Surge in PV Systems: A Complete Guide to Its Role, Risks, and Selection

# Surge in PV Systems: A Complete Guide to Its Role, Risks, and Selection

Whether it’s a small residential solar panel setup or a commercial photovoltaic (PV) power plant, "surge" is an unavoidable key topic—but many people only realize its importance when equipment malfunctions. This note breaks down the core role of surges in PV systems and how to select a Surge Protective Device (SPD) that suits your system, making it easy for beginners to understand.

## I. First, Understand: What Exactly Is a Surge in a PV System?

Simply put, a surge is a sudden "voltage/current shock wave" in a PV system, with three common sources:  

1. **External Impact**: The most typical is lightning strikes (direct or induced lightning). Discharge from clouds can instantly generate a high voltage of tens of thousands of volts in the lines;  

2. **System Start-Up/Shutdown**: When PV inverters or combiner boxes start up or shut down, sudden changes in current trigger "operational surges";  

3. **Grid Fluctuations**: Sudden increases in grid voltage (e.g., when a grid fault is repaired) can cause reverse impacts on the grid-connected PV system.  

These surges are characterized by being "short but intense"—they may last only a few microseconds, but the voltage can soar to more than 10 times the system’s rated voltage, which ordinary PV modules and inverters cannot withstand.

## II. Core Role of Surge Protective Devices (SPDs): Installing a "Safety Valve" for PV Systems

Surges themselves are not "useful"; what really works is the **Surge Protective Device (SPD, also known as a lightning arrester)**. Its core task is to "block dangerous surges," specifically in three aspects:  

1. **Protect Core Equipment from Breakdown**  

Components in PV inverters, PV panel junction boxes, and combiner boxes have upper limits on voltage withstand capacity (e.g., the DC-side voltage withstand of inverters is usually 1000V-1500V). Once the surge voltage exceeds this limit, components will burn out instantly, with maintenance costs often ranging from thousands to tens of thousands of yuan. SPDs can conduct electricity in the blink of an eye (usually ≤25 nanoseconds) when a surge occurs, diverting excess voltage/current to the ground—equivalent to "blocking bullets" for the equipment.  

2. **Prevent Sudden System Shutdown or Malfunction**  

Even if a surge does not directly burn the equipment, it may interfere with the inverter’s control chip, causing the inverter to falsely report faults and disconnect from the grid. For example, after a thunderstorm, many residential PV systems suddenly stop generating electricity—this is likely due to surges affecting the inverter. Installing the right SPD can reduce such "unwarranted troubles" and ensure the stable power generation of the system.  

3. **Extend the Overall Lifespan of the PV System**  

Frequent small surges (e.g., those caused by daily grid fluctuations) can damage the circuits of modules and inverters "over time," such as accelerating capacitor aging. SPDs can filter out these small surges, indirectly extending the service life of the entire PV system (usually by an additional 3-5 years).

## III. Key Step: How to Select an SPD Suitable for Your PV System?

Selecting an SPD is not about "the bigger the better" or "the cheaper the more cost-effective." It requires focusing on three core parameters of your system and following four steps:  

### Step 1: First, Clarify the System’s "Voltage Level"

This is the most basic prerequisite—the rated voltage of the SPD must match the DC-side and AC-side voltages of the PV system:  

- **Residential PV (usually 3-10kW)**: The DC-side voltage is generally 300V-800V; select an SPD with a rated DC voltage (Uc) ≥800V. The AC side is connected to a 220V grid; select an SPD with a rated AC voltage (Uc) ≥250V.  

- **Commercial/Industrial PV (usually 50kW and above)**: The DC-side voltage may reach 1000V-1500V; the SPD’s Uc should be ≥1500V. The AC side is connected to a 380V three-phase power grid; select an SPD with Uc ≥420V.  

*Note: If the SPD’s rated voltage is lower than the system voltage, it will burn out itself; if it is too high, it cannot activate protection in a timely manner.*

### Step 2: Select "Current-Carrying Capacity" Based on System Power

Current-carrying capacity (Iimp or In) represents the maximum surge current that an SPD can withstand. If it is too small, the SPD will be broken down by the surge; if it is too large, it will be a waste of money:  

- **Residential Systems (3-10kW)**: If there are no tall buildings nearby and the lightning strike probability is low, an SPD with In=20kA (8/20μs waveform) is sufficient; if located in mountainous areas or thunderstorm-prone regions, an SPD with In=40kA is more reliable.  

- **Commercial/Industrial Systems (50kW and above)**: For SPDs at the front end of combiner boxes and inverters, it is recommended to select those with In=40kA-60kA; for large-scale power plants (MW-level), an additional primary SPD with In≥100kA is required on the high-voltage side.  

*Fun Fact: 8/20μs is the most common surge waveform in PV systems, meaning it takes 8 microseconds for the surge current to rise from 0 to its peak and 20 microseconds to drop to half of the peak.*

### Step 3: Check the "Protection Level" and Match the Installation Location

SPDs in PV systems require "hierarchical protection," and different levels of SPDs should be selected for different locations:  

- **Primary Protection (System Inlet)**: For example, the main distribution box of the PV array and the front end of the grid-connected cabinet. Select a "Class B" SPD (capable of withstanding large currents from direct lightning strikes) with a large current-carrying capacity (above 40kA).  

- **Secondary Protection (Equipment Front End)**: For example, the input ends of inverters and combiner boxes. Select a "Class C" SPD (protects against induced lightning and operational surges) with a current-carrying capacity of 20kA-40kA.  

- **Tertiary Protection (Component Front End)**: For example, the internal circuit boards of inverters and monitoring equipment. Select a "Class D" SPD (protects against small surges) with a current-carrying capacity of 10kA-20kA.  

*Residential systems must have at least secondary protection (front end of the inverter + grid-connected cabinet), while commercial systems must be equipped with all three levels of protection.*

### Step 4: Don’t Overlook "Certification and Compatibility"

- **Certification**: Be sure to select SPDs with international or domestic certifications, such as the EU’s CE certification and China’s CQC certification. Avoid buying "three-no products" (many low-quality SPDs fail after a few months of use).  

- **Compatibility**: Pay attention to whether the SPD’s interface type (e.g., terminal block or plug) matches the PV cables. At the same time, confirm that the SPD’s installation size can fit into the distribution box (residential distribution boxes have limited space, so do not buy oversized ones).

## IV. Final Reminder: Correct Installation Is as Important as Correct Selection

1. **Install Close to the Protected Equipment**: The SPD should be installed as close as possible to the protected equipment (e.g., within 1 meter of the inverter’s front end). The shorter the cable, the better the protection effect;  

2. **Reliable Grounding**: The grounding resistance of the SPD’s grounding wire must be ≤4Ω. Poor grounding will prevent surge current from being diverted, making the SPD useless;  

3. **Regular Inspection**: Before the thunderstorm season every year, check the SPD’s indicator light (it should be green under normal conditions; if it turns red or goes out, replacement is required). Residential SPDs are recommended to be replaced every 3-5 years, and commercial ones every 2-3 years.

If your PV system has specific parameters (such as power or installation location), you can add them in the comments, and I can help you refine the selection suggestions! You are also welcome to share any surge-related issues you have encountered, so we can avoid pitfalls together!