Surge Protection in PV Systems: Functions and Selection Guide

The Role of Surges in PV Systems

A surge, defined as an instantaneous and significant voltage fluctuation, plays a crucial role in photovoltaic (PV) systems. Since most PV systems are installed outdoors and exposed to the natural environment for long periods, they are highly susceptible to surges caused by factors such as lightning strikes, grid voltage fluctuations, and internal system faults. The primary function of a Surge Protective Device (SPD) is to withstand these surge voltages and ensure the safe and stable operation of the PV system.

Protection Principle

Surge protectors typically incorporate key components like Metal Oxide Varistors (MOVs). Under normal voltage conditions, an MOV exhibits a high-impedance state, allowing almost no current to pass through and thus having no impact on system operation. However, when a surge voltage occurs and exceeds the MOV's trigger threshold, the MOV's resistance drops sharply and instantaneously, switching to a low-impedance state. This creates a low-resistance path through which the surge current is quickly diverted to the ground, preventing high voltages from damaging sensitive electronic components within the PV system. It acts like a robust barrier, shielding the system from surge damage.

Protecting Critical Equipment

In PV systems, expensive and operationally vital equipment such as inverters, PV panels, and controllers are highly dependent on stable voltage. Surge protectors directly safeguard these critical devices from damage caused by surge voltages. For instance, inverters—core components that convert direct current (DC) to alternating current (AC)—have strict requirements for voltage stability. Surge protectors effectively prevent inverter failures due to surge impacts, ensuring their normal operation and thus guaranteeing the power conversion and output of the entire PV system.

Ensuring Continuous System Operation

The installation of surge protectors significantly reduces equipment damage and downtime caused by surges, thereby enhancing the PV system's power generation efficiency and return on investment. Without surge protection, surge-induced equipment damage would require repair or replacement, resulting in high maintenance costs and prolonged system downtime. This downtime leads to lost power generation and reduced revenue. Surge protectors mitigate these risks, ensuring the system generates power continuously and stably, providing users with a reliable power supply and economic benefits.

Fire and Explosion Prevention

On the DC side of PV systems, surge voltages can cause arcing—a highly dangerous phenomenon that easily triggers fires and explosions. Surge protectors suppress this arcing risk, reducing fire hazards, ensuring the safe operation of the PV system, and protecting personnel and the surrounding environment.

How to Select the Right Surge Protector

Determine the Protection Level

Level 1 Protection (Class B): Primarily used in the main distribution cabinets of buildings to withstand direct lightning strikes and strong lightning electromagnetic pulses. In large-scale PV systems located in areas with frequent lightning activity, a Level 1 surge protector can be installed at the system's main incoming line. This provides the first line of defense, preventing powerful lightning energy from directly infiltrating the system.

Level 2 Protection (Class C): Suitable for distribution cabinets, it mainly protects against lightning-induced overvoltages and switching operation overvoltages. In PV systems, Class C SPDs can be installed at the input and output terminals of inverters, as well as in combiner boxes. They further reduce residual surge voltages after Level 1 protection, shielding downstream equipment from induced and operational overvoltages.

Level 3 Protection (Class D): Typically installed before end-use equipment to protect against residual lightning-induced overvoltages and switching operation overvoltages. In PV systems, for small voltage-sensitive devices such as monitoring modules and data loggers, a Level 3 surge protector can be installed at their front end to provide more precise protection and ensure stable operation.

Match the Voltage Level

The maximum continuous operating voltage (Uc) of the surge protector must match the rated voltage of the PV system. For example, a common 1000V PV system requires a surge protector with a Uc of no less than 1000V. If the selected SPD has an excessively low voltage rating, it may be damaged during normal operation due to inability to withstand the system voltage. Conversely, an excessively high voltage rating may prevent the SPD from activating promptly during a surge, rendering it ineffective. Accurately matching the voltage level is therefore a critical step in SPD selection.

Consider the Current-Carrying Capacity

The current-carrying capacity (Imax) refers to the maximum lightning current that a surge protector can withstand. In practical applications, the SPD's Imax should be slightly higher than the expected lightning current, based on local lightning activity and the potential surge intensity the system may encounter. In areas with frequent lightning, SPDs with larger current-carrying capacities are required to ensure they can reliably divert current to the ground during strong surges without failing due to overload, thus guaranteeing the SPD's reliability and longevity.

Pay Attention to the Response Time

The response time of a surge protector should be as short as possible, ideally in the nanosecond range. Only with a sufficiently fast response time can the SPD conduct quickly the moment lightning current arrives, diverting the surge current to the ground promptly and protecting equipment from excessive voltage damage. A delayed response would mean the SPD activates only after the surge voltage has already damaged the equipment, failing to fulfill its protective function.

Adapt to Installation Location and Method

Centralized PV Systems: Surge protectors can be installed at both the input and output terminals of inverters. Installing an SPD at the inverter input protects the inverter from surges originating from the PV panel side, while an SPD at the output terminal shields grid-side equipment from surges generated by the inverter. Additionally, suitable surge protectors should be installed in the combiner boxes of large centralized PV power plants to ensure electrical safety during the current combining process.

Distributed PV Systems: Due to their smaller scale and scattered distribution, distributed PV systems can have surge protectors installed in the combiner box of each PV array to protect individual power-generation units. If a distributed PV system is connected to the internal power grid of a building, a corresponding surge protector must also be installed at the building's distribution box to prevent surge voltages from spreading to other electrical equipment via the grid.