Due to the unique electrical configuration on the DC side of photovoltaic (PV) systems, only surge protective devices (SPDs) that are specifically designed for PV applications should be used to protect this part of the system.
1. Selection of the Voltage Protection Level (Up) for DC-side SPDs
To make sure the connected equipment is properly protected, the SPD's voltage protection level (Up) must be lower than the impulse withstand voltage (Uw) of the equipment. In most cases, the Up value should not be more than 80% of the Uw. This gives at least a 20% safety margin. You can find more details in IEC 61643-12 and IEC 62305-4.
It's also important to route the PE (Protective Earth) conductor close to the DC conductors. This helps reduce loop impedance and limits the risk of overvoltage.
If there is no specific data available about the equipment’s Uw, you can refer to the recommended values in Table 1. Please note that these reference values might differ slightly from the ones given in GB/T 16895.32-2021, which is the Chinese standard for solar PV power supply installations.
Table 1 – Rated Impulse Withstand Voltage (Uw) of Equipment Between PV Array and Inverter (excluding other influencing factors)
1. Selection of Nominal Discharge Current (In) and Impulse Discharge Current (Iimp) for DC-side SPDs
Choosing the right SPD ratings depends on the lightning protection level and the wiring layout of the PV DC side. SPDs can be used in different configurations. You can use individual protection modes as shown in Figures 1 and 2, or go with a multi-pole SPD. Inside the SPD, the protective components may be voltage-limiting, voltage-switching, or a mix of both.
A typical example is a system with a non-isolated external lightning protection system (LPS) and an ungrounded Y-connection on the DC side, as shown in Figure 3. According to IEC test results and simulations, the SPD requirements will vary depending on the type used. Under a lightning current of I₁₀/₃₅₀, if you use a Type I voltage-switching SPD, the required values are shown in Table 2. If you use a combined SPD that includes both voltage-switching and voltage-limiting components in parallel, the values are listed in Table 3. For a rooftop PV system that has an external LPS with at least four down conductors and follows Class III lightning protection standards, the SPD should have an Iimp of at least 12.5 kA if it's a voltage-switching type. If you're using a combined SPD, the required Iimp can be lower—at least 10 kA. Another option is to use a combination SPD with one path rated at Iimp ≥ 5 kA and the other at Iimp ≥ 10 kA.
