Selection Surge Protection Devices Selection for PV Systems - Types of SPDs

Photovoltaic (PV) power generation is a key source of renewable energy and is highly competitive economically compared to traditional power generation. Small distributed PV systems, such as rooftop solar panels, are becoming increasingly popular. Rooftop PV systems involve both AC and DC distribution with voltages reaching up to 1500V. The DC side, especially the PV panels, can be directly exposed to lightning strikes in high-risk areas, making them vulnerable to lightning damage.

Lightning protection for buildings is divided into external protection (Lightning Protection System, LPS) and internal protection (Surge Protective Measures, SPM), based on lightning risk. Surge Protective Devices (SPDs), as part of the internal protection, safeguard against transient overvoltages caused by atmospheric lightning or switching operations. SPDs are installed outside the protected equipment and mainly function as follows: when there is no surge in the power system, the SPD does not significantly affect the normal operation of the system it protects. When a surge occurs, the SPD offers low impedance, diverting the surge current through itself and limiting the voltage to a safe level. After the surge has passed and any residual current has died out, the SPD returns to a high impedance state.

1.The Installation Location of Surge Protective Devices (SPD)

The installation location of SPDs is determined according to the degree of lightning threat and based on the Lightning Protection Zones (LPZ) concept in IEC 62305. Transient overvoltages are progressively reduced to a safe level, which must be below the withstand voltage of the protected equipment. As illustrated in Figure, SPDs are installed at the boundaries of these zones, giving rise to the concept of multi-level surge protection used in low-voltage systems. For PV systems, the focus is on preventing lightning surges from entering through the AC and DC sides, thereby protecting critical components such as inverters.

Image: https://ecdn6.globalso.com/upload/p/1627/image_other/2025-07/lightning-strike-diagram.png

2. Surge Protective Devices (SPD)Test Classes

According to IEC 61643-11, SPDs are classified into three test categories based on the type of lightning current impulse they are designed to withstand. Type I tests (marked as T1) are intended to simulate partial lightning currents that may be conducted into a building. These use a 10/350 s waveform, as shown in Figure blow, and are typically applied at the boundary between LPZ0 and LPZ1-such as at main distribution boards or low-voltage transformer incomers. SPDs for this level are usually of the voltage-switching type, with components like gas discharge tubes or spark gaps (e.g., horn gaps or graphite gaps).

Type II (T2) and Type III (T3) tests use shorter-duration impulses. Type II SPDs are usually voltage-limiting devices that use components such as metal oxide varistors (MOVs). They are tested with a nominal discharge current using an 8/20 s current waveform (see Figure blow), and are responsible for further limiting the residual surge voltage coming from the upstream protection device. Type III tests use a combination wave generator with a 1.2/50 s voltage and 8/20 s current impulse (see Figure below), simulating surges closer to end-use equipment.

Image: https://ecdn6.globalso.com/upload/p/1627/image_other/2025-07/test- style=

3. Connection Type of Surge Protective Device (SPDs)

There are two main modes of protection against transient overvoltages. The first is common-mode protection (CT1), which is designed to protect against surges between live conductors and PE (protective earth). Lightning strikes, for instance, can introduce high voltages relative to ground into a system. Common-mode protection helps mitigate the impact of such external disturbances, like lightning, as illustrated below.

Image: https://ecdn6.globalso.com/upload/p/1627/image_other/2025-07/tn-c-or-tn-s-systems-t1-spd-4-0-wiring-configuration.png

The second is differential-mode protection (CT2), which safeguards against surges between the line conductor (L) and the neutral conductor (N). This type of protection is especially important for addressing internal disturbances, such as electrical noise or interference generated within the system itself, as shown in the diagram below.

Image: https://ecdn6.globalso.com/upload/p/1627/image_other/2025-07/connection-of-spd-spd-wiring-method-connection-method.png

By implementing one or both of these protection modes, electrical systems can be better shielded from potential surge sources, ultimately enhancing the longevity and reliability of connected equipment.

It's important to note that the selection of SPD protection modes should align with the grounding system in place. For TN systems, both CT1 and CT2 protection modes can be used. However, in TT systems, CT1 can only be applied downstream of an RCD. In IT systems-particularly those without a neutral conductor-CT2 protection is not applicable. This is a critical consideration in DC distribution systems that use IT grounding configurations. Details can be found in the table below.

Image: https://ecdn6.globalso.com/upload/p/1627/image_other/2025-07/grounding-system-selection.png

4. Key Parameters ofSurge Protective Devices (SPD)

According to the international standard IEC 61643-11, the characteristics and tests of SPDs connected to low-voltage power distribution systems are defined, as shown in Figure 7.

(1) Voltage Protection Level (Up)

Image: https://ecdn6.globalso.com/upload/p/1627/image_other/2025-07/wechat-image_20250715103621.png

The most important aspect in selecting an SPD is its voltage protection level (Up), which characterizes the SPD's performance in limiting the voltage between terminals. This value should be higher than the maximum clamping voltage. It is reached when the current flowing through the SPD equals the nominal discharge current In. The selected voltage protection level must be lower than the impulse withstand voltage Uw of the load. In the case of lightning strikes, the voltage across the SPD terminals is generally kept below Up. For PV DC systems, the load usually refers to PV modules and inverters.

(2) Maximum Continuous Operating Voltage (Uc)

Uc is the maximum DC voltage that can be continuously applied to the SPD protection mode. It is selected based on the rated voltage and the system's grounding configuration and serves as the activation threshold of the SPD. For the DC side of PV systems, Uc should be greater than or equal to the PV array's Uoc Max. Uoc Max refers to the highest open-circuit voltage between the live terminals and between the live terminal and ground at the designated point of the PV array.

(3) Nominal Discharge Current (In)

This is the peak value of an 8/20 s waveform current flowing through the SPD, used for Type II tests and for preconditioning tests in Type I and Type II [https://www.leikexing.com/type-2-dc-surge-protection/]. IEC requires that the SPD can withstand at least 19 discharges of 8/20 s waveform current. The higher the In value, the longer the SPD lifespan, but the cost also increases.

(4) Impulse Current (Iimp)

Defined by three parameters: current peak (Ipeak), charge (Q), and specific energy (W/R), this current is used in Type I [https://www.leikexing.com/type-2-dc-surge-protection/] tests. The typical waveform is 10/350 s.

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