21, rue d’Artois, F-75008 PARIS C1-108 CIGRE 2012 http : //www.cigre.org
Te Uku Wind Farm – Planning and Operation of a Deeply Embedded Power Plant with Advanced Ancillary Services
RAY W BROWN HAYDEN N SCOTT-DYE Meridian Energy Ltd Meridian Energy Ltd New Zealand New Zealand
SUMMARY
Meridian completed commissioning of the 64.4 MW Te Uku Wind Power Plant (WPP) in 2011. The WPP is embedded within a rural 33 kV distribution network and located approximately 25 km from the 220 kV Grid Exit Point (GXP). Before the WPP was developed, the 220 kV GXP voltage could move out of Grid Code requirements during transmission outages. Planning studies have shown that the WPP is able to reliably control voltage levels across the 33 kV distribution network and maintain the 220 kV GXP 220 kV voltage within Grid Code requirements during contingencies.
The WPP is the largest deeply embedded wind farm in New Zealand. Integrating it with the 33 kV distribution network provided a number of challenges and these are discussed.
New Zealand does not have subsidies for renewable energy power plants. It has a lightly regulated competitive electricity market with relatively low electricity prices. Environmental approvals allowed for a larger wind farm however the optimal commercial development of the WPP was required in order for its owner to achieve adequate returns in the electricity market. Various technical performance requirements were also placed on the WPP. Transmission integration options to address economic and technical requirements are discussed in the paper. The methodology and simulation studies performed to arrive at the optimal solution are described.
The WPP uses full converter output wind turbine generators. The converters provide isolation between the power system and the turbine generator resulting in exceptional grid integration performance. The WPP can provide voltage support even when there is no wind, and modelling has shown that this can solve Grid Code non-compliant 220 kV voltage issues. The WPP is therefore able to provide voltage support ancillary services similar to a STATCON or a synchronous generator operating in synchronous condenser mode.
New Zealand is an island system that has high penetration of renewables with over 70% of electricity supply coming from renewable sources. NZ’s energy strategy is to generate 90% of
NZ’s electricity from renewable energy by 2025. It is conceivable that within ten years, North Island wind generation levels may exceed 50% of North Island load during light load periods. As wind penetration grows, wind power plants will need to provide ancillary services that have traditionally been provided from other sources.
Frequency related services have historically been provided from grid connected plant. The Te Uku WPP has governor systems and the performance and integration of these will be discussed. Instantaneous reserve performance from the wind farm in particular appears to be superior to conventional plant. The use of the wind farm to provide frequency support ancillary services is explored in the paper.
In summary, this paper provides an overview of the electrical and grid integration design of Te Uku WPP, and also explores the ability of the WPP to provide ancillary services. Practical examples from simulations and commissioning tests are also shown.
KEYWORDS
Ancillary-Services, DER, Renewable-Energy-Resources, Wind-Power, Optimisation, Frequency- Response
1 THE NEW ZEALAND POWER SYSTEM AND ANCILLARY SERVICES
The NZ power system consists of two island power systems (the North Island and the South Island) connected by an HVDC (High Voltage Direct Current) link.
The North Island’s electricity is supplied by many fuel sources, and approximately 1500 GWh p.a. is transferred north over the HVDC link during years when water is plentiful in the South Island. The South Island’s electricity generation is 100% renewable, coming from hydro generation and small WPPs (Wind Power Plants).
Demand Demand Peak Generation Wind Capacity Capacity North Island 24,400 GWh 4,500 MW 5,652 MW 428 MW South Island 14,600 GWh 2,300 MW 3,408 MW 58 MW Total NZ 39,000 GWh 6,500 MW 9,060 MW 486 MW Table 1 - NZ Load and Generation 2009/2010 [1]
Due to the low amounts of load and generation, the frequency can change quickly when generation or the HVDC link trips. The frequency range in the North Island is 47 Hz to 52 Hz.
NZ’s wind energy resource is extensive because the country lies across the “Roaring 40s”. NZ WPPs generally operate at an average of 42% of maximum capacity, making them commercially competitive with other forms of electricity generation in NZ without subsidies.
Wind energy is in its infancy in NZ with annual energy penetration having reached only approximately 4 %. With a NZ light load of approximately 3000 MW in summer, wind generation can reach levels up to approximately 20 % during brief periods in summer.
The governments’ energy strategic goals have been to generate 90 % of NZ’s electricity from renewable energy by 2025. In 2010 the government introduced an Emissions Trading Scheme that will favour renewables over greenhouse gas-emitting power stations over time [2].
It is foreseeable that within 10 years, North Island wind generation levels may exceed 50 % of North Island load during light load, high wind periods. It is anticipated that during these periods some WPPs that are unable to provide ancillary services such as frequency and voltage support will not be dispatched and conventional gas, hydro or coal plants that are able to provide ancillary services will be dispatched instead. The revenue for owners of these undispatched WPPs will be reduced as wind penetration grows.
In order to enable high wind penetration, WPPs will need to provide instantaneous reserves (IR) and frequency keeping services. IR and frequency keeping services earn revenue in the ancillary services market [3]. Governors have been installed in recent NZ WPPs [4] to enable them to provide IR and frequency keeping services. When it becomes economically optimal to use WPPs for frequency support services, market rules and mechanisms will be developed to enable WPPs to offer these services into the ancillary service market.
The structure of the electricity market is a point of difference between NZ and many jurisdictions. NZ has full nodal pricing, security-constrained economic dispatch. There is a reserves market and commercial arrangements for ancillary services such as voltage support.
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This paper provides an overview of the electrical and grid integration design of Te Uku WPP, a deeply embedded wind farm that has been designed to provide both voltage and frequency ancillary services to the transmission grid.
2 EXISTING TRANSMISSION SYSTEM DESCRIPTION
The Te Uku WPP site is located approximately 20 km west of Hamilton in the North Island of New Zealand between the Raglan and Aotea Harbours. The WPP Te Uku covers an area close to 56 km 2
The Distribution Network Operator (DNO) in the region, obtained planning permits for up to 28 Wind Turbine Generators (WTGs) on the generation site which could have a capacity up to 3 MW each, providing for a potential wind farm capacity up to 84 MW. The site wind resource is Class IIB with a high average wind speed of 8.24 m/s at hub height. This provided potential for a generation capacity factor in the order of 40 %. Figure 1 – Te Uku Geographical Overview
The centre of the site is approximately 20 km from the Te Kowhai 220 kV / 33 kV substation that supplies a large proportion of Waikato’s West Hamilton region through a single circuit 33 kV line which extended as far as the town of Raglan. The Te Kowhai offtake has generally averaged around 20 MW however it has peaked up to 90 MW during times of low existing embedded generation output (Te Rapa cogeneration) and the switching of load from other parts of the Hamilton distribution network. The nearest 33 kV line was approximately 8 km from the centre of the site and this single circuit line was the primary supply to Raglan.
The Te Kowhai substation has two 100 MVA 220 kV / 33 kV transformers. It is connected to one circuit of the 220 kV double circuit Stratford – Taumarunui – Huntly line (see figure 2). This is a long line that has capacitance charging issues when the connection to Huntly is lost. In this case the 220 kV voltage at Te Kowhai can rise to approximately 1.125 pu, outside of Grid Code requirements, particularly when the offtake at Te Kowhai is light.
The challenge presented to Meridian and the DNO was to complete an optimised design for the site that could be commercialised. After much detailed analysis, 28 WTGs with 2.3 MW generators and 101 m diameter turbines and a 33 kV transmission solution was adopted. This provides an estimated output of 248 GWh p.a. from the wind farm’s 64.4 MW capacity operating at a capacity factor of 43.9 %
3 PERFORMANCE REQUIREMENTS
The initial phase of the transmission solution design process required the establishment of appropriate power quality standards for a large embedded wind farm. This required interpretation of the Grid Codes applicable for the development, and negotiation of standards with the DNO and the Transmission System Operator (TSO).
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Key
Te Uku
Figure 2 – Waikato Transmission Network Overview
The DNO was most interested in the power quality experienced by customers distributed around its network. The DNO’s requirements were power quality performance based. The TSO was more prescriptive with its targets requiring fault ride through and adequate power system response following specific transmission faults. At the time of design, New Zealand did not have generation fault ride through code requirements.
The wind farm is required to control the steady state voltage at its 33 kV switchboard so that the network operates such that the regulated 11 kV voltage to supply customers is within obligations. Studies showed that this could be achieved if the distant Te Kowhai and nearby DNO substation 33 kV buses were regulated within +/- 5 % of nominal by the wind farm.
The expected steady state voltage range at Te Kowhai 220 kV (TWH220) is from -10% to +12.5% of nominal. Although the Code has +/-10 % limits on the 220 kV grid, in some grid contingencies without Te Uku WPP operating, the Te Kowhai voltage may rise to 1.125 pu.
At the request of the TSO, the WPP was designed to operate during weak system conditions that included transmission outages with low North Island generation. Some of these scenarios were subsequently found to be inherently unstable without the WPP. The WPP’s design must also be such that the power quality within the network is within standards after faults at the WPP or within the DNO or TSO networks. Fault ride through was required for 33 kV and 11 kV network faults, and distant TSO 220 kV and 110 kV Grid faults, and close-in grid faults during conditions where all network components are in service pre-fault. The WPP is required to lift its switchboard voltage during undervoltage faults or suppress it during overvoltage faults, and to control voltage post fault.
Unlike conventional synchronous generators, the WPP can remain connected during close-in earth faults. During situations where the WPP farm becomes islanded (with no connection through to the TWH220 bus), including auto-reclose events it was decided to design the WPP to trip. This was decided in order to maintain simplicity and ensure safety of the network as the remaining islanded 33 kV network might not have a star/earth reference point.
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Comprehensive power system studies were performed to design a transmission solution and test the wind farm’s performance under numerous grid scenarios.
The reactive power capability requirements in the Grid Code were originally intended to exclude embedded generation, so the developer was surprised to learn that even deeply embedded generation are now required to comply with the Code 50% export and 33% import reactive power rules. The wind farm’s export capability was just outside code requirements and a dispensation from this requirement was applied for. Fortunately steady state undervoltages are not an issue in this part of the grid whereas over voltages are, and therefore the non-compliance does not prevent network owners to perform within their obligations.
4 TRANSMISSION OPTIONS
The two primary options considered were a new 110 kV spur line from Te Kowhai to the WPP, and enhancement of the existing 33 kV network in order to integrate the WPP into it. To minimise environmental effects, a 220 kV line was not considered. The DNO had initially designed a 110 kV connection solution for the wind farm, however with the adoption of smaller WTGs by Meridian, other more economic options could be considered.
Figure 3 – Transmission solution in the DNO Network
A 33 kV solution for the 64.4 MW WPP was studied in detail. It was found that building a new 33 kV line from the WPP directly to Te Kowhai, and connecting the existing 33 kV radial network to the WPP and bussing the two circuits together there to create a 33 kV ring
6 from the WPP back to Te Kowhai within the DNOs’ network, was a cost effective and environmentally benign solution.
The new 25 km 33 kV line consists of four sections of underground cable with a total length of 8.38 km and three sections of overhead line with a total length of 16.9 km. The first 5.6 km of overhead line from the WPP was double circuit so that one circuit could connect the WPP to the existing 33 kV network at Cogswell Road, approximately 10 km from the WPP.
The existing 33 kV network also required substantial upgrades in order to support the wind farm. Approximately 7.8 km of overhead line was reconductored and 5.6 km of new underground cable replaced an existing circuit. New Ring Main Units were also added to the network to create two new switching stations.
The combination of new line, circuit upgrades, additional switching stations, the creation of the network ring, protection upgrades and the wind farm’s voltage control at the remote end of the network have substantially increased security of supply and power quality to the region West of Hamilton, and provided for considerable demand growth.
The two 33 kV circuits connecting the WPP to the network can only carry a proportion of the WPP’s maximum power output. Fast runbacks were installed that detect overloads in the 33 kV network and ramp back the WPP power output within seconds to within the stressed circuit’s rating. The amount of runback required depends on zone substation demand and WPP generation at the time of the outage, and varies automatically and dynamically. After the WPP has ramped its output down such that distribution network equipment is no longer overloaded, the runback signal is reset. Periodically the WPP runback system lifts the WPP output and the runback signal from the distribution network will again be set if equipment is overloaded. In this way the WPP shall automatically maximise its utilisation of the capacity of the DNO’s network.
Protection upgrades were also made in order to ensure good power quality during all contingencies. Ride through of the WPP was particularly important. The primary 33 kV network protections now consist mainly of unit (differential) protections. The high level goal was to clear most 33 kV faults within 200 ms.
5 ON SITE RETICULATION AND VOLTAGE REGULATION
Meridian selected 2.3 MW full scale converter WTGs. Due to the decoupling of the turbine speed from the grid frequency that these generators provide, power system integration and excellent power system performance was achievable.
Full Scale Frequency Converter Capacitor Ba nk Gearbox Genera Generator to r Grid
Figure 4 – Full Scale Converter Wind Turbine Generator Configuration
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The 690 V output from the frequency converters is stepped up to 33 kV via outdoor WTG transformers. Three primary cable strings connect the WTGs back to the centralised switchboard. Figure 5 shows the WPP line diagram. The WPP switchboard does not have a main power transformer due to the direct connection to the DNO 33 kV network.
Figure 5 – Te Uku WPP Single Line Diagram
The WTGs provide STATCOM-like dynamic reactive power support. Each 2.3 MW converter is capable of approximately 1.0 Mvar export and 2.5 Mvar import at nominal voltage, to give a total wind farm reactive power capacity of approximately 28 Mvar export and 70 Mvar import. Reactive power output capability is dependent on the reticulation network voltage and WTG active power output.
When the wind farm is not generating active power, the wind farm can act as a STATCOM. In this mode each WTG can support network voltage stability by varying its reactive power output. This is the first application of this mode in New Zealand and it is anticipated that a voltage support contract will be necessary to enable its operation in the market.
The WPP controls act to control the network steady state 33 kV voltage by managing the WTG reactive power output. The controls can act in voltage, power factor or Var set-point modes. Voltage control with a 1.02 pu set point at the WPP switchboard was found to be the best mode. This maintains voltages in the distribution network and at the 220 kV grid exit point within 5% of nominal during varying network and generation conditions. During transient events, the WTG voltage controllers take priority over the 33 kV voltage control and modify the WTG reactive power output in order to maintain the WTG 690V voltage level close to unity so that the WTGs ride through (do not trip) due to power system earth faults.
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6 POWER SYSTEM STUDIES
The cornerstone of the modelling was the development of an accurate power system model. The WTG supplier provided a model for its WTGs in a form suitable for DigSilent PowerFactory. A PowerFactory model of the North Island was provided by the TSO and a distribution network PowerFactory model was translated and developed from DSO models.
The wind farm is required to perform under a large range of fault types and locations: • Three phase faults and single phase line and bus faults as far as 360 km away and as close as the wind farm switchboard; • Auto reclose events within the transmission and distribution network; • Circuit breaker failure with long fault clearance times; • Fault clearance with back-up protection clearance times; • Light load, high load.
Initial dynamic studies ascertained the critical power system scenarios and events that should be tested. This determined that there were 36 dynamic scenarios requiring examination. The WPP was found to generally improve power quality in all fault scenarios studied, except those where the power system model was already unstable and would not converge without the WPP in operation. In these inherently unstable cases WPP operation could not be simulated.
Following dynamic studies, steady state studies were undertaken to design the WPP voltage control methodology. Figure 6 shows some results from this analysis where it was found that 33 kV voltages within the distribution network could be kept within requirements with a WPP Voltage Control set point between 0.96 and 1.04 pu. It can be seen that the network voltages decay as generation increases due to voltage and reactive power drop across the network.
Figure 6 –Voltage Variation at Points Around the Distribution Network at Peak Load
The reactive power exchange at Te Kowhai was also examined and it was found (figure 7) that a higher than nominal WPP voltage setpoint minimised the reactive power exchange at the Grid Exit / Grid Injection Point.
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Further study showed that a higher than nominal WPP voltage control set point would minimise network losses. Contingency analysis was then successfully undertaken to ensure that network voltages would be well within limits during outage scenarios.
Figure 7 – Reactive Power Variation at the Grid Exit Point at Minimum Load
7 VOLTAGE SUPPORT ANCILLARY SERVICES
As discussed above, in some grid contingencies without Te Uku WPP operating, the remote 220 kV Te Kowhai Grid Exit Point voltage may rise to 1.125 pu. The Grid Code requires this voltage to stay within +/-10% of nominal.
The Code also requires power stations to have a voltage control system that regulates reactive power output in order to control power system voltages. It was found that with a local voltage set point of 1.02 pu the WPP can bring the 220 kV voltage within code requirements at all levels of active power generation. Existing high voltage when HLY-TWH out (+10%)
TUK wind farm lowers 220 kV
Figure 8 – Light load Contingencies and positive Effect on Overvoltages
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The Code does not require power stations to control the power system’s voltage when they are not generating active power. It was found that when the WPP is operating in reactive compensation mode with no active power generation, it could also bring the 220 kV voltage within code requirements when it had a voltage set point of 1.02 pu (see figure 8). This provides the opportunity for voltage support ancillary service revenue for the WPP, should the TSO wish to maintain the Te Kowhai voltages within Code requirements. The TSO is considering whether it will keep the 220 kV voltages within Code requirements by contracting the WPP to provide voltage ancillary services.
A number of real system tests and actual faults have proven the responsiveness of the WPP and its ability to control the remote 220 kV voltage in weak grid scenarios.
8 WIND POWER PLANT FREQUENCY RESPONSE
The WPP control system can modulate turbine pitch and therefore the active power as well, at a very fast rate, allowing quick response to frequency deviations. Figure 9 shows a result from an actual active power step response test. With sufficient wind, the WPP can step from zero production to 90 % output within 6 seconds, from a 100 % power step order.
30 MW Manual Step 40
30
20 MW
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0 22:22:48 22:22:49 22:22:50 22:22:51 22:22:52 22:22:53 22:22:54 22:22:55 22:22:56 22:22:57 22:22:58 22:22:59 22:23:00 PP1ActivePower PP1Pscheduled
Figure 9 – Actual Step Response to 30 MW Command from 0 MW
In order for WPPs to provide Instantaneous Reserves (IR), WPP generation is curtailed to provide head room. This is not ideal with wind generation as this will result in loss of energy revenue, however in order to enable high wind penetration levels, WPPs will need to provide IR and frequency keeping services. IR and frequency keeping services earn revenue in the ancillary services market.
The New Zealand government has a strategic goal to generate 90 % of NZ’s electricity from renewable energy by 2025. It is probable that within ten years, North Island wind generation levels may exceed 50 % of North Island load during light load periods. It is anticipated that during these periods WPPs that are unable to provide ancillary services such as frequency and voltage support will not be dispatched and conventional gas, hydro or coal plants that are able
11 to provide ancillary services will be dispatched instead. The owners of these WPPs will suffer significant commercial losses as wind penetration grows.
In order to achieve high penetrations of wind generation in New Zealand, WPPs are being developed that provide frequency and voltage support ancillary services. Some pitch controlled WTGs are able to provide superior frequency support services than conventional generation due to their faster speed of response, and therefore WPP frequency support services will add a useful diversity to the ancillary services market in future years.
The Te Uku WPP is equipped with a frequency governor. As WPP frequency governors were new to NZ, the WPP owner engaged with the TSO to develop and agree the test procedure for testing WPP governors on the NZ grid. A test plan was subsequently developed and followed.
Figure 10 shows the WPP response to a frequency step of +0.5 Hz. The WPP quickly reduced its output. The WPP was then manually ramped up to pre-fault power levels and left in frequency keeping mode with a delta, in order to regulate frequency continuously.
The frequency controller can operate in delta mode, where the active power output is curtailed by an absolute value (a “delta”) below the calculated available output. When an under frequency event occurs, the frequency controller uses the delta to stabilise the grid frequency as in Figure 11.
Droop 4%, Deadband 0.03, +0.5Hz step
50.000 50.6 50.5 50.4 45.000 50.3 50.2 50.1 40.000 50 49.9 49.8 35.000 49.7 49.6 Hz
MW 49.5 30.000 49.4 49.3 49.2 25.000 49.1 49 48.9 20.000 48.8 48.7 48.6 15.000 48.5 21:47:30 21:48:01 21:48:31 21:49:01 21:49:31 21:50:02 21:50:32 21:51:02 PP1ActivePower PP1Frequency Figure 10 – Actual WPP Response to +0.5Hz step
The standard underfrequency characteristic for testing governor systems in NZ is Freq (t) = 49.25 + (0.75 - 0.8055 t ) e -0.1973 t [5]. This curve represents the typical response of the 12 power system to a severe underfrequency event. A stepped composite of this curve (as shown in figure 11) was used extensively during Te Uku governor tests. Figure 11 shows the WPPs response to the frequency curve being injected into the WPP controls when the WPP had a delta of 100 % and an available production level of approximately 62 MW.
Te Uku 14/09/2011 HPPP Frequency Response - Standard Underfrequency Curve Injection Wind 14m/s, Droop 4%, Deadband +/-15mHz, Available Power approx 62MW Frequency Sensitive Mode
65.0 50.2 60.0 50.0 55.0 49.8 50.0 49.6 45.0 49.4 40.0 49.2 35.0 49.0 30.0 48.8 Hz MW 25.0 48.6 20.0 48.4 15.0 48.2 10.0 48.0 5.0 47.8 0.0 47.6 -5.0 47.4 18:28:30 18:28:34 18:28:38 18:28:43 18:28:47 18:28:51 18:28:56 18:29:00 18:29:04 18:29:09 18:29:13 18:29:17 18:29:22 18:29:26 18:29:30 18:29:35 18:29:39 18:29:43 18:29:48 18:29:52 18:29:56 18:30:01 18:30:05 18:30:09 18:30:14 PP1ActivePower PP1Frequency
Figure 11 – Governor response to standard under frequency test signal in delta control
It can be seen that the WPP can respond to an extreme underfrequency event by moving from 0 MW production to approximately 85 % power output in 4 seconds. In real system underfrequency events however the response would be slightly different. During an event in December 2011 the trip of the Huntly 220 kV bus with approximately 850 MW of generation at Huntly power station caused the frequency to drop briefly to 47.5 Hz. The WPP was operating without a delta but due to inertia being released from the turbines the WPP increased its output by approximately 15 % for a few seconds. The WPP also assisted to stabilise voltages in the region. It is therefore anticipated that the WPP would respond slightly more quickly and with a higher initial output in a real underfrequency event of the magnitude shown in Figure 11.
In March 2011 the WPP owner completed the commissioning of the final WTG at Te Uku WPP and later in the same year completed commissioning tests sufficient for providing frequency ancillary services in the NZ electricity market.
9 CONCLUSION
State-of-the-art Wind Turbine Generators and control systems at Te Uku wind farm have resulted in excellent power quality within the Waikato power system, West of Hamilton.
Comprehensive power system studies were required to test and design the transmission solution and the wind farm’s performance.
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The wind farm has been designed to enable the provision of voltage and frequency response ancillary services. Governor tests have proven the ability of the wind farm to govern, and show promise for offering this ancillary service to the market. The wind farm’s no-wind voltage response has shown its ability during actual events to keep remote grid voltages within code compliance levels. Its performance in these areas provides an indication of the opportunities available for ancillary services as wind’s participation in the market grows.
As more WPPs are developed that provide superior short term frequency support services than conventional generation, wind farm frequency support services will add a useful diversity to the ancillary services market.
BIBLIOGRAPHY
[1] Electricity Commission, Statement of Opportunities, 2010. [2] New Zealand Ministry for the Environment, Major Design Features of the Emissions Trading Scheme, Factsheet 16 INFO 318, 2008. [3] Energy Link and MWH NZ, Wind Energy Integration in NZ, Ministry of Economic Development and Energy Efficioency and Conservation Authority, 2005. [4] R.W. Brown, West Wind, Grid Integration, and Keeping the Lights On, NZ Wind Energy Association conference, 2010 [5] Transpower NZ Ltd, Companion Guide for Testing of Assets, 2010.
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