried out to allow grid connected PV systems to obtain “approved” status from the UK electricity supply industry, in the form of "type testing".

The concept of an “ESI Approved” inverter is an important one, since it shows that the inverter design has considered the relevant factors related to grid connected operation and the inverter meets the criteria laid down by the electricity industry to

11 ensure safe operation, with the correct protection functions and can operate without significantly degrading power quality to the PV owner/operator and other consumers.

When evaluating the likely effect that small-scale single phase PV generators might have on the electricity distribution network, the following principles are considered:-

• ensuring that the statutory requirements set out in the Electricity Supply Regulations 1988 are met • meeting the requirements of the Distribution Code (and Grid Code, if appropriate) • ensuring the security of supply to other customers is not prejudiced • ensuring quality of supply to other customers is not prejudiced. • ensuring plant and equipment already on the distribution system will operate within their rating • ensuring the grid inter-connection meets appropriate technical standards

The issues which are of primary concern include the effect that PV-inverter generators connected to the network will have on:-

• network capacity • network security • voltage regulation • power quality • earthing • fault level • asset utilisation • losses

It is part of the network operator's licence that the design of the network connection for any generator must be achieved at minimum cost, whilst meeting all of the relevant legal, regulatory and safety requirements.

3.2 Protection

Protection can mean different things to different people, depending on perspective and viewpoint. In general, protection is necessary to fulfil several different requirements and in the context of electrical safety of PV systems these requirements include:

• protection of people (including electricity company personnel, customers and the general public) • protection of the electrical network • protection of the PV inverter system (including the installation)

The voltage and frequency protection trip limits specified in the guidelines document are fairly straightforward, being identical to the G59/1 limits (which are in turn directly related to the supply limits set in the Electricity Supply Regulations (1988)).

12 The issue which causes the most discussion and commands the least consensus is the issue of loss of mains (LOM) protection under an “islanding” condition. Islanding is a phenomenon which can only occur when generation is embedded within the distribution network. The islanding phenomenon refers to the condition which can exist whereby part of the network has become electrically isolated from the main network and all the loads within that network are being supplied by the generators embedded within that network. The electricity company (or more accurately the public electricity supplier, or PES) has no way of controlling the voltage and frequency of an islanded network, since it has no control over the generation within the islanded section of network Since the PES is responsible for the safety of the network and for maintaining satisfactory power quality, it will require all generation within the 'island' to be shut down to ensure the system is in a safe state and to prevent customers receiving a supply that could jeopardise their equipment.

However, power quality is not the only issue which is raised by islanding. The debate surrounding the risks of islanding is normally associated with megawatt scale generators. With these larger generators, the energy stored by virtue of the inertial mass of the rotor shaft means that the consequences of an "out-of-sync" circuit breaker reclosure on a multi-megawatt generating set can be quite dramatic. Such events have been recorded and have resulted in the generator shaft being torn out of its bearings, allowing it to make contact with the generator stator and thereby completely destroying the generator. An out-of-sync reclosure is still an event that could effect grid connected PV inverters, but the lack of mechanical inertia within the inverter should mean that the inverted is in a better position to survive such an event without damage to itself.

3.3 Operation and safety

The UK enjoys a relatively good safety record with respect to electrical safety, mainly due to high safety standards, and good education about the dangers of electric shock. Thus, within the electricity industry there are fewer accidents and injuries related to electric shock than caused by other physical causes (i.e. motor vehicle accidents, falls from 11kV wood poles, etc.). Similarly, within the domestic environment there are more deaths caused each year by fire due to electrical faults (such as faulty earthing) than are due to electric shock.

The concern for safety is the main reason for the clauses within the technical guidelines document stating the need for various switches, including a manual isolation switch and a mechanical isolation switch (often referred to as the automatic disconnection switch). There has been some confusion over the various terms used and this has resulted in confusion in understanding the purpose and the reasons behind the requirement for each switch. The following paragraphs attempt to clarify the situation.

Firstly, there is a requirement for a manual isolation switch, which is "easily accessible" and "lockable". This switch should isolate the PV inverter output from both the domestic circuit and the grid supply to allow maintenance to be done to either. The switch needs to be lockable to allow it to be a true point of isolation,

13 otherwise someone could close the switch whilst someone else was still working on the installation. The use of lockable isolation switches is standard practice within the ESI.

The second requirement is for the automatic protection (including islanding protection) to be via a mechanical relay to IEC 255. There are two reasons for this. Firstly, the requirement for a mechanical relay relates to the IEE Wiring Regulations, which requires any point of isolation to have a physical gap between conductors. Secondly, the IEC 255 (electrical relays) standard refers to the insulation and isolation performance of relays between the relay CONTROL pins and its INPUT/OUTPUT pins (and NOT between the INPUT-OUTPUT pins when the relay contacts are open). The purpose of this requirement is to prevent voltage transients present on the electrical supply (such caused by local lighting activity) and which will also appear on the relay I/O terminals from effecting the integrity of the controller which is controlling the relay (and likely to be controlling other peripheral devices as well). Thus, the automatic protection is not required solely for the islanding protection function, but to disconnect the equipment under any “fault” condition (such as PV array failure, inverter failure, inverter power transistor failure, over/under voltage, over/under frequency, islanding, etc).

In practice, it is likely that any LV network which is likely to have PV inverters connected to it will have to be treated as if it were still live by anyone called upon to work on it. Maintenance staff would feel no more re-assured that a PV inverter was isolated by an automatic mechanical relay than they would if the inverter were isolated by a solid-state switch. Live-line working at LV voltages is carried out for certain maintenance and repair functions already, but under very strict, controlled procedures and only if there is no other way of carrying out the work. There is constant pressure from the Health and Safety Executive (HSE) to reduce the amount of live-line working carried out within the ESI and so a likely outcome for LV networks with grid-connected PV inverters is that the HSE will insist on ensuring as far as possible that the line is not energised, but to call for live-line working techniques to be adopted in any case. This does have implications for the costs of network maintenance, since live-line working techniques are more costly than normal working techniques.

3.4 Power quality

The level of power quality experienced by any consumer is the responsibility of the contracted public electricity supplier (PES). This is governed by the PES licence, which is a legally binding document, formally linked to the Electricity Supply Regulations (which defines such things as nominal supply voltage) and other industry documents, such as the Distribution Code and Grid Code, both of which are further supplemented by engineering recommendations (such as G59/1, etc.) and engineering equipment approvals lists. Thus, the PES license holder has responsibility for ensuring adequate power quality for its customers and it is because of this responsibility that power quality is included within the scope of the technical guidelines document.

14 In the context of the technical guidelines document, power quality includes harmonics, power factor, flicker, EMC and DC injection.

3.4.1 Harmonics

Levels of harmonic generation by non-linear loads have been subject to limits for many years. Harmonics can be viewed as a form of electrical pollution on the distribution network, since they provide no useful benefits but their existence can interfere with the correct operation of certain types of systems connected to the network. Such systems include line-commutated rectifiers and mains-synchronised clock circuits, etc. Excess harmonics can also result in network plant having to be operated at reduced ratings. Thus, it is in the interest of both distribution network operator (DNO) and consumers alike to minimise the levels of harmonics present on the network. The most notable incident related to excess harmonics occurred in the late 1960's and was caused as a result of the rapid expansion of television. The power supplies used within the TV sets of that period were very crude, with the cheapest using half-wave rectification (which saves the cost of three diodes, compared with full-wave rectification!) and produces high levels of even-order harmonic distortion. The effect on the network of the high level of harmonics was: a) some household clocks which were synchronised with the mains frequency ran too fast b) large harmonic currents were present at the LV distribution transformer, causing localised overheating

The problem was cured mainly by simple, minor changes in the manufacturing procedures used in the TV's power supplies. The principle change in the manufacturing process allowed for a random orientation of the half-wave rectifier diode, which allowed for a statistical spread of load between the positive and negative sine wave cycles of the mains supply, thus balancing out the even-order harmonic currents.

3.4.2 Power factor

Power factor is a technical term defining the relative displacement in time between the mains voltage waveform and the current waveform. (Both of these waveforms are sinusoidal, but the peaks will only be co-incident at unity power factor).. “Real power" relates to the current component at peak voltage, whilst “reactive power" relates to the current component at voltage zero. Many domestic loads (most notably induction motors, commonly found in refrigerator compressor units etc.), require a reactive power component to operate correctly. In this context, the network is often referred to as “supplying Vars”. Approximately 30-50% of domestic load (excluding space heating) is made up of loads requiring reactive power (i.e. induction motors).

"Useful work” is only done by the “real” part of the power (the domestic meter measures only “real” power). So, since for a sustained island to occur there needs to be a match of both real and reactive power components, restricting grid connected PV inverters to only exporting real power dramatically reduces the chances of the necessary conditions for islanding being met.. This is why the technical guidelines

15 document restricts PV inverters to export between unity power factor and 0.95 leading (i.e. importing Vars). Note, this refers to reactive current "exported” to the network, and does not restrict the inverter from supplying reactive power to the loads within the same premises as the PV generator. However, the simpler inverter designs are not likely to be able to distinguish between power exported to the network and power taken by the domestic loads. In this case, the inverter output will be restricted to operate between unity power factor and 0.95 leading.

3.4.4 Flicker

Flicker problems are normally associated with non-linear or pulsed loads, such as welding equipment. As its name suggests, flicker is associated with lighting and is caused by repetitive, periodic voltage fluctuations. The eye is particularly sensitive to fluctuations in the range once per second to ten times per second, where voltage fluctuations of 1% or less can be detected by the human eye. Concerns over flicker is much greater in other European countries than it is in the UK, mainly because of differences in electrical network configurations. However, there is general concern that because of the time-varying output of PV systems, they have the potential to cause flicker problems. Thus, flicker is given consideration in the connection guidelines document.

3.4.5 DC injection

The purpose of the electricity distribution network is to distribute electrical power at 50Hz AC. The presence of anything else on the network, such as harmonics, transients, and DC is therefore regarded as pollution. However, current standards make no reference to DC, other than to say its presence is "deprecated" (i.e. not approved). This effectively says that DC should be zero. This requirement can be easily met if all connections to the network are made to linear devices (such as resistors, capacitors or inductors) or via a transformer. However, with the development of modern power electronic devices it is possible to design PV inverters whose power devices can interface directly to the electrical network. This can remove a costly component from the inverter design, but it does mean that there is likely to be some residual DC output component from the inverter, however small. Thus, in the absence of a defined limit for DC within existing standards, the technical guidelines document states a maximum value of 5mA DC for a single installation. This is in line with limits set for harmonic currents and is less than the sensitivity levels used in RCD protection devices (typically 25mA AC).

The 5mA limit could be changed in the future (either up or down), dependent on operational experience, further research or additional information on the likely effects of DC on the network.

3.5 Commissioning and acceptance testing

Under current regulations, most notably G59/1, the electricity company asks to either witness the commissioning of generators or to see copies of the commissioning test results. This requirement for witnessing of commissioning tests was aimed at the

16 larger embedded generator (of typically 1 to 5 MW size). Such a procedure is obviously not practical for grid-connected PV inverters at the domestic-level and so the technical guidelines document aims to simplify the process by agreeing to a minimum number of commissioning tests for an “Approved” inverter. DNOs have reserved the right to have additional commissioning tests carried out for “Non- approved” inverters, and may wish to witness the commissioning also.

3.5.1 Metering

Much has been written about metering for PV systems and so only a brief review will be given here. Needless to say, the metering aspect will form part of the Connection Agreement. "Net metering", where exported units run the meter backwards (making the PV-generated unit selling price effectively the same as the purchase rate for utility- supplied electricity), is seen as the most attractive option for PV system owners and gives quite an attractive rate of return (some estimates have shown that this method can contribute approximately 15% of the PV system cost recovery). However, for any electricity import/export arrangement, a meter with two separate metering registers is required. It is not satisfactory to allow the meter to simply run backwards for exported units, since this leaves the meter open to fraudulent abuse. In any case, the commonly used Ferraris electro-mechanical meter is mechanically damped to prevent the meter from rotating backwards. Also, modern electronic meters can be programmed to run only in one direction (i.e. only forwards), again to combat meter fraud. A more conventional approach is to use a PV-generated unit selling price 1 equivalent to the electricity pool price (approximately /3 of the domestic purchase rate). However, the cost of the two-way import/export meter becomes a major cost factor for smaller PV installations . For smaller systems the value of units exported is likely to be small and so it may be beneficial to ensure that as much of the PV output as possible is used within the house. A final option may be to negotiate a fixed, unmetered, annual payment from the local PES.

3.5.2 Labelling

Adequate labelling of the installation provides a very simple but effective way of ensuring improved safety of PV inverter installations. This is particularly important in the early development of the grid connected PV market, since these types of systems will be unfamiliar to many people until they become much more widely used. Simply indicating the presence of on-site generation at the key electrical locations (service termination, meter position and isolation switch - normally in the same place anyway) will encourage people to proceed with additional care. For electricians and electricity company personnel, having a circuit diagram available of the PV system wiring as well as summaries of protection settings within the equipment can save valuable time on-site. It is also helpful to have contact details for the supplier/installer/maintainer of the equipment should queries arise, and so labels on the equipment giving such information would also improve maintenance efficiency.

17 4 Summary of Task V Activities

The International Energy Agency (IEA), headquarters in Paris, is an autonomous body within the framework of the Organisation for Economic Co-operation and Development (OECD). The Agency was established in November 1974 to carry out a programme of energy co-operation among its 23 member countries: Australia, Austria, Belgium, Canada, Denmark, Finland, France, Germany, Greece, Ireland, Italy, Japan, Luxembourg, the Netherlands, New Zealand, Norway, Portugal, Spain, Sweden, Switzerland, Turkey, the United Kingdom and the United States.

An important part of the IEA’s programme is collaboration in research, development and the demonstration of new energy technologies. This work is carried out through a series of collaborative programmes to which countries opting to sign up have to agree to commit a minimum level of resources. The national resource is usually a mixture of direct government support, research funds and contributions from relevant industries and agencies.

The International Energy Agency (IEA) Implementing Agreement on Photovoltaic Power Systems (PVPS) Task V Working Group is entitled: “Grid Interconnection of Building Integrated and Other Dispersed Photovoltaic Power Systems" and since its formation in 1993 has investigated the grid interconnection issues through international collaboration. The main objective was to develop and verify technical requirements which may serve as technical guidelines for grid interconnection of building integrated and other dispersed PV systems. The development of these technical guidelines will enable safe and reliable interconnection of PV systems to the utility grid at the lowest possible cost.

Comprehensive Task Reports have been issued under the auspices of the International Photovoltaic Power Systems Programme and so those readers interested in the details of the work and would like further reading are invited to obtain copies of other reports, as listed in the Reference section of this report, with supplementary material listed in Appendix F. Task V has proved to be one of the most productive and focused groups within the IEA PV Implementing Agreement.

A brief summary of the Task V activities is given below. A more comprehensive summary can be found in the report “Grid-connected Photovoltaic Power Systems: Summary of Task V Activities from 1993 to 1998”, IEA-PVPS Report, IEA-PVPS T5- 03: 1999, March 1999.

4.1 Task V Activities

In order to achieve the objectives of Task V, three sub-tasks were established and collaborative work between participating experts in Task V was conducted.

• Sub-task 10 : Review of existing PV grid interconnection guidelines, grid structure and previously installed PV experience

18 • Sub-task 20 : Theoretical studies on various aspects for grid interconnection and configuration of PV systems

• Sub-task 30 : Experimental tests using the Rokko Island and/or other test facilities

4.1.1 Subtask 10

Sub-task 10 had four items of study. The first item was a review of existing technical guidelines or local regulations for the interconnection of PV systems with the utility network. This survey enabled an understanding of the general requirements for safe and reliable interconnection to be obtained. The result of the survey was summarised as Task V internal report, “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries (Revised Version)”, Task V Report IEA-PVPS V-1-03, March 1998. The second study item reviewed utility distribution network configurations in order to understand the differences in grid interconnection requirements from country to country. The result of the survey was also summarised as a Task V internal report, “Information on electrical distribution systems in related IEA countries (Revised Version)”, Task V Report IEA-PVPS V-1-04, March 1998. The third study item reviewed the status of technology for interconnecting equipment (inverters and protective devices) in order to identify the applied criteria to satisfy the requirement for grid interconnection. Specifications and characteristics of many inverters and protective devices have been collected and summarised as a Task V internal document. The fourth item was a review of the operating experience of grid interconnected PV systems to identify any known problems relating to PV system operation.

4.1.2 Subtask 20

The scope of work for sub-task 20 was to analyse present day and possible future problems relating to the grid interconnection of PV systems and to draft possible recommendations for improvement. The following subjects were considered.

• Harmonics • AC-Modules • Multiple inverters and AC grid • Grounding of equipment in PV systems • Ground-fault detection and array disabled for PV systems • Overvoltage protection • Islanding • Electro-magnetic compatibility (EMC) of inverter • External disconnect • Re-closing • Isolation transformer and DC injection

19 The results of the work were given in the formal IEA PVPS report, “Utility aspects of grid connected photovoltaic power systems”, Task V Report IEA-PVPS-T5-01, December 1998. The scope of the work, theoretical results, experimental results, conclusions and recommendation for future work were described for each subject. The information within the subtask 20 report is easy to access and useful for readers with a technical background.

4.1.3 Subtask 30

In sub-task 30, actual experimental tests of PV system grid interconnection were conducted using the Rokko Island Test Facility in Japan. These tests were intended to show the actual phenomena of grid interconnection problems and to be the reference for subtask 20 work. In reality, only a few tests were actually directly related to subtask 20 work. Subjects of the tests conducted at Rokko Island are listed below.

• Output power variation of many PV systems • Harmonics from PV systems • Temperature measurement of PV array • DC injection • Islanding • Short circuit fault in distribution system • Output power variation of PV system with battery

The results of the work was given in the formal IEA PVPS Task V report, “Demonstration Tests of Grid-Connected PV Power Systems”, Task V Report IEA- PVPS- V-3-01, May 1999.

5 Results

A major output from the project has been the Draft G77 “UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA” which is currently being prepared for publication as formal guidance to the ESI. The provisional draft guidelines were launched to an audience of ESI representatives and PV industry representatives at a workshop hosted by EA Technology in February 1999. As well as this, the sharing of experiences between the members of the REC consultative group and the PV specialists has helped to smooth the path for new PV installations during the life of the project.

The UK technical guidelines document was itself based on the foundation work carried out within the international Task V working group activities. Several Task V reports produced the background information which was subsequently used to identify areas of common concern related to grid-connected PV systems. The information gathered together in the reports was also helpful in understanding the reasons behind any differences in approach adopted by different countries.

20 Firstly, the fundamental parameters and characteristics of electrical networks were compared from each country in order to identify common features and differences between countries. This information is contained in the following reports:

• “Information on electrical distribution systems in related IEA countries”, Task V Internal Report, IEA-PVPS V-1-02, July 1996.

• “Information on electrical distribution systems in related IEA countries (Revised Version)”, Task V Internal Report, IEA-PVPS V-1-04, March 1998.

Other reports included several survey reports on the status of guidelines development, including:-

• “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries”, Task V Internal Report, IEA- PVPS V-1-01, July 1996.

• “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries (Revised Version)”, Task V Internal Report, IEA-PVPS V-1-03, March 1998.

• “Proceedings of the IEA Workshop on Existing and Future Rules and Safety Guidelines for Grid Interconnection of Photovoltaic Systems”, Zurich, September 1997.

A report of a more technical nature was then produced which examined the technical issues of grid-connected PV systems in more detail. This report was entitled “Utility Aspects of Grid Interconnected PV systems”, IEA-PVPS Report, IEA-PVPS T5-01: 1998, December 1998. Those readers who wish to develop a deeper understanding of the technical issues of grid-connected PV systems are encouraged to read this report.

Finally, the results of experimental work carried out by members of the Task V working group (principally Japan - Rokko Island) are detailed in the report “Demonstration Tests of Grid Connected Photovoltaic Power Systems”, IEA-PVPS Report, IEA-PVPS T5-02: 1999, March 1999.

From the above, it can be clearly seen that the development of the technical grid connection guidelines document illustrates the successful interaction that has taken place between the REC consultative group and the Task V working group, where a wealth of international experience has been tapped into as well as the UK experts contributing to the developing international knowledge on the subject of grid- connected PV systems.

21 6 Conclusions

In general there is a need to look at the effects of a range of PV penetration scenarios on system operation and performance, with concentrations of multiple PV systems operating in UK climatic conditions. The development of the technical guidelines document highlighted the main areas of concern for electricity companies. These areas of concern are:

• voltage level control • power quality, including harmonics, power factor and DC injection • the real risks associated with islanding • safety, including earthing and generator isolation • rate of change conditions (e.g. fast moving clouds) and flicker • capacity of the existing LV network to accept PV systems • future design of LV distribution networks containing PV systems • the impact on infrastructure costs • the impact on operating and maintenance costs • effects on spinning reserve and unloading of conventional plant, • effects of renewable energy systems with unsecure, variable generation characteristics • the value of the energy from PV to the integrated system, • implications on distribution price control (i.e. DuoS) • connection agreements with PV generators.

22 on grid-connected PV standards because of the differences in electrical network topologies in different countries. These differences have been magnified by some PV manufacturers who naturally endeavour to develop standards which are advantageous to their own products. However, a major step forward from the UK perspective has been the recent development of the UK's "Action Plan for PV Standards". The overall action plan covers PV cells and modules, stand-alone systems and building integration issues as well as grid connection issues. The main issues which were identified include the development of a type approval testing methodology (this is actually being carried out as part of the UK PV Experimental Programme), including type approval accreditation and installation testing and commissioning procedure specification.

It is anticipated that by the time this report is published, G77 will have reached official approval status as an ESI standard engineering recommendation. In view of the pioneering nature of G77, it is fully anticipated that it will need to undergo formal revision early in its lifetime (say, after about 18 months) in the light of practical experience in order to gain a long-lasting mutual acceptance from both the ESI and the PV industry.

The success of the present project and related work in reducing technical and non- technical barriers to an enlarged PV market have resulted in the confidence to continue activities in this area. Phase II of the ‘Co-ordinated Experimental Research into PV Power Interaction with the Supply Network’ Project was established in 1998 to carry out research to progress the type-testing requirements for PV power conditioners to be connected to the UK network and draft the procedures for testing. It also provides the mechanism to continue the successful collaboration with the Consultative Group from the Regional Electricity Companies (RECs) and prepare for constructive UK participation in the international work on inverter standards.

At the end of these projects, it is hoped that the majority of the technical barriers will have been researched and where feasible measures put in place to remove or reduce them.

7 Future Work

Many of the issues raised in the previous section of this report will be tackled by a new project supported by the DTI, entitled "Strategic PV Network Study". This study will concentrate on the main areas of technical uncertainty which will need to be resolved before grid-connected PV systems become widespread. Consideration in this study will be given to possible roll-out scenarios for grid-connected PV inverter systems as a pre-cursor to developing models for PV/grid interaction (primarily looking at voltage levels and profiles on the LV distribution network). Another important element of the study will be a risk analysis of islanding. The results of the modelling work and the risk analysis will then be used to produce an impact study, examining the effects likely effects multiple grid-connected PV inverters on conventional electricity supply.

23 Part of the UK activity will involve liaison with the activities of the IEA “Task V” working group, who will be continuing their successful work carried out to date within a new subtask, Subtask 50. The objective of the new subtask, called “Study on Highly Concentrated Penetration of Grid-Interconnected PV Systems”, is to assess the net impact of highly concentrated PV systems on electricity distribution systems and to establish recommendations for both distribution and PV inverter systems to enable widespread deployment of solar energy. Subtask 50 can be broken down into the following smaller subtasks:-

Subtask 51: Guidelines for Grid Interconnection, Certification Test Methods and New Technologies Subtask 52: The Islanding Phenomena, Probability of Occurrence, Detection Methods, Impact of Islanding and Mitigation. Subtask 53: Multiple PV Systems and their effect on power quality and power system design and operation. Subtask 54: Capacity of PV Systems

The activities of the Task V group are arranged with different countries taking a “lead role” and additional countries acting as “co-worker” on some of the identified subtasks. The UK has agreed to take a lead role in co-ordinating the activities of Subtask 51. In addition, the UK has agreed in principle to act as co-worker on the following subtasks:

• Subtask 51 (guidelines and testing certification methods only) • Subtask 52 (probability of islanding, detection methods and mitigation only) • Subtask 53 (effect of multiple PV connection on power system design and operation only) • Subtask 54 (capacity of PV systems - penetration levels only)

The UK ranked these subtasks in order of importance to the UK, based on current and future predicted areas of interest (1= highest, 5 = least important).

Subtask 51, Guidelines for Grid Interconnection, Certification Test Methods and New Technologies (UK ranking = 2)

The status of grid connection guidelines in each country will be reviewed at each meeting and the internal Task V report will be updated accordingly every year. This is relevant to the UK PV connection guidelines, which are shortly to be launched as an “Engineering Recommendation” by the UK Electricity Association. It is important that the progress and experiences of other countries are monitored and fed back to the UK electricity industry during the early life of the official PV guidelines document.

Related to the review of national guidelines, a review of testing certification methods used in each participating country will be carried out. This activity can serve both as a source of information for the UK PV Experimental Programme as well as a forum for validation of planned UK activities.

24 A third, smaller, element of this subtask is the “new technologies” activity. As well as being a product market survey, it is also a survey of new technologies, including novel system configurations. This sub activity was ranked low by the UK representatives and so will involve only a “watching brief” from the UK.

Subtask 52, The Islanding Phenomena, Probability of Occurrence, Detection Methods, Impact of Islanding and Mitigation (UK ranking = 1).

The aim of this subtask is to quantify the islanding issue in terms of the likelihood of part of the network becoming disconnected from the grid (due to a local fault, etc.) at the same time as there is a match between PV generation and local load requirements. The task will rely heavily on results from an international study being co-ordinated by the Dutch representative within Task V. The likelihood of islanding will be quantified using statistical probability studies. The task will also assess the operation of islanding detection methods (including ‘active’ techniques), the likely impact of an islanding event (i.e. effect on the network, equipment connected to the network, the inverter itself, and people (electricity company personnel and the general public). The task will also look at what practical steps could be taken to mitigate the impact of islanding (i.e. can inverters be made to withstand an out-of-sync reclose operation?, new LV live-line working practices (isolation and earthing of supplies), etc.).

Subtask 53: Multiple PV Systems and their effect on power quality and power system design and operation (UK ranking = 3).

This subtask will investigate as far as possible through experimental experiences safety issues, protection, fault currents (fault infeed), negative power flow and reclosing. It will also investigate as far as possible issues related to power quality, such as voltage fluctuations (including flicker), voltage profiles, harmonics and customer service. In view of the fact that the UK currently has very few PV systems connected to the domestic LV network, it is not possible for the UK to contribute much in the way of experimental data. However, the UK will keep a watching brief on the activities taking place in other countries.

Subtask 54: Capacity of PV Systems (UK ranking = 2/3/4)

This subtask is made up of our main issues:

• network issues (power quality, voltage, etc.) • capacity issues (i.e. need for generation, power balance) • space constraints (this is the work of IEA, PVPS Task VII) • Cost of PV

The network issues are effectively covered from a UK perspective by Subtasks 52 and 53. Thus, the main new area of interest to the UK are the capacity issues. The subtask will look at how PV can be accredited with some kind of capacity rating.

25 It is the purpose of this project to develop the theoretical and practical understanding of the likely effects that large-scale connection of dispersed photovoltaic power systems could have on the electricity supply network. 8 Acknowledgements

EA Technology would like to thank the many people who have assisted in the production of this report, including all of the Task V working group members, especially the chairman, Mr Tadao Ishikawa, CRIEPI, Japan, the members of the REC’s Consultative Group especially the chairmen, formally Geoff Finlay and now John Sinclair of the Electricity Association, the members of STP Module 5, Embedded Generation especially the chairman, Pete Thomas, Manweb. Additionally, I would like to thank our colleagues Rod Hacker and Jim Thornycroft from HGa who did such sterling work on developing the technical guidelines document. Finally, we would like to thank Harry Edwards of ETSU, for his encouragement in helping to guide the "vision" of grid-connected PV in the UK.

26 References

[1] Task I Report, “Photovoltaic Power Systems in Selected IEA Member Countries”, Report IEA PVPS ExCo: T1 1997:1, March 1997.

[2] Halcrow Gilbert Associates Ltd, “Grid Connection of Photovoltaic Systems”, ETSU Report S1394-P1, 1993.

[3] Halcrow Gilbert Associates Ltd, “Co-ordinated experimental research into PV power interaction with the supply network - phase 1”, ETSU Report S/P2/00233/00/REP, 1999.

[4] Newcastle Photovoltaic Applications Centre , “The Potential Generating Capacity of PV-Clad Buildings in the UK”, ETSU Report S1365-P1, 1992.

[5] Altechnica, ‘Update of the Database of Photovoltaic Installations in the UK”, ETSU S/P2/00301/REP.

[6] Electriciy Association Engineering Recommendation G77, “UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA”, Draft July 1999.

[7] Task V Internal Report, “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries”, IEA- PVPS V-1-01, July 1996.

[8] Task V Internal Report, “Information on electrical distribution systems in related IEA countries”, IEA-PVPS V-1-02, July 1996.

[9] “Proceedings of the IEA Workshop, “Existing and Future Rules and Safety Guidelines for Grid Interconnection of Photovoltaic Systems”, Zurich, September 1997.

[10] Task V Internal Report, “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries (Revised Version)”, IEA-PVPS V-1-03, March 1998.

[11] Task V Internal Report, “Information on electrical distribution systems in related IEA countries (Revised Version)”, IEA-PVPS V-1-04, March 1998.

[12] IEA-PVPS Report, “Utility Aspects of Grid Interconnected PV systems”, IEA- PVPS T5-01: 1998, December 1998.

[13] IEA-PVPS Report, “Demonstration Tests of Grid Connected Photovoltaic Power Systems”, IEA-PVPS T5-02: 1999, March 1999.

[14] IEA-PVPS Report, “Grid-connected Photovoltaic Power Systems: Summary of Task V Activities from 1993 to 1998”, IEA-PVPS T5-03: 1999, March 1999.

27 Appendix A:

UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA

Generally speaking, UK electricity company personnel will not be very familiar with photovoltaic power generation systems, especially grid-connected systems. Therefore, the UK technical guidelines were originally developed as an unoffical document to assist electricity company personnel in dealing efficiently with requests for the connection of grid-connected PV systems. The guidelines would also be useful to PV system designers and installers in providing information on the electrical network requirements for grid-connected PV systems.

Subsequently, in Autumn 1998, the technical guidelines document was taken on board by the electricity supply industry under the chairmanship of the Electricity Association and developed into its current form as “G77”, an official ESI Engineering Recommendation. The document is currently undergoing the final stages of formal review and it is anticipated that the final version of G77 will be formally approved in Autumn 1999. It is also anticipated that the final version of G77 will not be significantly different from the draft version contained here. However, it is recommended that once it is officially published, an official version should be obtained directly from the Electriciy Association, Millbank, rather than relying on the draft version reproduced for information here.

28 DRAFT (Dated: 15/7/1999)

UK TECHNICAL GUIDELINES FOR INVERTER CONNECTED SINGLE PHASE PHOTOVOLTAIC (PV) GENERATORS UP TO 5 kVA

SUMMARY

Recommendations for the connection of single phase inverter connected photovoltaic (PV) generation equipment of up to 5kVA in parallel with a Distribution Network Operators (DNO's) distribution system.

This document also contains guidance on the approval and type testing of inverters. The aim of this document is to encourage the use of “approved” inverter equipment and recognised connection procedures in order to lessen the need for DNO personnel to perform local tests.

The guidelines do not cover practical or safety issues related to the customer’s installation.

29 CONTENTS

1 INTRODUCTION

2 DEFINITIONS

3 PROTECTION 3.1 Automatic Protection 3.2 Automatic Disconnection

4 SUPPLY POWER QUALITY 4.1 Harmonics 4.2 Power Factor 4.3 Voltage Flicker 4.4 Electromagnetic Compatibility (EMC) 4.5 DC Injection

5 OPERATION & SAFETY 5.1 Connection Arrangements 5.2 Labelling 5.3 Maintenance & Routine Testing 5.4 Earthing

6 COMMISSIONING / ACCEPTANCE TESTING 6.1 Connection Requirements 6.2 Interaction between PV Inverter Generators

FIGURE 1 TYPICAL CIRCUIT DIAGRAM

APPENDIX APPROVAL & TYPE TESTING

30 1. INTRODUCTION

The following principles underlie the national technical guidelines for inverter connected single phase photovoltaic (PV) generators to the DNO’s distribution system:

1.1 It is anticipated that DNOs will use these guidelines as a basis for formulating their specific requirements for connection of inverter-connected PV generators in conjunction with:

Engineering Recommendation G.59/1, Amendment 1 (1995) Recommendations for the Connection of Embedded Generating Plant to the Regional Electricity Companies’ Distribution Systems.

Engineering Technical Report No. 113, Revision 1 (1995) Notes of Guidance for the Protection of Embedded Generating Plant up to 5 MW for Operation in Parallel with Public Electricity Suppliers’ distribution systems.

BS EN 61727:1996, (IEC 1727:1995) Photovoltaic (PV) Systems Characteristics of the Utility Interface.

1.2 The Guidelines are only concerned with the interface between the DNO distribution system and the inverter connected PV generator.

1.3 The PV inverter should be an ‘Approved Inverter for PV Generators’ as defined in the Approval and Type Testing Appendix.

1.4 The guidelines are written from a functional perspective. They are not technology specific, i.e. they do not require specific technologies to be used for connection to the DNO’s distribution system or the energy generation schemes themselves. This approach allows for ongoing development to continually improve the cost and technical performance of the equipment connected to the DNO’s distribution system.

1.5 This document is only applicable for single installations where the total installed PV generation capacity is 5 kVA or below.

31 2. DEFINITIONS

2.1 Inverter A device for conversion from DC to nominal frequency AC.

2.2 Approved Inverter for PV Generators At this stage, until suitable national standards are developed, an Approved Inverter for PV Generators is one:

• constructed and demonstrated to an accepted specification such as in the Approval and Type Testing Appendix; and • accepted by a DNO for connection to its distribution system

2.3 Inverter Connected Photovoltaic (PV) Generators One or more single phase inverter connected photovoltaic (PV) generation systems up to 5 kVA total, connected to the DNO’s distribution system at the nominal voltage and frequency.

2.4 Nominal Voltage and Frequency Low voltage: 230 volts (+10/-6%) single phase, 50 Hz (+/- 1%).

2.5 Distribution Network Operator (DNO) The company responsible for making technical connection agreements with consumers seeking connection of equipment to its distribution network. A DNO may be a PES which owns and operates a distribution network.

2.6 Publc Electricity Supplier(s) (PES) A public electricity supplier or suppliers who hold licences granted under section 6(1)(c) of the Electricity Act 1989 or the Electricity (Northern Ireland) Order 1992.

2.7 Islanding Islanding of inverter connected PV generator systems means any situation where the source of power from the DNO’s distribution system is disconnected from the network section in which the generator is connected, and one or more inverters maintains a supply to that section of the distribution system or consumers installation.

32 3 PROTECTION

3.1 Automatic Protection Protection shall be provided to isolate the inverter connected PV generator from the DNO’s distribution system when:

• operating voltage is greater than 253V phase to neutral (230V +10%) • operating voltage is less than 207V phase to neutral (230V - 10%) • operating frequency is greater than 50.5 Hz (50Hz +1%) • operating frequency is less than 47 Hz (50Hz - 6%) • the mains supply is lost

The voltage and frequency limit settings should not be capable of adjustment by the user.

The inverter should incorporate a recognised technique for providing loss of mains protection (such as frequency shift or vector shift). Active techniques that distort the voltage waveform beyond the limits specified in section 4.1 or inject current pulses into the DNO’s network are not approved.

This protection must ensure that the inverter disconnects from the DNO’s distribution system within 5 seconds, and does not reconnect until at least 3 minutes after the supply from the DNO system has been restored to within the voltage and frequency limits already specified.

As some distribution systems employ automatic reclosing, the inverter equipment might be re-energised from the mains within the 5 second period. The inverter must therefore be capable of withstanding connection to a non-synchronised mains supply – refer to section A2.4.

3.2 Automatic Disconnection The protection function can either be incorporated in the inverter or in separate relays. In either case it shall be type tested for compliance with the above requirements and the Approval and Type Testing Appendix.

When the system is disconnected, this must be achieved by the separation of mechanical contacts to IEC 255. Electronic disconnection alone is not sufficient. The electrical relays used for the automatic disconnection shall meet:

IEC 255-5: 1977 (BS 5992 Part 3) Electrical Relays Electrical Relays: Specification for the Insulation Testing of Electrical Relays

33 4. SUPPLY POWER QUALITY

4.1 Harmonics All equipment shall meet:

BS EN 61000-3-2 (1995) + A12 January 1996 Electromagnetic Compatibility (EMC) Part 3, Limits Section 2: Limits for harmonic current emissions (equipment input current ≤16 A per phase). Equiv. IEC 1000-3-2: 1995

For equipment rated >16 A, the emission currents shall not exceed the limits quoted in Technical Report IEC 61000-3-4 when the PV inverter generator is supplying a purely resistive load of equal rating to the output of the PV inverter generator.

Note: for the purpose of assessing emission currents in accordance with 61000-3-4, the ‘Short Circuit Power’ Scc, shall be assumed to be 548 KVA - derived using maximum values for open circuit voltage and network impedance i.e. (2532/0.35) x 3.

Connection of the equipment shall be subject to compliance with EA Engineering Recommendation G.5/3 (1976) Limits for Harmonics in the United Kingdom Electricity Supply System or any superseding recommendation.

4.2 Power Factor Power Factor shall be within the range of 0.95 leading to unity relative to the DNO supply, unless otherwise agreed with the DNO. Note: Leading power factor is VARs absorbed by the inverter.

4.3 Voltage Flicker All equipment shall meet:

BS EN 61000-3-3 (1995) Electromagnetic Compatibility (EMC) Part 3 Limitations of voltage fluctuations and flicker in low voltage supply systems for equipment with rated current less than 16 A. Equiv. IEC 1000-3-3: 1994

Connection of the equipment shall be subject to compliance with: Engineering Recommendation P28 (1989) Planning limits for voltage fluctuations caused by industrial, commercial and domestic equipment in the United Kingdom

Note: the automatic protection settings of section 3.1 afford adequate immunity for operation of the PV inverter generator, against : voltage dips, short supply interruptions (up to 5 seconds, dependent on protection time setting) and frequency dips.

4.4 Electromagnetic Compatibility (EMC) All equipment shall be CE marked in accordance with the UK EMC regulations and meet the relevant EMC standards:

BS EN 50081-1: 1992 Electromagnetic Compatibility Generic Emission Standard

34 BS EN 50082-1: 1998 Electromagnetic Compatibility Generic Immunity Standard

4.5 DC Injection DC currents entering the AC distribution system can give rise to technical problems. G5/3 deprecates the existence of DC currents on the UK distribution system but does not specify levels.

It is recommended that a transformer be installed between the inverter and the DNO’s distribution system to prevent DC from entering the distribution system. However if a DC detection device is installed at the point of connection on the AC side then the transformer may be omitted; provided that the output of the inverter(s) is disconnected if the level of DC injection exceeds 5mA (present regulation is zero).

Where fitted the DC isolation transformer will normally be located adjacent to the inverter, which will either be in close proximity to or an integral part of the PV array.

5 OPERATION & SAFETY

5.1 Connection Arrangement Connection of an Inverter Connected PV Generator will be subject to the requirements of section 6.1 of this document.

Inverter connected PV generators should be connected directly to a double-pole manual isolation switch that is located in an accessible position, and lockable in the open position.

5.2 Labelling There shall be labelling at the service termination, meter position and isolation switch to indicate the presence of on-site generation and the point of isolation. The Health and Safety (Safety Signs & Signals) Regulations 1996 stipulates that labels should display the prescribed triangular shape, using black on yellow colouring. A typical label is shown below:

In addition to the safety labelling, Schedule 3 of the Electricity Supply Regulations 1988 requires certain information to be displayed at the point of interconnection. In

35 the context of small single phase generators, which will typically be installed in domestic environments, it is envisaged that this requirement can be met by displaying:

1) a circuit diagram showing the relationship between the inverter equipment and supply 2) a summary of the protection settings incorporated within the equipment 3) a contact telephone number for the supplier/installer/maintainer of the equipment

A typical circuit diagram is shown in Figure 1.

5.3 Maintenance & Routine Testing The DNO shall have the right to carry out tests pursuant to Regulation 27 of the Electricity Supply Regulations 1988. The DNO may require the customer to re-test the inverter equipment in association with supply quality and/or safety investigations.

5.4 Earthing The system shall meet:

BS 7430 (1991) Earthing Code of Practice for Earthing

BS 7671 (1992) Requirements for Electrical Installations IEE Wiring Regulations Sixteenth Edition

In the UK, the majority of new low voltage (LV) electrical supplies are of the Protective Multiple Earthing (PME) type. These supply cables have a Combined Neutral and Earth (CNE) metallic outer conductor, which is earthed at multiple points on the supplier’s Terra-Neutral-Combined (TNC) distribution network. Separate earth and neutral terminals are then provided within the customer’s premises (TNC-S). Generally the PME earthing facility is not allowed to be extended outside of the equipotential zone and so exposed metalwork on the outside of the building must be earthed independently using a local earth electrode.

In the case of older premises supplied from Terra-Neutral-Separate (TNS) networks or service lines, the requirements of the PME approvals do not apply, although all bonding should still comply with BS7671: 1992.

36 6 COMMISSIONING / ACCEPTANCE TESTING

6.1 Type Tested Equipment An ‘Approved Inverter for PV Generators’ may be connected without further testing. subject to:

• Satisfactory on site commissioning tests as agreed with the local DNO • Opportunity for a DNO representative to witness the commissioning tests • A satisfactory Connection Agreement is in place • The installation is in accordance with IEE Wiring Regulations BS 7671 1992 and any other regulations specific to the installation in question.

Non-approved inverter equipment may also be connected if additional commissioning and acceptance tests are carried out, as defined by the local DNO.

6.2 Interaction between PV Inverter Generators For multiple inverter installations a statement is required from the manufacturer detailing how the design of his inverter avoids adverse interaction and interference with the operation of the automatic protection functions of other inverter installations. In particular the manufacturer should address the issue of inverters that employ a loss of mains (LoM) protection that could interfere with other units on the same network that employ contrary means of detecting LoM, e.g. frequincy shift up/down techniques.

37 FIGURE 1 - TYPICAL CIRCUIT DIAGRAM

The PV array shall be connected to earth by PV array, inverter means of: and transformer (1) a local earth electrode if the array is outside the equi-potential zone, or (1) (2) (2) a bonding conductor connecting the array to the main earthing terminal of the installation if the array single phase fused switch is within the equi-potential to be lockable in the open zone. position for isolation

single phase consumer unit fused switch export meter import meter (only if reqd.)

PME earthing terminal blocks service cutout terminal customer's earthing block

equi-potential bonds to other metallic services service cable

Typical installation arrangements for a PV array connected in a residential premises

38 APPENDIX - APPROVAL & TYPE TESTING

The following points will be checked and tests carried out as necessary to establish if an inverter can be classed as an ‘Approved Inverter for PV Generators’ for the purposes of the UK Technical Guidelines for Inverter Connected Single Phase Photovoltaic (PV) Generators up to 5kVA.

A1 CE Marking and Certification

In the first instance, the equipment should be checked for compliance with European CE Mark Regulations. The following table can be used to identify equipment specifications which may be covered by European Norm (EN) standards, those which are covered by British Standards (BS), those which are covered by Electricity Supply Industry (ESI) standards and those which in the absence of official standards will require additional verification:

Function Standard/Specification Reference

Protection

General “Electrical relays” IEC 255, Part 5 (1977) Under/over “Recommendations for the connection of embedded ...” G59/1 voltage “Notes of Guidance for the protection of ...” ETR113 Under/over frequency Loss of mains

Supply Quality

Harmonics “Electromagnetic Compatibility (EMC) Part 3 - Limits” BS EN 61000-3-2 (1995) + A12 (1996) “Limits for harmonics in the UK electricity supply system” G5/3 Voltage Flicker “Electromagnetic Compatibility (EMC) Part 3 - Limits” BS EN 61000-3-3 (1995) “Planning limits for voltage fluctuations ..... ” ER P28 Electromagnetic “Generic Emission Standard (EMC)” BS EN 50081-1 (1992) Compatibility “Generic Immunity Standard (EMC)” BS EN 50082-1 (1992) DC Injection “Limits for harmonics in the UK electricity supply system” G5/3 Safety

Earthing “Code of practice for earthing” BS 7430 (1991)

39 A2 TESTING OF AUTOMATIC PROTECTION

A2.1 Over / Under Voltage

Trip times and settings shall be tested as follows:

The inverter equipment shall be tested by operation into a variable AC voltage test supply system, whilst being fed from a DC source (which is simulating the PV DC output). The set points for over and under voltage at which the inverter system disconnects from the supply will be established by varying the AC supply voltage. These set points and trip times shall be within the guideline requirements of section 3.1.

Check reconnection feature as in A2.4.

DC Source Variable AC (simulating Inverter Voltage Test PV generator) Supply

A2.2 Over / Under Frequency

Trip times and settings shall be tested as follows:

The inverter equipment shall be tested by operation into a variable frequency test supply system) whilst being fed from a DC source (which is simulating the PV DC output). The set points for over and under frequency at which the inverter system disconnects from the supply will be established by varying the supply frequency. These set points and trip times shall be within the guideline requirements of section 3.1.

Check reconnection feature as in A2.4.

DC Source Variable (simulating Inverter Frequency PV generator) Test Supply

Note: It may be necessary to disable any loss of mains protection function built in to the inverter in order to perform this test.

40 A2.3 Loss of Mains Protection

The time to trip upon loss of mains will be verified to be less than the time required by the guidelines under the conditions below:

The inverter equipment shall be fed from a DC source to simulate the output from a PV source. The inverter output shall be connected to a variable impedance test set to model the local load (at the inverter output terminals) and a switch placed between the local load/inverter combination and the DNO’s distribution system as shown below:-

DC Source DNO’s (simulating PV Inverter Variable Distribution generator) Impedance Test System Circuit to Model Local Load

The inverter equipment is to be tested at three output powers and two local load conditions, giving six tests in all (see table below):

Local Load Þ Load Match 10% greater than load match Inverter output ß 10% 50% 100%

Load match conditions are defined as being when the current from the inverter connected generator meets the requirements of the local load i.e. there is no export or import of current to or from the DNO’s distribution system. (It is assumed that the local load will be mostly resistive. If other power factors are allowed these tests would have to be modified depending on the controllability of the inverter).

The tests will record the inverter output voltage and frequency from at least 2 cycles before the switch is opened until the inverter protection system operates and isolates itself from the DNO’s distribution system. The time from the switch opening until the protection isolation occurs is to be measured and must comply with the guidelines under all conditions of output power and local load.

Check reconnection feature as in A2.4.

41 A2.4 Re-connection

Further tests will be carried out with the three test circuits to check the inverter time- out feature prior to automatic network reconnection. This test will confirm that once the AC supply voltage and frequency have returned to their nominal values following an automatic protection trip operation there is a minimum time delay (as specified in the guidelines) before the inverter output is restored (i.e. before the inverter automatically reconnects to the network). The inverter shall be tested for connection to a mains supply which is 180 degrees out of phase and at peak voltage to ensure no damage occurs.

A3 POWER QUALITY

A3.1 Harmonics

In addition to compliance with BS EN 61000-3-2 (limits for harmonic current emissions), harmonics shall be measured in accordance with Engineering Recommendation G5/3.

A3.2 Power Factor

For this test, the inverter can be fed by a DC source to simulate the DC output of a PV generator. The inverter generator supplies full load to the DNO system via the power factor (pf) meter and the variac shown below. The inverter pf should be within the limits given in 4.2, for the three test voltages:

DC Source DNO’s (simulating PV Inverter pf Variac Distribution generator) System

Note: for reasons of clarity, points of isolation are not shown.

Test Voltage (supplied by variac) Inverter PF (measured)

230V +8% 230V 230V-8%

42 A3.3 Voltage Flicker

In addition to compliance with BS EN 61000-3-3 (Limitations of Voltage Fluctuations and Flicker etc.), voltage flicker shall be measured in accordance with Engineering Recommendation P28.

A3.4 Electromagnetic Compatibility

The manufacturer shall ensure that the equipment is tested to BS EN 50081-1:1992 (emissions) and BS EN 50082-1: 1998 (immunity).

A3.5 DC Injection

The level of DC injection from the inverter connected PV generator in to the DNO network shall not exceed 5mA when measured during tests A2.1, A2.2, A2.3 and A3.2. This condition is satisfied by installation of a transformer on the AC side of the inverter connected PV generator.

A4 SAFETY

A4.1 Earthing

The manufacturer shall ensure that the equipment is tested to the relevant British standards.

A4.2 Labelling

There shall be labels supplied with the inverter for use at the service termination, meter position and isolation switch as outlined in the guidelines.

43 Appendix B:

Summary of IEA Task V Activities

44 Summary of IEA Task V Activities

1 Task V organisational structure

The International Energy Agency (IEA), founded in November 1974, is an autonomous body within the framework of the Organisation for Economic Co- operation and Development (OECD) which carries out a comprehensive programme of energy co-operation among its 23 member countries. The European Commission also participates in the work of the Agency. More information about the IEA is available on their web site at:

http://www.iea.org

The IEA Implementing Agreement on Photovoltaic Power Systems (PVPS) is one of a number of collaborative R&D agreements established within the IEA, and since 1993 its participants have been conducting a variety of joint projects in the application of photovoltaic conversion of solar energy into electricity.

The PVPS Programme has members from 20 countries (Australia, Austria, Canada, Denmark, Finland, France, Germany, Israel, Italy, Japan, Korea, Mexico, the Netherlands, Portugal, Spain, Sweden, Switzerland, Turkey, the United Kingdom and the United States) and the European Commission.

The PVPS Programme is divided up into a number of Task Sharing group activities, each focusing on different aspects of PV.

• Task I is entitled “Exchange and Dissemination of Information on Photovoltaic Power Systems” and is the parent task taking the overall strategic view, promoting and facilitating the overall gathering and dissemination of information related to photovoltaic power systems.

• Task II is entitled “Operational Performance and Design of Photovoltaic Power Systems and Subsystems” and its objectives are to develop an information system on design characteristics and technical performance of PV systems as well as examining procedures for the measurement of PV power plant performance. The UK does not participate in this task.

• Task III is entitled “Use of Photovoltaic Power Systems in Stand-Alone and Island Applications” and as its name suggests it objective is to advance the state- of- the-art for PV systems located remotely from any grid supply. Again, the UK does not participate in this task.

• Task IV is entitled “Modelling of Distributed Photovoltaic Power Generation in Support of the Electric Grid” and has the aim of developing methods for evaluating the benefits of grid-connected PV systems. This task has not yet been implemented.

45 • Task V is entitled “Grid Connection of Building Integrated and Other Dispersed Photovoltaic Power Systems” and has the overall objective to identify the technical requirements and develop technical guidelines for grid-connected PV systems. This task forms much of the background to this report.

• Task VI is entitled “Design and Operation of Modular Photovoltaic Plants for Large Scale Power Generation”. This task has not yet been implemented.

• Task VII is entitled “Photovoltaic Power Systems in the Built Environment” and aims to improve the architectural design and implementation of PV in buildings, including residential and commercial buildings as well as infrastructure installations.

Not all countries participating in the PVPS Implementing Agreement participate in every Task. For example, the UK only participates in Task I, Task V and Task VII. Thus, only a subset of eleven of the twenty previously named countries participate in Task V. These countries are Australia, Austria, Denmark, Germany, Italy, Japan, Mexico, The Netherlands, Switzerland, the United Kingdom and the United States.

2 Task V work programme

The UK was instrumental in forming the IEA Task V Working Group. The first stage of the PVPS Task V activity was carried out between 1993 and 1998 with the participation of experts from 11 countries. The objectives of this task were the identification of technical issues related to small-scale building-integrated PV systems connected to low voltage distribution networks, with a view to propose possible solutions to the problems identified.

More specifically, the main objective was to identify the technical requirements in pre-normative technical guidelines for the network interconnection of building- integrated and other dispersed photovoltaic (PV) systems. The aims of the guidelines were to allow a safe, reliable and low cost interconnection of PV systems to the electric power network.

The Task V work programme consisted of three main subtasks:-

• Subtask 10: “Review of Systems & Guidelines” • Subtask 20: “Utility Aspects of Grid-Connected PV Systems” • Subtask 30: “Demonstration Tests of Grid-Connected PV Power Systems”

46 Specifications and characteristics of many inverters and protective devices have been collected and summarised as a Task V internal document. The fourth item was a review of the operating experience of grid interconnected PV systems to identify any known problems relating to PV system operation.

The scope of work for sub-task 20 was to analyse present day and possible future problems relating to the grid interconnection of PV systems and to draft possible recommendations for improvement.

In sub-task 30, actual experimental tests of PV system grid interconnection were conducted using the test facility in the Rokko Island Test Facility in Japan. These tests were intended to show the actual phenomena of grid interconnection problems and to be the reference for subtask 20 work. In reality, only a few tests were actually directly related to the subtask 20 work.

47 Appendix C:

Review of Systems and Guidelines

(Subtask 10)

48 1 Introduction to Subtask 10

For Sub-task 10 the following subjects were studied:-

• Existing guidelines and regulations for grid interconnection of PV systems in Task V participating countries.

This was summarised as Task V internal report, “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries (Revised Version)”, Task V Report IEA-PVPS V-1-03.

• The differences between distribution systems in each Task V member country.

This was summarised as Task V internal report “Information on electrical distribution systems in related IEA countries (Revised Version)”, Task V Report IEA-PVPS V-1- 04.

• The actual characteristics of inverters and related protection equipment and actual operating experience of grid interconnected PV systems.

These were summarised as Task V internal documents.

1.1 Grid-Interconnection Guidelines

The following information relating to the guidelines was clarified according to the research on the guidelines of each country participating in Task V.

- Type of generation (Energy sources covered by the guidelines document) - Classification of interconnection voltage (differences between guidelines with interconnection voltage) - Limitation of generation capacity per customer - Correspondence to reverse power flow (i.e. export power) - Requirements for facility Electrical system Power factor Harmonics HF Noise Flicker Protection co-ordination Protection and safety Necessity of isolation transformer Restrictiononinverter External disconnect switch Location of switch Reclosing procedure

49 Detection of islanding phenomena Voltage fluctuation Short-circuit capacity - Safety and wiring of DC side - Metering - Lightning protection - Authorisation procedure - Standard configuration and electrical layout - Islanding Protection

In general it can be said that no common guidelines for the connection of PV systems to the utility grid exist in the participating countries. Every country has its own set of rules, and in many countries these rules vary in different regions. This reflects the fact that in many countries it is up to the individual utilities to define rules for connecting independent power generators to their grid.

Safety is the most important issue in all of the regulations. At this moment it seems that the problem of possible islanding (i.e. continued operation of a PV inverter even when the grid is off) is a crucial issue and needs further study as it could immediately lead to severe accidents. Topics such as cabling and earth fault detection seem to be well known and can be readily adopted from guidelines governing conventional electrical systems. However, the potential for DC currents to flow from the solar panels leads to possible dangers that are different from those associated with AC currents.

To enable successful promotion of photovoltaics it is of great importance to set internationally recognised standards which allow identical PV systems to be connected to the grids of different countries. During the progress of Task V activities, both utilities and manufactures have been aware of the problems of grid interconnection of PV system leading to revision of guidelines and regulations in some countries. The guidelines summary report is important in that it contains information that allows comparisons to be made of the various approaches to the problems and dangers specific to PV.

1.2 Distribution System Configuration

It is important that the configuration, equipment, required protection relays and so on of the distribution system are known because they are strongly related to the requirement for grid interconnection equipment. Therefore, the distribution system configuration for each country participating in Task V were studied and summarised.

The following information was obtained: - Voltage level and network structure HV transmission HV distribution MV distribution LV distribution

50 - Capacity of transformers - Feeders - Capacitors Transformer Feeders per transformer Impedance Average length Number of switches per feeder Number of sectionalisers per feeder Capacitor for p.f. improvement Average number of customer per feeder per phase for LV Power rating of customer for LV - Protective devices Protective device installed in the public network Reclosing Protective co-ordination with independent producer - Type and setting levels of the interface devices installed in the independent producer’s network - Operation criteria Voltage fluctuation Voltage regulation Temporary supply Work method for fault repair

It is worth noting that some data contained in the report must be considered as referring to mean or typical conditions. As a consequence data referring to countries where many local utilities exist and network characteristics and operating procedures differ from region to region, should be treated carefully.

In general, it can be said that the LV grids in Europe have similar voltage levels while USA and Japan have a different voltage level for the LV grid. Distribution system configuration are different from country to country, and protection schemes also vary. This is one of the reasons why the grid interconnection guidelines for PV systems are different in each country.

This electrical distribution system report aids understanding of the differing electrical grid configurations and the different approaches to the grid interconnection requirements.

1.3 Inverter and Related Protection Equipment

Characteristics of grid-interconnected PV systems are very much dependent on the performance of inverters and protective devices. These interconnecting devices should be designed to comply with existing guidelines and regulations on grid interconnection. Therefore, the survey of interconnection devices makes it possible to identify the status of existing applied measures for PV system grid interconnection. This survey is also intended to identify remaining technical issues on interconnection equipment including cost reduction and to clarify the concepts for requirements and devices for grid interconnection in the future. The survey was carried out by circulating a questionnaire on interconnecting equipment including inverter

51 specifications, protective device specifications etc. More than 50 responses were obtained including 5 inverters for AC modules, a new PV system concept in which an inverter is integrated into a PV module. The summarised results are as follows:-

1.3.1 Inverter design and specifications

Type of Conversion

Most converters are self-commutated PWM inverters with current control. Some voltage controlled self-commutated PWM inverters and line-commutated inverters are listed.

Nominal AC voltage & Standard grid connection

- Europe

230 V (1 phase/3 wires) 380 V (3 phase/3 or 4 wires) 400 V (3 phase/3 or 4 wires 50 Hz

- Japan

100 V (1 phase/2 wires) 200 V (1 phase/2 or 3 wires, 3 phase/3 wires) 50/60 Hz

-USA

120 V (1 phase/3 wires) 240 V (1 phase/2 or 3 wires) 480 V (3 phase/4 wires) 60 Hz

Harmonic current

Less than 5% in total and less than 3% for each harmonic. There are inverters compliant with EN/IEC/VDE or G5/3 standards.

Control methods of harmonics

Mostly current controlled PWM with or without filtering.

EMC standards/directives with which inverter complies

Europe: EN (CENELEC), IEC, VDE, CISPR Japan: VCCI (Japanese Voluntary Directives)

Value of AC power factor in each power output point

52 Almost constant in each power output point except for very low power (around 1).

Control method of power factor

Mostly synchronising current phase with line voltage at zero crossing.

Control method of voltage fluctuation

Japanese inverters have reactive power supply or output power suppression option. Most of the other inverters have no control on voltage fluctuation.

Isolation transformer

Utility power frequency transformer or high frequency transformer are employed.

Control system

Almost all employ Maximum Power Tracking. Many inverters have power factor control (optional on some).

Operations AC voltage and frequency ranges

Voltage: nominal rated voltage +/- 10% Frequency: Majority is utility frequency +/- 1%, Maximum +/- 10%. Others are +/- 1 Hz, +/- 3%, +/- 5%, etc.

Conditions for start-up and stop

Start-up: Mainly by DC voltage level increase. Stop: Mainly by DC or AC output power decrease. DC current and DC voltage decrease are also employed.

Control power source

Depend on the inverter design. Either DC side or AC side is used. Some inverters can be powered from both DC side and AC side.

53 1.3.2 Protective Devices

Required protective devices, protection level and operation time

AC side: Overvoltage, Undervoltage, Over frequency, Under frequency, Overcurrent, Fuse (in Europe), Metal oxide varistor (in Europe and USA), Optional AC disconnecter (in USA)

DC side: Ground fault detection, Over current, Fuse (in Europe), Over/Under voltage, Metal oxide varistor (in Europe and USA), Optional DC disconnecter (in USA)

Presence and specification of overvoltage protection/device

Transient: Varistor, TSV-Diodes, Metal oxide varistor (in Europe and USA) Dynamic: Over-voltage relay or software detection

Necessary protective devices for preventing islanding phenomenon

Europe - Required in most countries: frequency relay, voltage relay, harmonic increase, frequency shift

Japan -Yes: combination of both passive measure and active measures

USA - Not required yet (although P929 will require active frequency shift technique) some inverters already use the frequency shift technique

Disconnection procedure at abnormal state

Gate blocking and trip of breaker (or opening of contactor).

Restart

Most are automatic. Typical restart times are:

30 sec (Austria), 180 sec, adjustable (Italy), 150 sec, 160 sec, 0-300 sec adjustable (Japan), first zero crossing, 6 sec, 120 sec after utility return to spec. (USA), 5 sec, 15 sec, 2 to 4 min (Switzerland), 3 minutes (UK)

54 Location of protective device

Most included in inverter.

Prices of inverter and protective devices

Depends on output power and quantity to be sold. Cost 1000 to 3000 (5000) $/kW for under 5 kW devices. Less than 1000 $/kW for large power inverters. AC module could be 1$/W.

1.3.3 Other issues on interconnection devices

- Noise reduction - Full evaluation of islanding prevention measures - Transformer-less design - To customise control board and other components - Islanding prevention measures with a number of interactive inverters - Minimisation of wiring in construction - Minimisation of heat dissipation - Minimisation of certification procedure - Inclusion of external disconnecter into the inverter - External transient protection may be needed in some installation - Standard EMI specifications could define minimum standards

1.3.4 Operating experiences for existing PV systems

It is useful to review the operating experience of existing grid connected PV systems to identify the problems actually encountered. The survey was mainly focused on a review of faults and failures. By understanding the cause of a failure countermeasures and required design modification could be clarified. The survey was carried out by circulating a questionnaire on faults in low voltage grids and faults in PV systems. A summary of the results follows.

1.3.4.1 Grid conditions (low voltage grid) Frequency and duration of power outage

From some seconds to some hours per outage, few times per year

Failure or malfunction of protective device(s) of an interconnected power plant

No information available

1.3.4.2 Faults and failures for PV Systems

PV array

55 total number of incidents: around 90. Typical reasons given are:-

human error (string not connected, wiring error, loose terminal) design error (partial shading, module overrating) manufacturing error (insulation failure, defective diode) other (condensation in junction box, corrosion, theft, broken glass, lightning)

Inverter

total number of incidents: over 470. Typical reasons given are:-

human error design error (poor MPPT, malfunction of control circuit) manufacturing (defective fuse) Other (unspecified internal cause, grid transients, power limitation)

(Note: many problems occur at the pre-production stage.)

Utility interface

total number of incidents: 8. Typical reasons given are:- Static switch, MV grid, frequency shift due to thermal failure

Other component failure

loose terminal in junction boxes, varistor burnt, burned contacts in circuit breaker in front of battery (stand alone system), break of battery case (stand alone system)

Problems caused for the local grid or loads by introduction of PV systems

voltage rise after long feeder

Transient overvotage protection

damaged inverter

Lightning strike

module destroyed

Grounding fault

many faults in systems with old module design

Grounding fault detectors or insulation monitors

frequent tripping under high humidity, GFCI worked well with transformerless inverter

56 Experience of islanding

No actual islanding incidents were reported

Problems due to DC injection

No problems were reported

Problems due to reclosing

No problems were reported

Problems due to electromagnetic interference

Some old inverters caused disturbances on radio and TV, no problem with new design. Disturbance on telephone

Experience of multiple inverters on one feeder of the grid

Several countries have experience and working well so far

It was found that inverter failure is the most frequent incident. This is mostly caused by the lack of experience in first generation equipment and newly designed inverters have good reliability. Only a few faults or failures caused by interconnection of PV systems have been reported so far. However, this fact does not always indicate that there is no problem for grid interconnection of PV systems. It is difficult to identify the cause of the fault in some cases due to the absence of precise measuring equipment. Some unexplained inverter failures may have been caused by grid disturbances, reclosing and other interconnection issues.

57 Appendix D:

Utility Aspects of of Grid-Connected PV Systems

(Subtask 20)

58 1 Introduction to Subtask 20

In subtask 20, several topics were studied on the utility interface of grid connected PV systems and summaries of the selected topics are described below. Detailed information on the topics is given in the subtask 20 report “Utility aspects of grid connected photovoltaic power systems”, Task V Report IEA-PVPS T5-01: 1998.

1.1 Harmonics

Problems

The harmonic problem has assumed a particular relevance starting from the 1960s with the increasing use of static converters, which directly effect the quality of the electricity supply.

In general, the harmonic problem can be defined as that particular disturbance that is caused by the presence of non-linear loads which cause a modification of the voltage and current sinusoidal waveforms in terms of sinusoidal components at a frequency which is an integer multiple of the fundamental frequency.

Findings

It was found that since PV generators are connected to the distribution network through solid state inverters they are potentially liable to cause harmonics, thereby downgrading the quality of electricity and altering the performance of other equipment sensitive to voltage harmonics. In addition, static converters themselves can be sensitive to harmonics and may operate incorrectly as a result of harmonic voltage distortion. An investigation of the harmonic phenomena as applied to PV systems took into account aspects relevant to the generation (emission) and effects on (susceptibility) the PV systems.

The work done has also showed the necessity to further investigate the effects on harmonics in the case of multiple PV systems.

1.2 AC Modules

Problems

An AC Module is an integrated combination of a single solar module and a single inverter. The inverter converts the DC energy from the module into an AC energy and feeds this energy into the AC network. The main advantage of AC Modules is the modularity. Complicated DC wiring is not required and the solar power is directly available as AC power.

59 This modularity allows for very simple systems that can easily be expanded by simply paralleling several AC Modules at the AC side. An AC Module is an electric product and comparable to other appliances. It is anticipated that AC Modules will become easily available at local retail outlets, which will enable people to buy and install them without consulting a certified electrical engineer.

This “plug and play “ idea of AC Modules has raised several questions by experts from electrical safety bodies and utilities. Plug and play means that the AC Module is equipped with a standard AC-plug that allows for a direct plug-in to a regular mains socket outlet in the electrical installation of a building. Some national and international safety standards do not allow this, other standards are unclear.

Findings

A survey revealed that only a restricted number of countries are actually developing and/or using AC Module systems. Other countries have no objections to AC Modules but wait for other countries to gain hands-on experience. Nevertheless it is expected that AC Modules will be used world-wide within a few years.

The most important unresolved question is how to connect an AC Module to the network. Manufacturers are in favour of allowing the AC Modules to be connected to a regular mains socket outlet. This allows easy installation and reduces cost. Safety standards, however, do not always allow this and/or utilities do not like the idea of having generators connected at normal feeders of an electrical installation. There is also a non-technical but realistic aspect to this discussion. When AC Modules become generally available, it can be expected that people will not install a separate feeder to the consumer unit/metering point just for one or two AC Modules, mandatory or not, and will connect the AC Module to a regular outlet.

The Netherlands have issued a pre-draft guideline that AC Modules (or other types of small generators) may be connected to normal feeders if the generated power is below approximately 500 W. This philosophy is also under discussion in Switzerland. However, some countries for example Australia and USA, have strict regulations not to allow AC Modules or other types of small generators to be connected to a regular feeder; a separate feeder is always necessary.

There are two certification standards for AC Modules available in the world. Both these standards provide a set of rules to guarantee the electrical, mechanical safety of an AC Module. The Dutch standard is issued by KEMA, in the USA the standard is issued by Underwriters Laboratories (UL). UL and KEMA are working to harmonise both these standards.

AC Modules have recently been introduced as a commercial product, application can be found in a limited number of countries. It is expected that this number will grow dramatically in the next few years.

Although AC Modules have been available on the market for some years now, there is still a lot of discussion on methods for interconnecting AC-Modules with the network. Also, the method of interconnection is an important topic; is there a need for an AC-

60 marshalling box or is an AC-cord with (special) plugs an option, and what about the connection to the AC-mains supply, bolted terminals or a separate plug?

If AC-Modules become as popular as expected, these issues have to be settled. Present day standards and guidelines need to be adapted to cater for AC-Modules.

1.3 Multiple Inverters and the AC Grid

Problems

As small PV power generation systems become more common, it will be necessary to investigate several effects that are not significant for single inverter systems. For example, if a large number of dispersed PV generators are connected to a branch of the low voltage distribution system, then the reverse power flow to the higher voltage power system will substantially increase during periods of light load and maximum daylight. This may cause a significant voltage rise in the distribution lines, particularly at the ends. Also, the PV systems will supply a part of the fault current in the event of a distribution line fault. This additional fault current will decrease the fault current flowing at the substations and might cause fault detection relays in substations to malfunction. It is thus necessary to identify effects that may occur when connecting large numbers of PV systems, and to establish countermeasures.

Findings

The voltage at the customer’s terminals may exceed the upper statutory limit because of reverse power flow from PV systems during light-load hours in the daytime. Leading power factor operation of the PV system is an effective countermeasure to prevent the voltage rise without reducing effective power.

If each customer supplied by a distribution transformer installs PV systems with a capacity equal to or above their contracted power, the reverse current flowing through the transformer could exceed the transformer. It would be necessary to consider replacement of the transformer or installation of an energy storage facility in such a situation. If a diversity figure is used such as After Diversity Maximum Demand (ADMD) for distribution system design purposes, restricting generation to the ADMD could be an effective countermeasure.

In the event of a short-circuit fault condition occuring in the distribution line, the increase in short-circuit capacity of the distribution line and the malfunction of OCRs or fuses in the distribution system may occur as part of short circuit current is supplied from PV systems. It would be necessary to develop a new fault detection system for the PV system. A method of detecting the voltage phase change occurring in the fault condition may be one useful option.

The effects anticipated to occur when a large number of PV power generation systems are interconnected with distribution lines were investigated. The theoretical results and experimental result regarding the effects and countermeasures are reported. Recommendations for future work are as follows.

61 Development of a new fault detection for PV systems to detect a short-circuit fault occurring at an end of a long distribution line, with a high resistance, or during distribution line overload.

Further studies on the effect on distribution line voltage variation caused by the widespread application of PV power generation, covering different application areas and the number of interconnected systems.

It would also be important to study and encourage the application of various distribution line support systems which make the best use of the added values offered by PV power generation.

1.4 Grounding of Equipment in PV Systems

Problems

When rules for early power generation and electrical distribution systems were being developed in the late 1890 to early 1900’s, grounding requirements were limited to lightning protection. In the United States, the National Electrical Code (NEC) and its grounding requirements were first published in 1897. Most other countries throughout the world, often independently, developed other versions of electrical codes to address safety and grounding issues for electrical generation and distribution systems. The resulting grounding techniques and requirements vary from country to country. Optimised grounding for personnel protection does not optimise fire safety of a system and grounding for fire safety does not optimise personnel safety. Grounding to provide protection for equipment would require a third set of requirements. Photovoltaic (PV) systems, as distributed current sources, require additional grounding considerations. Distributed leakage paths, multiple fault paths and new roles for fuses and circuit breakers are among a few of the new issues that need careful consideration for PV applications. Codes for PV have closely followed the national practice for AC power systems in each country, but many PV codes are being developed as separate documents, rather than being included into existing codes. Grounding of batteries associated with PV power sources adds another consideration when grounding the PV array.

Findings

System and equipment grounding practices and requirements vary widely with applications, among the countries, and sometimes within individual countries and a survey of participating IEA countries revealed requirements and practices. Codes in the USA require equipment grounding of all systems, and system grounding for systems with voltages over 50 volts (open circuit module voltage). European and Japanese codes require equipment grounding, but do not require system grounding and most of their PV systems do not have grounded current-carrying conductors on the DC side.

62 The grounding of power systems is complicated by the introduction of current-limited PV sources interconnected with batteries and conventional voltage-source electromechanical generators. Two universal conclusions for grounding were: a) most codes and standards generally require equipment grounds for all metal surfaces that might become energised, b) when system grounds are used, single-point grounds are required. The ungrounded system provides the best fire hazard reduction because multiple ground faults are needed to create a fire hazard. Ungrounded systems allow easy ground fault detection and simple PV array disable.

The grounded PV system generally provides the best personnel protection from electrical shock because the voltages to ground are well defined. The system grounding ensures a solid or known PV array ground through properly sized conductors. The distributed capacitance to ground, of the PV modules and wiring, does not build static charges and the system voltage is stable and known in the grounded PV system. With proper design, both grounded and ungrounded PV systems can achieve good personnel, fire and equipment safety.

1.5 Ground-Fault Detection and Array Disable

Problems

Installed PV systems rarely perform exactly in the manner indicated by electrical schematics. Accumulative leakage currents associated with the large PV array, long runs of wiring, surge protection, diodes, junction boxes that collect moisture, and conduit often make actual ground-fault detection difficult. Leakage currents in early PV systems were often sufficient to cause false indications of ground faults and contributed to many hours of system down time. The leakage currents associated with all of the distributed PV source components and wiring also pose unseen and unfamiliar hazards to personnel, or may contribute to ground faults that increase fire danger and personnel hazards.

Fault currents may occur between active conductors in the circuit called line-to-line or bolted faults, and active circuit conductors-to-ground called ground faults. Utility- interconnected PV systems are often installed in close proximity to utility power lines and accidental cross connection is a possibility that must be addressed. Unintentional connections or faults may result in insulation failures and line-to-line (bolted) faults or line-to-ground (ground) faults. The ground-fault protection of the PV system must be consistent with the ground-fault protection used on the connected AC power system. The AC circuit ground-fault protection requirements are generally part of electrical system installation codes for the application.

Findings

A review of PV system experiences and requirements related to ground faults for grid- connected applications was included as part of a survey of participating IEA countries. The survey included hardware compatibility reviews and ground-fault detection requirements as well as detection methods and disable methods. New developments such as the rapidly evolving AC PV module will not require the use of ground-fault

63 detection on the PV-side DC circuits, since the DC voltage is self-contained within the module and inverter, and there is no external access to the DC circuits. Additionally, the tests associated with listing or certifying the self-contained AC PV modules will assure both fire and personnel safety. It is very unlikely that any conditions will require DC ground-fault detection in AC PV module applications.

The evolution of building-integrated PV systems using DC wiring circuits, PV source circuit combiners and inverters will require ground-fault detection and PV array disable devices for fire and personnel safety. Issues such as backfeeding that may result from inadvertent four-quadrant operation of an inverter, transformer insulation breakdowns or internal circuit failures must be addressed for building-integrated systems

Comparisons of the fire and personnel safety of the grounded and ungrounded PV systems along with considerable research, showed the advantages and disadvantages of each with respect to ground-fault detection. Users and operators must be aware of the grounding methods used and the ground-fault detection and array disable methods. The work included comparisons of PV array ground-fault detection requirements and array disable experience, along with hardware, standards, listing guidelines and practices used for PV system installations. The results of simulated ground faults, simulated transients and lightning, and measured performance for the selected grounding methods are reported and referenced.

1.6 Overvoltage Protection

Problems

PV systems are installed on roof tops, facades of buildings and special applications such as sound barriers on motorways. PV-systems have, by definition, a large exposure to the external environment and are therefore subjected to atmospheric influences. A lightning strike is one of the most severe atmospheric influences. To protect a PV-system for a direct lightning strike is not possible due to the very high energy content of the lightning strike. However, a PV system must and can be designed to withstand the effects of an indirect lightning strike.

Another cause for transient overvoltages in grid-connected PV systems is the AC- network itself. These overvoltages originate from switching phenomena, fault clearance in the power network, and/or lightning induced voltages in overhead lines of the utility. These transient overvoltages are not special for PV systems and are applicable for all types of equipment connected to the distribution network.

Findings

Overvoltages due to indirect lightning strikes can be controlled with proper design of the grounding structure of the PV-system. The main objective is to reduce loops between the DC and AC wiring and the ground structure. This can be solved by having a grounding wire running down from the metal support structure of the array, DC wiring, inverter, AC wiring to the ground structure at the main fuse box. If an external lightning protection system is available this should be connected to the metal

64 support structure of the array. This deliberately formed ground loop allows currents to flow, but reduces the presence of overvoltages to a minimum. Since these currents flow in a well defined path, no hazards are present.

1.7 Islanding

Problems

Islanding is the continued operation of a grid-coupled inverter (or generator in general) in cases where the utility grid has been switched off, cut off or the distribution lines have been damaged so that no electric energy is delivered from the utility side. In such a situation the safety of persons and/or the safety of equipment might no longer be guaranteed.

Findings

A lot of anti-islanding methods have been identified in the literature and have been tested in practice. They can be divided into 2 groups:

• Passive methods: a detection circuit monitors grid parameters (e.g. voltage, frequency, voltage phase jumps, voltage harmonics); these methods do not have any influence on grid power quality.

• Active methods: a detection circuit deliberately introduces disturbances (e.g. active or reactive power variation, frequency shift) and deduces from the reaction to these disturbances if the grid is still present. Active methods can therefore directly effect local power quality.

Currently, no two countries have the same rules as far as islanding is concerned, but there is some common ground: in all countries a PV inverter (or some external protective device) is required to monitor voltage and frequency. However, the set- points for shutdown and disconnection from the grid are not generally agreed upon.

Islanding seems to be the most controversial topic with grid-coupled PV systems. However, theoretical studies show that islanding can only happen under very special and unlikely circumstances if basic safety methods are implemented. These basic methods are

• monitoring of grid voltage • monitoring of grid frequency

As these parameters can be monitored very easily it is recommended to include the sensing circuits in the inverter electronics to reduce system costs. Some countries like The Netherlands, Germany, Switzerland and Austria have tried this approach and would say that they have had good experiences.

65 It is further recommended to perform a scientific risk analysis based on real load patterns in real distribution systems to determine the probability of islanding. Such an analysis could form the common ground from where generally accepted anti-islanding methods could be derived. At present the dangers of islanding seem to be over- estimated; in some countries this has led to legislation demanding very costly or too sensitive anti-islanding methods.

From the technical point of view it seems to be possible to include effective and reliable anti-islanding methods in the inverter control software which would make PV systems more simple to install and bring costs down.

1.8 Electro-Magnetic Compatibility

Problems

Electro-magnetic compatibility is the ability of an electric or electronic device or system to operate according to its purpose in its electromagnetic environment without negatively influencing other equipment by conducted or radiated electromagnetic emissions.

Therefore a manufacturer of PV inverters has to make sure that his device has a certain immunity against external electromagnetic phenomena. At the same time it must not produce emissions disturbing other electronic devices.

Findings

All industrialised nations have some form of legislation which sets limits to the maximum allowable level of electromagnetic emissions. These limits are usually a result of long discussions and are well-proven in practice. The compliance with these limits is tested in well-defined test set-ups with standardised test instruments. The relevant standards for Australia, Europe, Japan and the US have been compiled and referenced. Where possible, the relevant immunity standards (if such exist) have also been cited.

The problem of EMC is not a PV-specific topic. Therefore it does not make sense to create new standards for PV equipment like inverters as the existing standards are generally valid. The only remaining topic: the test set-up for measurements of emissions on the DC lines has to be defined more clearly as conventional devices usually do not have DC connections.

1.9 External disconnect

Problems

This topic examined the necessity for PV systems to have an external manual AC disconnect switch to allow the Utility to disconnect the PV system in the case of maintenance on the AC network or fire hazard etc., to comply with Health and Safety Regulations

66 Findings

Nearly all countries required a means of physically disconnecting the PV generator from the mains for maintenance of the inverter and the AC network to which it is connected. The traditional means to achieve this was a mechanical switch mounted in an external position such that the Utility could operate it before carrying out maintenance on the AC network. This had evolved for a situation of a relatively small number of large generators.

It was generally agreed that as small generators became more common, the task of isolating every unit at an external switch would become impossible to implement reliably, and moreover it did not take into account units that were illegally connected, and thus not registered with the Utility. For this reason, and the relatively high installation costs for such a switch, it was proposed to investigate other solutions.

Some countries, such as Germany the Netherlands and Austria, were coming to the view that in certain circumstances protection relays and operational procedures could be relied upon. This was backed up by a risk assessment study in Germany by the Employer’s Liability Insurance Association to IEC guidelines.

It was generally agreed that if anti-islanding devices were used for the external disconnect function, they would have to be relay devices with a physical opening of contacts rather than an electronic semiconductor switch.

For PV it is anticipated that the situation evolving in Germany and the Netherlands will become more widely adopted, where the external switch is not mandatory, and the Utility relies on the relay 'islanding' protection and their practices for checking and grounding the conductors, assuming that they are live, before carrying out maintenance. This relies on the involvement of the Utilities.

It is important to recognise that the problem is not specific to PV but also applies to other embedded generators, and so it is sensible to harmonise with other work being carried out in this area.

When more information is available from the anti-islanding work, then these devices should be assessed for their suitability to provide the function of the external disconnect also.

1.10 Re-closing

Problems

By re-closing, it is meant the automatic procedure used by the distribution network operator to reduce the duration of the power supply interruption to the users caused by network faults.

67 Findings

Re-closing is utilised by the distributor on the MV network but, as MV networks are usually operated in an open-ring scheme, it has consequences on the downstream LV network where PV systems are connected.

In fact, the re-closing procedure may lead to out-of-phase parallel conditions with consequent potentially dangerous stress for the inverters, for the loads, for the line- breakers and for the transformers installed on the utility network.

1.11 Isolation Transformer and DC Injection

Problems

Transformerless inverters gain increasing importance for grid-connected PV systems due to technical and economical advantages. Contrary to current technology, which mostly relies on transformers built into the inverter, transformerless inverters offer no inherent protection against a dc component fed into the utility’s network.

A dc current fed from the customer's side into the grid can disturb the regular operation of the upstream distribution transformer. It can shift the transformers operating point and cause saturation. This would result in high primary current peaks, which might trip the input fuse and thus cause a power outage to that specific section of the grid. It would furthermore cause increased harmonics.

An overview is given how the participating countries view the requirement for an isolation transformer. The possible impact of a dc current on the operation of a distribution transformer was assessed using literature review and laboratory experiments.

Findings

From the references it can be concluded that dc components from a transformerless inverter may cause saturation effects in the local distribution transformer. However, a disruption of the utility service is unlikely. The experiments showed that primary currents from secondary ac and pulsed dc components linearly superimpose.

Under high dc components high primary current peaks occur. Also, a high level of harmonics is generated. The pulsed dc component may reach levels around 10 % of rated current without jeopardising the proper operation of the transformer.

Conclusions

The hazard of dc currents from small PV systems for the local distribution transformer seems to be negligible, therefore a general requirement for isolation transformers for PV inverters is not justified on these grounds. However, dc currents could cause problems in other areas, such as accelerated corrosion of underground cables due to cathodic erosion, adverse effects on residual current protection devices (RCD's) as

68 well as adverse effects on the mains transformers within the consumers other mains appliances.

69 Appendix E:

Demonstration Tests of Grid-Connected PV Systems

(Subtask 30)

70 1 Introduction to Subtask 30

In subtask 30, experimental studies using the Rokko Test Centre for Advanced Energy Systems test facility were conducted. Experiments were conducted for many aspects like harmonics, islanding, PV system output variation, dc-ac mixing and others. Summaries of the experimental results are described below. Detailed information on the test results are given in the IEA PVPS report “Demonstration Tests of Grid Connected Photovoltaic Power Systems”, Task V Report IEA-PVPS T5-02.

1.1 Harmonic Distortion Caused by PV Systems

Problems and Objectives

A grid-connected PV system generates harmonics because it requires a DC/AC converter to convert the DC from the PV array to the AC of the grid. These harmonics may affect the quality of the power supply. As the number of grid-connected PV systems increase, the total harmonic content in the system may increase. The relationship between the number of interconnected PV units and total harmonic current was measured.

Findings and Conclusions

It was found in the experiments carried out at Rokko Island that the third and the fifth harmonic current increased with the increase in the number of connected inverters. However, the higher harmonics did not always increase or sometimes decreased with the number of connected units. From these results, it can be concluded that third and fifth harmonic current from inverters have almost the same phase displacement and the total harmonic current is superimposed, while higher harmonics from inverters have different phase displacement and total harmonic current is cancelled to a degree. The phenomenon that the third harmonic and the fifth harmonic increases with the number of connected units is considered to be caused by the exitation current of the isolation transformers.

1.2 Measurement of Islanding Characteristics

Problems and Objectives

Islanding may cause problems such as human and equipment safety if it continues for a long time. It is therefore important to clarify the conditions under which continued islanding occurs, and verify the necessity of measures for preventing islanding and the effectiveness of these countermeasures, especially when a large number of PV systems are interconnected to one distribution line.

Findings and Conclusions

The Rokko Island tests showed that when many PV systems whose inverters have only ordinary protective relays such as over/under voltage relays and over/under

71 frequency relays, islanding can be continued for a long time if the total output power the from PV systems is higher than total load on the local distribution system. This result shows that some measures for detecting islanding conditions are required. Various kinds of islanding detection or prevention techniques, including passive and active schemes, have been proposed.

Islanding does not continue for a long period of time if multiple PV systems having islanding detection functions for their inverters were interconnected to the power distribution line, especially when inverters manufactured by different manufacturers were interconnected together. This means that when different islanding detection schemes co-existed, islanding did not occur. It was confirmed that mainly the passive scheme detected islanding phenomenon, while the contribution of the active islanding detection scheme was not clarified. It was also found that islanding detection time increases when a load which can sustain a distribution line voltage such as induction motor load is connected.

1.3 Characteristics under Distribution Line Short Circuit

Problems and Objectives

PV systems may supply fault current under short circuit fault conditions in the distribution system. Fault current from PV systems could affect fault detection, causing a delay in the operation of protection. Therefore, it is necessary to verify the effect of the short circuit current from PV systems on system fault detection.

Fault current from PV systems were measured under short circuit conditions at the LV side of the distribution transformer. Measurements were conducted for various output power of the PV system.

Findings and Conclusions

The Rokko Island tests showed that some inverters do not supply fault current at all (only maintaining the current before short circuit fault) and stop operation quickly (within 1 or 2 cycles) by the under voltage relay. Even for inverters supplying fault current the magnitude of fault current is only twice the pre-fault value and lasts only 1 or 2 cycles.

It was concluded from the Rokko Island tests that PV systems do not significantly affect the protection for short circuit faults in distribution systems.

1.4 Characteristics under AC/DC Mixing Fault

Problems and Objectives

If a PV system has no isolation transformer, a DC current component may be injected into the AC circuit of the power distribution system (DC injection), resulting in magnetic saturation of utility transformer. Any magnetic saturation of the utility transformer will cause current distortion and therefore harmonics are generated and

72 injected into the distribution system. The effect of DC injection could be examined by more severe situation, such as an AC/DC mixing fault condition, in which the DC circuit of the PV array is directly connected to the AC system. In this AC/DC mixing fault condition, the effect of an AC current on the DC circuit of the solar cell array can be also examined.

The propagation range of harmonics generated at the AC-DC mixing fault in the power system, the effect on other transformers connected to the same high voltage distribution line and the effect on other inverters for photovoltaic power generation connected to the same low voltage distribution line are examined.

Findings and Conclusions

The exitation current of the utility transformer starts to increase immediately after AC/DC mixing fault and becomes stabilised (saturated) after several seconds. At this time, magnetic saturation occurs and harmonic current of even orders are generated on the high voltage side of the utility transformer. Distortion of the current waveform was also seen in other utility transformers connected to the same high voltage side of the distribution line and the isolation transformers of other photovoltaic power generation systems connected to the same low voltage distribution line.

However, even though the AC/DC mixing fault persisted for several minutes, no overheating, vibration or sound was observed for the utility transformers. Also, no effect was observed for the operation of PV systems connected to the low voltage side of utility transformers located in the vicinity of the utility transformer generating mixing fault. For conclusion, the effect of DC current injected into the AC system on the utility's distribution transformer is negligible.

1.5 Output Fluctuation of PV systems

Problems and Objectives

The output of photovoltaic power generation fluctuates with solar irradiance. Solar irradiance can vary from one second to the next owing to clouds passing overhead. The fluctuation in output of the photovoltaic power generation causes fluctuations in power flow or voltage in the connected distribution line. If many PV systems are interconnected to a limited area, output fluctuation of the PV systems occur simultaneously and voltage fluctuations in the distribution line increases, possibly becoming of greater significance than load fluctuations. Moreover, considering that the voltage fluctuations take places in a timeframe of seconds, it may cause flicker, which may become a technological issue for the future introduction of photovoltaic power generation.

Power fluctuation from large number of PV systems interconnected to one distribution line within limited area was measured and the relationship between power fluctuation for individual units and the whole system was obtained.

73 Findings and Conclusions

In the case that many PV systems are connected, even if the output fluctuation of each PV system is large, both the magnitude and speed of fluctuation decreases as a whole. Accordingly, distribution voltage fluctuations due to PV output fluctuations also decreases.

74 Appendix F: List of Task V Reports

The IEA PVPS Task V group has published the following reports

[1] “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries”, Task V Internal Report, IEA- PVPS V-1-01, July 1996.

[2] “Information on electrical distribution systems in related IEA countries”, Task V Internal Report, IEA-PVPS V-1-02, July 1996.

[3] “Proceedings of the IEA Workshop on Existing and Future Rules and Safety Guidelines for Grid Interconnection of Photovoltaic Systems”, Zurich, September 1997.

[4] “Grid-connected photovoltaic power systems: Status of existing guidelines and regulations in selected IEA member countries (Revised Version)”, Task V Internal Report, IEA-PVPS V-1-03, March 1998.

[5] “Information on electrical distribution systems in related IEA countries (Revised Version)”, Task V Internal Report, IEA-PVPS V-1-04, March 1998.

[6] “Utility Aspects of Grid Interconnected PV systems”, IEA-PVPS Report, IEA-PVPS T5-01: 1998, December 1998.

[7] “Demonstration Tests of Grid Connected Photovoltaic Power Systems”, IEA- PVPS Report, IEA-PVPS T5-02: 1999, March 1999.

[8] “Grid-connected Photovoltaic Power Systems: Summary of Task V Activities from 1993 to 1998”, IEA-PVPS Report, IEA-PVPS T5-03: 1999, March 1999.

Note:

“Internal” Task V reports are available from the UK Task V representative (Dr Alan Collinson, EA Technology, Capenhurst, Chester) and are available on request, while stocks last.

“Official” Task V reports are available from the Task V Operating Agent (Mr Takuo Yoshioka, NEDO, Sunshine 60, 29F, 1-1, 3-Chome, Higashi-Ikebukuro, Toshima-ku City, Tokyo, Japan).

Once the existing stocks of reports has been depleted it may be necessary for a small charge to be made to cover printing, packing, postage and administative costs.

There are plans in hand to make as many of the above reports as possible available on the Internet, from the web site at:

http://www.iea.org/impagr/imporg/iadesc/pvps

75 Appendix G: List of PV Systems Installed in the UK

Installation (Building Integrated) Installed Peak Power (kWp)

Riverside Housing Association <1kWp Oxford Solar House 4kWp Nottingham University Solar Building 65kWp Northumbria University Solar Building 40kWp Milton Keynes BP Petrol Station Retail 20kWp Liverpool HAT <1kWp Integer House <1kWp Hometon Grove Adventure Playground <1kWp Greenpeace Rooflight <1kWp G8 Solar Pavilion 15kWp Ford Factory <100kWp Equinox Office <200kWp Earth Centre Solar Canopy 110kWp Doxford Solar Office 75kWp Centre Alternative Technology PV Building <20kWp Centre Alternative Technology Powerhouse <1kWp Brighton Solar House <5kWp BRE Environmental Office <5kWp BP Sunbury Office 50kWp Bowater House <10kWp Bedford BP Petrol Station/ Retail 20kWp

76 Installation (Building Attached) Installed Peak Power (kW ) The Greenhouse <4kWp Southwall Autonomous house <3kWp Solagen Unit <1kWp Skelton PV <1kWp Silvertown Solar Houses <2kWp SERG Building Solar Façade <8kWp Scolar Schools 1 to 10 <1kWp each Sascombe Winery PV Unit <1kWp Reading University PV Façade <8kWp Portsmouth FOE Building <1kWp Milton Keynes Shenley PV Conservatory <1kWp Lakeland <1kWp Jolly Gardeners <1kWp Guardian Royal Exchange <4kWp Giroscope PV Project <4kWp East Anglia – House <5kWp Dyfed Visitors Centre <1kWp David’s House <3kWp City Farm PV Unit <1kWp Centre Renewable Technology EcoCabins <1kWp Carters Vineyard PV Unit <1kWp Cambridge County Hall <4kWp Camberwell Solar House <2kWp Centre Renewable Technology Tracking PV Array <1kWp Browne Residence <2kWp Beacon Energy PV Walkway <4kWp Basingstoke Domestic PV <1kWp Barking Energy House <1kWp Antrim Domestic PV Unit <1kWp CERST Loughborough University <3kWp