How to Select SPD for PV Solar Systems (IEC 61643 Guide)

The long-term investment in solar PV systems provides decades of clean, reliable power. One of the most frequently overlooked yet most damaging threats to solar PV systems has been surge damage caused by lightning or grid disturbances. Surge events can lead to expensive repairs, unplanned downtime, and, in some cases, the invalidation of warranties on critical equipment when not properly protected.

Surge Protection Devices (SPDs) will be the first line of defense in securing the photovoltaic (PV) systems against surge damage. When the appropriate SPD is correctly selected and installed, the PV system, including the solar panels, inverters, and batteries, will be protected from damage and fulfil all applicable standards, including the IEC 61643-11. Here is a summary of all you need to know about SPDs, including how to reduce risk, increase return on investment, and ensure reliable operation of your photovoltaic (PV) system.

SPDs: Essential for Protection of Photovoltaic (PV) Systems

Photovoltaic systems experience both direct current (DC) high voltages on the solar side and alternating current (AC) high voltages on the grid-tie side. Consequently, photovoltaic systems are very prone to experiencing electrical surges. The main types of surges affecting photovoltaic systems include:

• Direct or Indirect Lightning Strokes: Even when lightning strikes are not touching or near photovoltaic arrays, the electric fields generated can cause voltage transient surges on the photovoltaic arrays that could result in high-voltage transients traveling down the DC cables directly to the inverter.
• Grid Switching & Load Transients: Also from power surges on the utility power grid or large electrical devices being switched on or off, photovoltaic systems are at risk for experiencing high-level electrical energy that may cause damage to the DC equipment and the utility grid connected to the inverter.

Surge protection devices (SPDs) provide protection by redirecting electrical transient energy to the earth, thus preventing the hazardous electrical transients from reaching your equipment and causing damage.

IEC 61643-11: International Standard for Photovoltaic Surge Protective Devices (SPDs)

When selecting a surge protective device (SPD) for any PV system, ensuring compliance with the IEC 61643-11 standard is very important. The IEC 61643-11 standard specifies the performance and safety requirements for surge protective devices (SPDs) installed in low-voltage distribution systems. As you determine which type of surge protection device (SPD) would be best for your system, be sure to review each of the following parameters:

• Voltage Protection Level (Up): The maximum allowable voltage to pass through to the connected device from the SPD itself. To protect sensitive devices, such as inverters, the Up should be as low as possible (ideal is 2.5 kV or lower).
• Nominal Discharge Current (In): The maximum allowable discharge current to be passed by an SPD for multiple surges. The higher the nominal current, the more resilient the SPD.
• Maximum Continuous Operating Voltage (Uc): The maximum continuous operating voltage of an SPD should be able to withstand the maximum open circuit voltage of the system itself (e.g., Voc).
• Type of SPD: SPDs are classified as Types 1, 2, or 3 based on the type of electrical system being protected and the amount of protection needed.

Choosing an SPD that has been tested and approved to IEC 61643-11 ensures that the SPD has been tested in real-world conditions for its performance, which is extremely important for commercial and industrial solar PV systems.

Step 1: Assess the AC and DC Surge Risks of Your PV System

Two zones of vulnerability exist for PV systems, and an appropriate solution is needed for each.

DC Side (From Solar Array to Inverter): PV arrays are exposed to the elements, especially being susceptible to both direct and indirect lightning strikes. A DC SPD must have a maximum Voc rating that matches the Voc rating of the array and be able to withstand high-energy discharge and outdoor environments.

AC Side (From Inverter to Grid): Due to grid swells and lightning strikes, the section of the system between the inverter and grid can also be exposed to surges. Coordination with upstream devices such as Miniature Circuit Breakers (MCBs) is critical, along with matching AC surge protective devices (SPDs) with the nominal voltage of the grid.

Step 2: Selecting the Right Type of SPD

Three primary types of SPDs exist to provide the best degree of surge protection at different points within an electrical installation:

SPDs: For most grid-tied PV systems, you should install at least a Type 2 SPD on both the AC and DC sides. In high to medium-risk areas, you should consider adding a Type 1 SPD at the service entrance of the PV system installation.

Step 3: Technical Specifications to Consider

When evaluating which SPD surge protection device (SPD) to choose for your solar PV system, you need to pay close attention to these specifications:

1. For Surge Protection of DC Systems

• Maximum Continuous Operating Voltage (Uc): The Uc must equal or exceed the highest DC open-circuit voltage (Voc) of the given system; therefore, if you have a 1,000-V DC system, you will need an SPD rated for at least 1,200 V DC.
• Polarity: The SPD must be polarized to match the equipment/system (i.e., the positive and negative terminals must align).
• Response Time: Choose SPDs with a response time of < 25 ns for DC surges.

2. For AC Surge Protection

• Nominal Voltage (Un): Match this to your grid voltage—e.g., 230/400V AC.
• Short-Circuit Withstand Rating (Iscw): Ensure it aligns with the Available Fault Current (AFC).
• Coordination with MCBs: Ensure compatibility between SPD and upstream MCBs.

3. Environmental Considerations

• Ingress protection: IP65 or higher for outdoor installations.
• Temperature ratings: Operates within -40°C to +85°C.
• Durable housing: Resistant to UV and weather damage.

Step 4: Avoid Common SPD Selection Mistakes

As a PV system owner/operator, you can make a small investment in purchasing the appropriate SPDs that will help ensure your long-term success. Here are five common mistakes that can be made when selecting an SPD for your PV system:

• Using the Wrong Voltage Rating: Be sure to verify that the voltage rating for the SPD matches the Voc of the system.
• Not Clarifying Coordination with an MCB: Verify that the SPD and MCB have been tested together to avoid unnecessary risks.
• Not Installing an SPD on the DC Side of the System: The PV array is very susceptible to surges; DC SPDs should be your number one installation priority.
• Using Uncertified Devices: Avoid untested devices that are not compliant with IEC 61643.
• Failing to Install a Grounding System: Ensure that the SPD is connected to a low impedance grounding system.

Final Checklist Before Purchasing an SPD for Your Solar PV System

Before purchasing an SPD for your solar PV system, be sure to confirm that it meets the following criteria:

• The SPD must be compliant with IEC 61643-11.
• The SPD must have the same system voltage rating for both the DC and AC sides of your PV system.
• The SPD must have the appropriate type rating (Types 1, 2, or 3) relative to how your system uses protection.
• The SPD must be properly coordinated with the upstream MCBs, making sure that they provide maximum safety and reliability.
• The SPD is rated for outdoor, harsh environments with a minimum IP65 rating.
• The SPD has a Low Voltage Protection Level (Up) low enough to protect sensitive items, such as inverters.
• The SPD is certified by a reputable 3rd party laboratory (e.g., TÜV or UL).

Conclusion

Protecting your solar PV system with the appropriate SPD is more than a compliance issue; it is a smart investment for a reliable long-term solution. By following this guide and understanding the various components of the SPD selection, you will ensure your PV system is protected against surges, reduce your risk of equipment failure, and improve the amount of energy produced from your PV installation.

If you are looking to protect your investment or get informed advice on the best SPD for your PV installation, then please contact our team. An appropriate SPD will ensure that your PV system will provide you with dependable and clean energy for many years to come.