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Lightning and surge protection for photovoltaic (PV) systems
The guaranteed 20 years service life for photovoltaic generators and their exposed installation sites, as well as the sensitive electronics of the inverter, require effective lightning and surge protection.
It is not only home owners that install a PV system on their rooftop, private companies are also making more and more investments in shared systems, which are erected on large-surface roofs, on various control structures, or unused open areas. Because of the big space requirements of the photovoltaic generator, PV systems are especially threatened by lightning discharges during thunderstorms.
Causes for surges in PV systems are inductive or capacitive voltages deriving from lightning discharges as well as lightning surges and switching operations in the upstream power supply system. Lightning surges in the PV system can damage PV modules and inverters. This can have serious consequences for the operation of the system. Firstly, high repair costs, for example, those of the inverter, have a negative effect, and secondly, the system failure can result in considerable profit cuts for the operator of the plant.
Necessity of lightning protection
Before installing a PV system, it must be decided whether the installation will be on a building with or without lightning protection. For public buildings such as halls or churches, schools or hospitals, some countries’ building regulations request lightning protection systems for safety reasons. For this purpose, buildings or structures are differentiated according to their location, construction type, or utilisation, and whether a lightning strike could easily have severe consequences. Such buildings or structures in need of protection have to be provided with a permanently effective lightning protection system.
In case of privately used buildings, lightning protection is often not installed. This happens partly due to financial reasons, but also because of a lack of knowledge with respect to this topic. If a building without external lightning protection was selected as a location for a PV system, the question arises, if, with the additional installation of the PV generator on the roof, lightning protection should be provided for the entire structure.
According to recent studies, the installation of PV modules on buildings does not increase the risk of a lightning strike, so the request for lightning protection cannot be derived directly from the mere existence of a PV system. However, there may be an increased danger for the electric facilities of the building in the event of a lightning strike. This is based on the fact that, due to the wiring of the PV lines inside the building in existing risers and cable runs, strong conducted and radiated interferences may result from lightning currents. Therefore, it is necessary, to estimate the risk by lightning strikes, and to take the results from this into account for the design.
IEC 62305-2 (EN 62305-2) states procedures and data for the calculation of the risk resulting from lightning strikes into structures and for the choice of lightning protection systems. For this purpose DEHN + SOHNE offers the DEHNsupport software. The risk analysis presented ensures that it is possible to draw up a lightning protection concept, which is understood by all parties involved, and which meets optimum technical and economic requirements, i.e. the necessary protection can be guaranteed with as little expenditure as possible.
A lightning protection system (LPS) designed for Class III meets the usual requirements for PV and solar thermal systems. Photovoltaic and solar thermal systems on buildings must not interfere with the existing lightning protection measures. Photovoltaic and solar thermal systems shall be protected by isolated air-termination systems according to 5.2 and 6.3 of IEC 62305-3 (EN 62305-3) against direct lightning strikes. If a direct connection cannot be avoided, the effects of partial lightning currents entering the building have to be taken into consideration.
Protection of photovoltaic inverters against surges also in case of direct lightning strikes
If a PV system is installed on a building with an external lightning protection system, one of the basic requirements is that the PV modules are within the protective area of an isolated air-termination system. Additionally, the separation distance between the PV supporting frame and the external lightning protection system has to be kept in order to prevent uncontrolled sparkover. Otherwise, considerable partial lightning currents can be carried into the building or structure.
Often the operator wants the entire roof to be covered with PV modules in order to gain a possibly high profit. In these cases the separation distance often cannot be realised and the PV supporting frame has to be integrated into the external lightning protection. Here, the effects of the currents coupled into the building or structure have to be taken into consideration and a lightning equipotential bonding has to be provided. Meaning that lightning equipotential bonding is also necessary for the dc conductors carrying lightning current.
According to IEC 62305-3 the dc conductors have to be protected by Type 1 surge protective devices (SPDs). Up to now, surge protective devices Type 1, in conjunction with a spark gap, for use on the dc voltage side, were not available. The problem was that the spark gap once being tripped, could not be quenched again and hence the arc persisted. With the combined lightning current and surge arrester DEHNlimit PV 1000 DEHN + SOHNE succeeded in developing a direct current extinguishing spark gap arrester.
Thus DEHNlimit PV 1000 is the ideal arrester for use in photovoltaic power plants. The encapsulated creeping spark gap technology provides a safe protection of the PV generator and the inverter also in case of direct lightning currents. This combined arrester is applicable for PV systems up to 1 000 V UOC STC . DEHNlimit PV 1000 has a high lightning current discharge capability of 50 kA 10/350 μs.
Single pole photovoltaic arrester Type 2 with integrated short-circuiting device
The inner structure of the Type 2 DEHNguard PV 500 SCP surge arrester sets new patterns for safety. With this arrester the proven double effect of the monitoring and disconnecting device Thermo Dynamic Control has been combined with an additional short-circuiting device. This completely new method of arrester monitoring ensures operation safety without the risk of a fire hazard, even if the devices are overloaded for example at insulation faults in the PV generator circuit.
An essential part of a lightning protection system is the lightning equipotential bonding for all conductive systems entering the building from the outside. The requirements of lightning equipotential bonding are met by direct connection of all metal systems and by indirect connection of all live systems via lightning current arresters.
The lightning equipotential bonding should be performed preferably near the entrance of the structure in order to prevent a penetration of partial lightning currents into the building. The low voltage power supply of the building is protected by a DEHNventil ZP, a multi-pole combined lightning current and surge arrester with spark gap technology. It is designed for installation on 40 mm DIN rails in the electrical distribution board. The surge protective device has to be chosen according to the type of power supply system. This combined lightning current and surge arrester has no interaction limiting reactor and is available as a complete pre-wired unit for every low voltage system (TN-C, TN-S, TT).
There is sufficient protection without additional protective devices between DEHNventil and terminal equipment up to a cable length of < 5 m. For greater cable lengths SPDs Type 2 or 3 have to be used in addition. If the distance between the ac output of the inverter and the application site of DEHNventil is not greater than 5 m, no further protective devices are required for the ac side.
At the dc input of the inverter each of the incoming string conductors has to be protected to earth by a DEHNguard surge protective device Type PV500 SCP installed between plus and minus.