The remote locations, exposed surface areas and extensive layouts of solar power plants put them at high risk of damage from the elements, particularly electrical storms. A significant concern for photovoltaic (PV) power plant operators is equipment damage caused by direct or indirect lightning strikes. Damage from these events can bring a PV installation offline for days or perhaps weeks, resulting in power interruption and revenue losses. To avoid the destructive effects of lightning strikes, overvoltage protection must be installed at the inverter and at various other locations in the PV facility.

Surge Protection Device Selection and Installation for PV Systems

PV systems have unique characteristics, which therefore require the use of SPDs that are specifically designed for PV systems.

PV systems have high dc system voltages up to 1500 volts. Their maximum power point operates at only a few percentiles below the system’s short circuit current.

To determine the proper SPD module for the PV system and its installation, you must know:

• the lightning round flash density;
• the system’s operating temperature;
• the system’s voltage;
• the system’s short circuit current rating;
• the level of waveform that is to be protected against (indirect or direct lightning); and
• the nominal discharge current

To have a protective effect, an SPD’s voltage protection level (Up) should be 20 % lower than the dielectric strength of the system’s terminal equipment.

It is important to use an SPD with a short circuit withstand current greater than the short circuit current of the solar array string that the SPD is connected to. The SPD that is provided on the dc output must have a dc MCOV equal to or greater than the maximum photovoltaic system voltage of the panel.

When a lightning strikes at point A (see Figure 1), the solar PV panel and the inverter are likely to be damaged. Only the inverter will be damaged if the lightning strikes at point B. However, the inverter is typically the most expensive component within a PV system, which is why it is essential to properly select and install the correct SPD on both the ac and dc lines. The closer the strike is to the inverter, the more damaged the inverter will be.

DC & AC Side SPDs for combiner boxes and inverters

Surge-protection devices are installed on the DC and AC sides of central and string inverters, in combiner boxes and to protect signal lines (measurements and communications). Depending on the atmospheric exposure of the PV plant to lightning, engineering, procurement and construction (EPC) contractors and designers may prescribe either standard type 2 IEC SPDs (indirect impact 8/20 μs) or more robust type 1+2 SPDs (tested to withstand direct lightning impacts in 10/350 μs waveform).

DC side protection at combiner boxes and inverters typically has a Y-connection DIN-rail configuration in order to provide common and differential mode protection.

On the AC side, overvoltage protection of the inverter is likewise required. Whether at the inverter itself or at the AC combiner box, exposure to voltage surges is generally addressed by using standard DIN-rail type 2 SPDs in TN S, IT or even TT configurations.

Moreover, depending on the PV installation, type 1+2 AC may be favored in some cases by EPC contractors and designers, in order to achieve a longer lifetime for SPDs and even protection where conditions are harsher in terms of the expected direct lightning impact. This will typically be the case in areas with a very high isoceraunic level, often associated with high altitudes.