Lightning and Overvoltage Protection

Highly excessive voltages and currents can threaten the operation of a PV plant. Such surges are mainly caused by lightning strikes, but also by faults in the grid.

In principal, a PV plant does not generally increase the risk of a building being struck by lightning. A separate lightning protection system does not necessarily need to be constructed simply because a PV plant has been installed. Nevertheless, VdS (the German institute for fire protection and security) recommends installing a lightning and overvoltage protection system for all plants with a capacity of ten kilowatts or more. In a given case, the risks should be assessed in order to enable a decision in favor of or against the construction of a lightning and overvoltage protection system. If the building on which the PV plant is constructed is already equipped with a lightning protection system (e.g. a public building), the PV plant must be integrated into the protection concept.

The standard DIN EN 62305 (VDE 0185-305):2006-10 provides a comprehensive approach to internal and external lightning protection for buildings and systems. In particular, the supplementary sheets to this European standard offer practical support when deciding whether or not to install a lightning protection system, as well as details on how to install such systems properly. Photovoltaic installations are primarily discussed in Supplement 5 "Lightning and surge protection for PV power supply systems". 

External lightning protection includes all measures for arresting lightning and conducting it to ground, and consists of a lightning current arrester, a down lead capable of carrying lightning and a grounding system which distributes the lightning current in the earth. 

Priority must be given to preventing the lightning from directly hitting the modules. This is first and foremost necessary when the PV generator has been installed in an exposed area (elevated on a flat roof, for example). Rods or wires are used as lightning current arresters, and the core shadow of these should not be cast on the modules as far as this is possible. Somewhat smaller air terminal rods are, therefore, placed in front of the solar modules and somewhat larger ones are placed behind the modules. The exact number and spacing of the air terminal rods is given by the class of protection desired and is calculated using methods such as the "rolling sphere method".

Indirect effects

The probability of indirect lightning effects occurring is significantly higher than that of a direct lightning strike. This is because every lightning strike within a one kilometer radius can generate current flow in the modules, module cables and in the main DC cable by means of induction. Conductive and capacitive coupling are also possible and can equally cause overvoltage. 

An integrated lightning protection system comprising measures and equipment within the PV plant and in the building is, therefore, required. Its fundamental purpose is to prevent inductive coupling and provide a path to earth for currents caused by overvoltage. 

In order to keep coupling in the module cables to a minimum, the area of the open conductor loops in the generator circuit must be as small as possible. The outgoing and return lines of the strings are, therefore, laid as close as possible to each other. The use of shielded single lines also reduces the risk of lightning effects. 

Surge protection devices (SPD) not only prevent inductive coupling but also the occurrence of grid-side overvoltage, and are normally built into the generator junction box. Because varistors used as voltage dependent resistors can age due to leakage currents, the combination of two varistors and a spark discharger in Y connection is considered the safest long term protection against overvoltage.

Reverse current and electric arcs

Increased currents can also occur if there is a voltage drop in a string, caused for example by shading or a short circuit. If this happens, the parallel-connected strings will function like an external power source which drives a fault current in the direction of consumption (reverse current) through the modules of the defective string. If the reverse current resistance of the modules is exceeded they will start to heat up, so string diodes are used to prevent such reverse currents. Many PV plants today are, however, built without string diodes, as most modules now have higher reverse current resistance and will easily withstand reverse current of 10 to 20 A. 

Since direct current and DC voltage are generated in a PV plant, there is a danger that non-self-extinguishing arcs could be created, which could cause fire. This danger is not present in an alternating current circuit because the regular zero crossing of the alternating current’s sine curve immediately extinguishes any electric arc created. The electrical connections in the DC circuit of a PV plant must, therefore, be extremely secure, because a loose connection can lead to sparking and, consequently, trigger an electric arc. As a result, when laying the DC cables of a PV plant it is standard to protect them from short circuit and ground leakages. This is achieved by tidy cable routing (e.g. not running unprotected over sharp edges) and the use of separate positive and negative cables, as well as double cable insulation. The DC cables used should be tested to "PV1-F" standards and marked accordingly. 

String fuses in the GJB can also generally prevent the cables from becoming overloaded in the event of faults. These are intended to reduce the risk of electric arcs.