Lightning and surge protection for rooftop PV systems

September 29th, 2014, Published in Articles: Energize

 

According to the South African Photovoltaic Industry Association (SAPVIA), photovoltaic (PV) is the fastest growing power generation technology in the world. Between 2006 and 2009 the installed capacity globally grew on average by 60% per annum. These systems are exposed to all weather conditions and are expected to be able to withstand them over many decades. The cables of PV systems frequently enter the building in question and extend over long distances until they reach the grid connection point.

More than 35 GW of PV is installed and operating worldwide, producing more than 30 TWh of clean energy per year. Bearing in mind that self-generated electricity is generally cheaper and provides a high degree of electrical independence from the grid, PV systems will become an integral part of electrical installations in the future.

Lightning discharges cause field-based and conducted electrical interference. This effect increases in relation with increasing cable lengths or conductor loops. Surges do not only damage the PV modules, inverters and their monitoring electronics, but also devices in the building installation. More importantly, production facilities of industrial buildings may also easily be damaged and production may come to a halt.

If surges are injected into systems which are far from the power grid, which are also referred to as stand-alone PV systems, the operation of equipment powered by solar electricity, such as medical equipment, water supply, and so on, may be disrupted.

Necessity of a rooftop lightning protection system

The energy released by a lightning discharge is one of the most frequent causes of fire. Therefore, personal and fire protection is of paramount importance in case of a direct lightning strike to the building.

The installation of PV modules does increase the risk of a lightning strike as the collection area increases and substantial lightning interference may be injected into the building through these systems. Therefore, it is necessary to determine the risk resulting from a lightning strike as per IEC 62305-2 (SANS 62305-2) and to take the results from this risk analysis into account when installing the PV system. For this purpose, DEHN offers a service through its consulting division, DEHNconcept, which can conduct the risk analysis and design a lightning protection system (LPS) for the site.

These standards require that a lightning protection system according to class of LPS III be installed for rooftop PV systems(>10 kW) and that surge protection measures be taken. As a general rule, rooftop PV systems must not interfere with the existing lightning protection measures.

Necessity of surge protection for PV systems

In case of a lightning discharge, surges are induced on electrical conductors. Surge protective devices (SPDs) which must be installed upstream of the devices to be protected on the alternating current (AC), direct current (DC) and data side, have proven very effective in safeguarding electrical systems from these destructive voltage peaks. Section 9.1 of the CLC/TS 50539-12 standard (Selection and application principles – SPDs connected to photovoltaic installations) calls for the installation of surge protective devices unless a risk analysis demonstrates that SPDs are not required. According to the IEC 60364-4-44, surge protective devices must also be installed for buildings without external lightning protection system such as commercial and industrial buildings.

Cable routing of PV systems

Cables must be routed in such a way that large conductor loops are avoided. This must be observed when combining the DC circuits to form a string and when interconnecting several strings. Moreover, data or sensor lines must not be routed over several strings and form large conductor loops with the string lines. This must also be observed when connecting the inverter to the grid connection. For this reason, the power (DC and AC) and data lines must be routed together with the equipotential bonding conductors along their entire route.

Earthing of PV systems

PV modules are typically fixed on metal mounting systems. The live PV components on the DC side feature double or reinforced insulation (comparable to the previous protective insulation) as required in the IEC 60364-4-41 standard. The combination of numerous technologies on the module and inverter side, with or without galvanic isolation, results in different earthing requirements. Moreover, the insulation monitoring system integrated in the inverters is only permanently effective if the mounting system is connected to earth. The metal substructure is functionally earthed if the PV system is located in the protected volume of the air-termination systems and the separation distance is maintained.

International guidelines require copper conductors, with a cross-section of at least 6 mm2 or equivalent, be used for functional earthing. The mounting rails also have to be permanently interconnected by means of conductors of this cross-section. If the mounting system is directly connected to the external lightning protection system, due to the fact that the separation distance cannot be maintained, these conductors become part of the lightning equipotential bonding system. Consequently, these elements must be capable of carrying lightning currents. The minimum requirement for a lightning protection system designed for class of LPS III is a copper conductor with a cross-section of 16 mm2 or equivalent. Also in this case, the mounting rails must be permanently interconnected by means of conductors of this cross-section. The functional earthing/lightning equipotential bonding conductor should be routed in parallel and as close as possible to the DC and AC cables or lines.

Earthing clamps can be fixed on all common mounting systems. They connect, for example, copper conductors with a cross-section of 6 or 16 mm2 and bare round wires with a diameter from 8 to 10 mm, to the mounting frame in such a way that they can carry lightning currents. The integrated stainless steel (V4A) contact plate ensures corrosion protection for the aluminium mounting systems.

Separation distances as per IEC 62305-3 (EN 62305-3)

A certain separation distance must be maintained between a lightning protection system and a PV system. It defines the distance required to avoid uncontrolled flashover to adjacent metal parts resulting from a lightning strike to the external lightning protection system. In the worst case, such an uncontrolled flashover can set a PV plant/farm on fire. The calculation of the separation distance can be easily and quickly calculated by DEHNconcept.

Core shadows on solar cells

The distance between the solar generator and the external lightning protection system is absolutely essential to prevent excessive shading. Diffuse shadows cast by, for example, overhead lines, do not significantly affect the PV system and the yield. However, in case of core shadows, a dark clearly outlined shadow is cast on the surface behind an object, changing the current flowing through the PV module. For this reason, solar cells and the associated bypass diodes must not be influenced by core shadows. This can be achieved by maintaining a sufficient distance. For example, if an air-termination rod with a diameter of 10 mm shades a module, the core shadow is steadily reduced as the distance from the module increases. After 1,08 m only a diffuse shadow is cast on the module (Fig. 1).

 

Fig. 1: Air termination rod with core shadow

Fig. 1: Air termination rod with core shadow.

 

Special surge protective devices (SPD) for the DC side of photovoltaic systems

The U/I characteristics of photovoltaic current sources are very different from that of conventional DC sources: They have a non-linear characteristic and cause long-term persistence of ignited arcs. This unique nature of PV current sources does not only require larger PV switches and PV fuses, but also a disconnector for the surge  protective device, which is adapted to this unique nature and capable of coping with PV currents.

Selection of SPDs according to the voltage protection level Up

The operating voltage on the DC side of PV systems differs from system to system. At present, values up to 1500 VDC are possible. Consequently, the dielectric strength of terminal equipment also differs. To ensure that the PV system is reliably protected, the voltage protection level Up of the SPD must be lower than the dielectric strength of the PV system it is supposed to protect. The CLC/TS 50539-12 standard requires that Up is at least 20% lower than the dielectric strength of the PV system. Type 1 or 2 SPDs must be energy-coordinated with the input of terminal equipment. If SPDs are already integrated in terminal equipment, coordination between the type 2 SPD and the input circuit of terminal equipment is ensured by the manufacturer.

Application example 1: Building without external lightning protection system

In a building without external lightning protection system, dangerous surges enter the PV system due to inductive coupling resulting from nearby lightning strikes or travel from the power supply system through the service entrance to the consumer’s installation. Type 2 SPDs are to be installed at all of the following locations:

  • DC-side of the modules and inverters
  • AC output of the inverter
  • Main low-voltage distribution board
  • Wired communication interfaces

Every DC input (MPP) of the inverter must be protected by a type 2 surge protective device. European standards require that an additional type 2 DC arrester be installed on the module side if the distance between the inverter input and the PV generator exceeds 10 m.

The AC output of the inverters are sufficiently protected if the distance between the PV inverters and the place of installation of the type 2 arrester at the grid connection point(low-voltage in-feed) is less than 10 m. In case of greater cable lengths, an additional type 2 surge protective device must be installed upstream of the AC input of the inverter.

Moreover, a type 2 surge protective device must be installed downstream of the meter of the low-voltage in-feed.

If inverters are connected to data and sensor lines to monitor the yield, suitable surge protective devices are required.

Application example 2: Building with external lightning protection system and sufficient separation distances (Fig. 2)

In this case, the primary protection goal is to avoid damage to persons and property (building fire) resulting from a lightning strike. Here it is important that the PV system does not interfere with the external lightning protection system. Moreover, the PV system itself must be protected from direct lightning strikes. This means that it must be installed in the protected volume of the external lightning protection system. This protected volume is formed by air-termination systems, such as air-termination rods, which prevent direct lightning strikes to the PV modules and cables. The protective angle method or rolling sphere method may be used to determine this protected volume.

 

Fig. 2: Building with lightning protection

Fig. 2: Lightning protection on a building.

 

A certain separation distance must be maintained between all conductive parts of the PV system and the lightning protection system. In this context, core shadows must be prevented by, for example, maintaining a sufficient distance between the air-termination rods and the PV module.

Lightning equipotential bonding is an integral part of a lightning protection system. It must be implemented for all conductive systems and lines entering the building which may carry lightning currents. This is achieved by directly connecting all metal systems and indirectly connecting all energised systems via type 1 lightning current arresters to the earth-termination system. Lightning equipotential bonding should be implemented as close as possible to the entrance point into the building to prevent partial lightning currents from entering the building. The grid connection point must be protected by a multipole spark-gap-based type 1 SPD. If the cable lengths between the arrester and inverter are less than 10 m, sufficient protection is provided. In case of greater cable lengths, additional type 2 surge protective devices must be installed upstream of the AC input of the inverters.

Every DC input of the inverter must be protected by a type 2 PV arrester. This also applies to transformerless devices. If the inverters are connected to data lines, for example to monitor the yield, surge protective devices must be installed to protect data transmission.

Another possibility to maintain the separation distance is to use high-voltage-resistant, insulated HVI conductors which maintain a separation distance up to 0,9 m in the air. HVI conductors may directly contact the PV system downstream of the sealing end range.

Application example 3: Building with external lightning protection system with insufficient protection distance

If the roofing is made of metal or is formed by the PV system itself, the separation distances cannot be maintained. The metal components of the PV mounting system must be connected to the external lightning protection system in such a way that they can carry lightning currents (copper conductor with a cross-section of at least 16 mm2 or equivalent). This means that lightning equipotential bonding must also be implemented for the PV lines entering the building from the outside.

Lightning equipotential bonding must also be implemented in the low-voltage in-feed. If the PV inverters are situated more than 10 m from the type 1 SPD installed at the grid connection point, an additional type 1 SPD must be installed on the AC side of the inverters. Suitable surge protective devices must also be installedto protect the relevant data lines for yield monitoring.

PV systems with micro-inverters

Micro-inverters require a different surge protection concept. To this end, the DC line of a module or a pair of modules is directly connected to the small-sized inverter. In this process, unnecessary conductor loops must be avoided. Inductive coupling into such small DC structures typically only has a low energetic destruction potential. The extensive cabling of a PV system with micro-inverters is located on the AC side. If the micro-inverters are directly fitted at the module, surge protective devices may only be installed on the AC side.

Conclusion

Solar power generation systems are an integral part of today’ s electrical systems. They should be equipped with adequate lightning current and surge arresters, thus ensuring long-term faultless operation of these sources of electricity.

Contact Alexis Barwise, DEHN Protection, Tel 011 704-1487, alexis.barwise@dehn-africa.com