TECHNICAL RENEWABLES

Back to the grid Recent figures suggest that more and more insulated single cables are used. electricity in the UK is being generated from The double insulated rubber- renewable sources – not least solar PV. With that sheathed cable of type H07 RN-F is permitted for operating in mind, Graham Wretham focuses on grid- temperatures not exceeding 60°C connected systems and, in particular, installing in its standard version. the electrical supplies – both AC and DC. Connection systems hotovoltaic (PV) • AC cabling Connection of the module string The author systems are generally • Metering equipment with cabling and other DC cabling divided into two groups, distribution board, supply etc should be undertaken with Graham Wretham is standalone systems extreme care. Poor contacts can Managing Director of Redford P and grid-connected systems. DC current cabling lead to arcing and increased Charles Ltd, an electrical training and consultancy However, this article only Module and string cables fire risk. To facilitate module company in Kent. considers the installation of grid For the electrical installation installation, plug-and-play Previously the NICEIC connected systems of up to 4kW of a photovoltaic system, only technology has become standard Inspecting Engineer for Kent, – for a small commercial unit or wiring and cabling that meets in grid connected systems. Graham was later promoted to domestic property. the requirements should be used Senior Engineer and was also for this application. A distinction DC load switch the NICEIC’s representative Grid-connected systems is made between module or (DC main switch) on various external and A grid-connected PV system string cables, the DC main cable In order to carry out maintenance international technical essentially comprises the following and the AC connection cable. and/or repair work, it must be panels. Graham gained further promotion to NICEIC components (see Figure 1): The electrical connecting cables possible to isolate the inverter from Operations Manager and then, • PV modules/array (multiple PV between the individual modules the PV generator. The DC main following a restructure, to modules connected in series of a solar generator and the switch is used for this purpose. Operations Director. and/or parallel) generator junction box are The DC main switch should be Graham left the NICEIC in • DC current cabling termed ‘module’ cables or ‘string’ rated for the open circuit voltage 2005 to start his own electrical • DC main isolator cables. These cables are generally of the solar generator, and for and consultancy business, • An inverter (to meet the installed outdoors. the maximum generator current Redford Charles Ltd, which requirements of Engineering In order to minimise the risk (short circuit current under has become one of the leading Recommendation G/83) of earth fault and short circuit, Standard Test Conditions). electrical training companies in the south east and works for • AC isolators (normally two, cables are installed so that the The DC switch should a number of blue chip clients. one adjacent to the inverter positive and negative poles are not preferably be located directly and another near the installed together – i.e. not in the before the inverter in a readily- distribution board) same cable. Accordingly, double accessible position.

41 TECHNICAL

As long as the irradiance • Recording of operating data is sufficient, the PV generator and signalling. supplies energy and is, therefore, • Establishment of DC and AC under load. When separating a protective devices (e.g. incorrect DC side isolation switch plug connector under load, the polarity, overvoltage, overload). PV array DC current can result in a long (usually building Inverter arc, which is a serious safety risk. AC switch disconnectors mounted) The AC switch disconnector AC side isolation switch Grid-connected inverters must be double pole, clearly Grid-connected inverters are labelled and only lockable in also known as grid-tied inverters. the ‘OFF’ position. A second The solar inverter is the link isolator is required adjacent to the between the PV array and the inverter if the inverter is installed AC grid and AC loads. Its basic in a separate location to the AC mains supply task is to convert the solar DC distribution board. Meter Main fusebox electricity generated by the PV To high array into AC electricity, and to efficiency AC cabling appliances adjust this to the frequency and The selection of a particular Figure 1. voltage level of the building’s wiring system for the AC supply Grid-connected PV system. electrical system (synchronisation). system needs to consider both In grid-connected PV systems, loading/rating of conductors and the inverter is linked to the mains cables and also environmental protective device within the PV testing should include: electricity grid via the building’s conditions. In commercial consumer unit. • Measurement of the open electrical installation, and, properties, it is likely that the Where the AC supply of the circuit voltage Voc. Open circuit therefore, the inverter must meet wiring system will match that of PV solar installation is connected voltage of the array (Voc) should the requirements of Engineering the existing installation – normally to the same side of an RCD as be the open circuit voltage Recommendation G/83. singles in plastic or metal final circuits protected by that (Voc) 3 ‘n’ (where n = the number With a coupling to the trunking. In domestic properties, RCD, the RCD must disconnect of modules in the string). building’s grid, the generated the wiring system is likely to be both the line and neutral • Measurement of the in-plane solar power is first consumed in PVC flat twin with earth cabling. conductors, which is the normal irradiance (Gi). the building then any surplus is configuration for most 17th • Measurement of the short fed to the mains electricity grid. AC connection Edition consumer units. circuit current (Isc). Modern grid-connected For a PV power supply The reason for this is that if inverters are able to perform the system, the connection to the the RCD does not disconnect the following functions: microgeneration (solar PV) neutral, protection does not rely • Conversion of DC generated by system must be from a dedicated just on the operation of the RCD ›› The AC switch the PV modules into mains AC. circuit from the distribution but also on the shut-down time disconnector must • Adjustment of the inverter’s board/consumer unit. The PV of the inverter. Wiring Regulations operating point to the MPP circuit needs to be connected to (BS 7671) requires a final circuit be double pole, of the PV Modules. a dedicated, outgoing overcurrent of up to 32A to be disconnected clearly labeled and within 0.4 seconds. However, only lockable in the shut-down of the inverter may be ‘OFF’ position.‹‹ TO SUM UP in the region of 3 to 5 seconds.

› Double insulated single cables are used on the DC side. Commissioning PV solar The frequently-used double insulated rubber-sheathed photovoltaic systems cable of type H07 RN-F being preferred. Commissioning comprises Inverter testing should include: › Plug-and-play technology has become standard in grid the following: • Inverter testing is to be connected systems. • Inspection completed after PV array • PV testing testing and with the grid › The DC main switch should be rated for the open circuit • Inverter testing connected. voltage of the solar generator and for the maximum • A check of the operational generator current (Isc). Inspection should include status of the inverter (e.g. status › An Inverter must meet the requirements of Engineering the following: indicator LEDs, check inverter Recommendation G/83. • PV generator and operation manual). › The AC switch disconnector must be double pole, clearly mounting construction • An awareness that for the labeled and only lockable in the ‘OFF’ position. • Corrosion protection measures first three minutes (for G83/1 • PV generator cabling compliant inverters), the › The wiring system for the AC supply system needs to • Junction boxes inverter will be in standby consider both loading/rating of conductors and cables, • Mains DC cables mode before it feeds any and also environmental conditions. • DC Isolators – set to ‘OFF’ power to the grid. › Where the AC supply of the PV solar installation is • Inverter(s) • Measurement of the DC array connected to the same side of an RCD as final circuits • Earthing arrangements and inverter power with the protected by that RCD, the RCD must disconnect both and requirements inverter operating and feeding the line and neutral conductors. • Lightning protection the grid. If the inverter does › Commissioning must include inspection, PV testing (if applicable) not feed power to the grid, the and inverter testing. • Labelling manufacturer’s manual should • AC isolators – set to ‘OFF’ be consulted for information • AC wiring on error conditions. n

42 › INSTALLER › April 2013