How to Size a DC SPD for a Solar PV Array

Introduction: The Cost of Getting it Wrong

Imagine this: a mid-sized solar farm, operational for just two years, suddenly goes dark. The operations team scrambles, and after hours of diagnostics, they find the culprit. It wasn’t a panel failure or a software glitch. It was a catastrophic failure of three central inverters, the heart of the operation. The root cause? A powerful surge from a nearby lightning strike, which the undersized and improperly installed surge protection devices (SPDs) were completely unable to handle. The result was over $100,000 in replacement hardware and a week of lost generation revenue, a costly lesson in the importance of a component that represents a fraction of the system’s total cost.

This scenario is not hypothetical; it’s a reality that plays out in solar installations worldwide. As highlighted in industry analyses, electrical events, including surges, are a leading cause of equipment failure and system downtime . A DC Surge Protection Device is not merely a checkbox item on a bill of materials; it is an essential insurance policy for the long-term health and financial return of your solar asset. For both the engineer designing the system and the manager procuring the parts, understanding how to correctly size, select, and place these devices is a non-negotiable aspect of responsible system design. This guide provides a definitive, step-by-step process for getting it right.

Part 1: The Unforgiving Nature of DC Surges

Solar PV systems are uniquely vulnerable to surge damage. Their large, exposed arrays act as massive antennas for lightning, while long DC cable runs can induce powerful transient overvoltages during a storm . However, the most critical factor to understand is the fundamental difference between AC and DC power.

In an AC system, the current naturally crosses zero 100 or 120 times per second. This zero-crossing provides a brief window where an electrical arc can extinguish itself. A DC system has no such zero-crossing. Once a DC arc is initiated—for example, inside a failing protective device—it can sustain itself, releasing immense thermal energy and creating a serious fire hazard.

This is why an AC-rated SPD must never be used in a DC application. As expert guides on the topic emphasize, AC SPDs lack the specialized arc-extinguishing mechanisms required to safely interrupt a DC fault current, leading to catastrophic failure . A proper DC SPD is engineered to handle the relentless nature of direct current. Its core function can be compared to a pressure relief valve; it remains passive during normal operation but opens in nanoseconds when it detects a dangerous voltage spike, safely diverting the damaging surge current to ground. This action “clamps” the system voltage at a safe level, protecting the sensitive electronics within the inverter and other components.

Part 2: Step-by-Step Sizing Calculations

Sizing a DC SPD is a precise engineering task. It requires a systematic approach based on the electrical characteristics of your PV array and the environmental conditions of the site. Following the methodology outlined in standards like IEC 61643-32 ensures a safe and effective protection scheme .

Step 1: Calculate Maximum Continuous Operating Voltage (MCOV / Uc)

The MCOV (often denoted as Uc or Ucpv) is the most critical parameter. It defines the maximum DC voltage the SPD can withstand continuously without activating. If the MCOV is too low, the SPD will see normal system voltage fluctuations as a fault, leading to premature wear and failure. If it’s too high, its protective performance will be compromised.

The MCOV must be higher than the maximum possible open-circuit voltage (Voc) of the solar array. This isn’t the Voc under standard test conditions (STC), but the Voc under the coldest expected temperatures at the site, as voltage increases as temperature drops.

The formula is: MCOV ≥ 1.25 × Voc(max)

Where:

• Voc(max) is the maximum string voltage, adjusted for the lowest historical temperature at the installation site.
• Le 1.25 factor is a crucial safety margin to account for voltage fluctuations and manufacturing tolerances.

Let’s walk through an example:

• System: A string of 20 PV modules.
• Module Specs: Voc at STC = 41.5V; Temperature coefficient of Voc = -0.28%/°C.
• Site Condition: Lowest expected ambient temperature = -10°C.
• Max String Voc at STC: 20 modules × 41.5V = 830V.

First, calculate the temperature-corrected Voc for a single module:
Voc(-10°C) = 41.5V × (1 + (-0.0028/°C) × (-10°C - 25°C))
Voc(-10°C) = 41.5V × (1 + 0.098) = 45.58V

Next, find the maximum string voltage:
Voc(max) = 20 modules × 45.58V = 911.6V

Finally, determine the required MCOV for the SPD:
MCOV ≥ 1.25 × 911.6V = 1139.5V

In this scenario, you would select a DC SPD with the next standard MCOV rating that is greater than or equal to this value, such as a 1200V or 1500V model .

Step 2: Determine Voltage Protection Level (Up)

The Voltage Protection Level (Up) indicates the residual voltage that will pass through the SPD and reach your equipment during a surge event. It is a measure of how well the SPD clamps the voltage.

The rule is simple: the SPD’s Up must be lower than the equipment’s impulse withstand voltage (Uw).

As a best practice, a safety margin of at least 20% is recommended (Up ≤ 0.8 × Uw). Inverters and other PV electronics typically have a Uw of around 2.5kV to 4kV. If an inverter has a Uw of 2.5kV, you would need an SPD with a Haut de la page well below 2.5kV, ideally less than or equal to 2.0kV.

Bold Takeaway: When comparing two otherwise equal SPDs, the one with the lower Haut de la page value offers better protection.

Step 3: Specify Discharge Current Capacity (In, Imax, Iimp)

This parameter defines the SPD’s robustness and lifespan.

• Nominal Discharge Current (In): The peak current an SPD can handle for a set number of surges (typically 15-20) without failing. This is a measure of durability. A common En rating for Type 2 SPDs used in PV systems is 20kA .
• Maximum Discharge Current (Imax): The maximum single surge current an SPD can handle one time without being destroyed. This is a measure of robustness. It is typically twice the En value (e.g., 40kA).
• Impulse Discharge Current (Iimp): This rating is specific to Type 1 SPDs and tests their ability to handle a portion of a direct lightning strike, using a more severe 10/350µs waveform.

The required ratings depend on the location’s lightning exposure risk and the type of SPD being used. For most DC-side applications at the inverter or combiner box, a Type 2 SPD with In = 20kA and Imax = 40kA is a standard and reliable choice .

A simplified workflow for sizing a DC SPD based on key system and site parameters.

Part 3: Choosing the Right Technology

Beyond the electrical ratings, the internal technology of the SPD matters. The two primary technologies used are Metal Oxide Varistors (MOV) and Gas Discharge Tubes (GDT), with many modern SPDs using a hybrid approach.

Comparison 1: Type 1 vs. Type 2 SPDs

The most fundamental choice is between a Type 1 and Type 2 SPD, which determines its application and robustness.

For the DC side of a typical solar installation, Type 2 SPDs are the standard choice for installation in combiner boxes and at the inverter DC input  A Type 1 device may be required at the main DC aggregator if the site has an external lightning rod system 

Comparison 2: MOV vs. GDT Technology

Bold Takeaway: Many high-performance DC SPDs are hybrid devices. They combine an MOV for its fast response and a GDT for its high-energy handling and isolation capabilities. This leverages the strengths of both technologies to provide superior protection .

Part 4: Strategic Installation: Where and How

A perfectly sized SPD is useless if installed incorrectly. Placement and wiring are just as critical as the device specifications. A “cascading” or layered protection strategy is best practice, installing SPDs at key transitions in the system .

For the DC side, the two most critical locations are:

1. In the string combiner box.
2. At the DC input of the central/string inverter.

This layered approach is guided by the “<10 Meter Rule,” a widely adopted industry standard. This rule states that if the length of the DC cable between the SPD and the equipment it is protecting (e.g., between the combiner box and the inverter) is greater than 10 meters (about 33 feet), a second SPD should be installed at the equipment end . This is because long cable runs have higher inductance, which can lead to large induced voltages during a surge, nullifying the protection of a distant SPD.

Furthermore, lead length is paramount. The wires connecting the SPD to the positive, negative, and ground terminals must be as short and straight as possible. Every inch of wire adds inductance, which increases the effective Haut de la page of the device. Long, looping wires can easily add enough voltage to render the SPD ineffective.

A diagram showing recommended SPD placement on the DC and AC sides of a PV system, incorporating the 10-meter rule.

Part 5: Maintenance and Troubleshooting

DC SPDs are designed to be sacrificial devices. They absorb damaging energy to protect more valuable equipment. Most modern SPDs feature a simple visual status indicator, often a small window on the front of the device.

• Green: The SPD is healthy and providing protection.
• Red: The SPD has reached its end-of-life and is no longer providing protection.

Bold Takeaway: A red indicator means the internal protective components (like the MOV) have disconnected due to degradation or a major surge event. The device has done its job and must be replaced immediately to restore protection.

These indicators should be checked as part of any routine operations and maintenance (O&M) visit. Many SPDs feature pluggable modules, allowing for quick and easy replacement without needing to rewire the entire base unit .

Part 6: Frequently Asked Questions (FAQ)

1. My inverter has built-in SPDs. Do I still need external ones?
Yes. While built-in SPDs offer a good baseline, they are often a final, low-level stage of protection. External SPDs installed in combiner boxes act as the primary, more robust first line of defense, absorbing the bulk of a surge before it ever reaches the inverter .

2. How many SPDs do I need for my system?
It depends on the system’s layout and size. At a minimum, you need one at the main DC combiner/inverter input. For larger systems with multiple string combiner boxes and cable runs over 10 meters, you will need additional SPDs at each box and again at the central inverter, following the cascading protection principle.

3. What happens if I use an AC SPD on the DC side?
It will fail, likely in a catastrophic and hazardous manner. It lacks the ability to extinguish a DC arc, which can lead to the device overheating and catching fire when it attempts to operate .

4. What does the MCOV (Uc) rating really mean?
It is the maximum continuous DC voltage the SPD can handle without conducting. Selecting an MCOV that is at least 1.25 times the array’s maximum cold-weather voltage is critical to prevent nuisance tripping and premature failure .

5. Why is the 10-meter rule so important?
Long cables have high inductance. During a fast-rising surge, this inductance creates a significant voltage drop along the cable, which adds to the SPD’s clamping voltage. If the cable is too long, this added voltage can be enough to damage the equipment you’re trying to protect .

6. Should I choose an SPD with the highest Imax rating?
Not necessarily. While a high Imax indicates robustness, the Nominal Discharge Current (In) is a better indicator of durability and lifespan. For most PV applications, a Type 2 SPD with In=20kA / Imax=40kA is a well-balanced and standard choice.

7. Does the grounding system affect my SPD choice?
Absolutely. The SPD diverts surge current to ground, so a low-impedance grounding system is essential for it to work effectively. The system’s grounding configuration (e.g., positive or negative ground, floating) also dictates the specific SPD connection scheme needed .

8. What certifications should I look for?
Ensure the SPD is certified to relevant standards. For PV applications, look for compliance with IEC 61643-31 or UL 1449. These certifications ensure the device has been tested for safety and performance in solar-specific scenarios .

Conclusion: A Critical Investment

Sizing and selecting a DC SPD is not a trivial task. It is a systematic process that balances electrical parameters, environmental conditions, and strategic placement. As we’ve seen, the key takeaways are clear:

• Calculate MCOV with a temperature-corrected Voc(max) and a safety factor.
• Choose a Haut de la page value well below your equipment’s withstand voltage.
• Use a cascading strategy, respecting the 10-meter rule.
• Keep connection leads as short as possible.
• Routinely inspect the status indicators.

The initial cost of a high-quality, properly specified DC SPD is minuscule compared to the cost of a replacement inverter and the associated generation losses. By treating surge protection as the critical investment it is, you safeguard the operational integrity and financial viability of your solar project for decades to come.