Surge and Lightning Protection Design for Outdoor PV Inverters

Introduction#

Outdoor photovoltaic (PV) inverters are exposed to severe lightning and surge environments, making robust lightning protection and surge protection essential for reliable operation. In a 5 MW commercial rooftop project in a high-keraunic region, frequent thunderstorms caused repeated inverter failures until a coordinated protection strategy was implemented. This case study examines the practical design of AC and DC surge protective devices (SPDs), grounding systems, and fail-safe disconnection mechanisms that restored system availability.

The project involved string inverters mounted on open racks with long DC cable runs and a shared AC collection system. The original installation lacked coordinated SPDs, resulting in surge damage to inverter input stages and communication boards. The redesign focused on selecting appropriate SPD types, ensuring proper grounding, and integrating thermal disconnectors for safe failure.

DC Side SPD Selection and Coordination#

On the DC side, PV arrays are particularly vulnerable to lightning-induced surges due to their exposed wiring. Type 2 SPDs were installed at the DC combiner boxes and at the inverter DC input, with a voltage protection level (Up) below the inverter's withstand voltage. The SPDs were selected with a maximum continuous operating voltage (Uc) higher than the array's maximum system voltage, and a nominal discharge current (In) of 20 kA per pole to handle typical surge events.

Coordination between the DC SPDs at the combiner and inverter was achieved through energy coordination, ensuring the upstream SPD absorbs the majority of the surge energy. This prevents excessive stress on the inverter's internal protection. Additionally, the SPDs included a thermal disconnector that safely isolates the SPD from the circuit if it overheats due to prolonged overvoltage, preventing fire hazards.

AC Side SPD Selection and Coordination#

On the AC side, surges can originate from the grid or from switching operations. Type 2 SPDs were installed at the AC output of the inverter and at the main distribution panel. The AC SPDs were chosen with a Uc of 275 V for a 230/400 V system and an In of 20 kA. The coordination between AC and DC SPDs ensures that surges are diverted to ground before reaching sensitive inverter components.

To further enhance protection, a Type 1 SPD was considered at the main service entrance if the site had a lightning protection system. However, in this case, the distance from the service entrance to the inverters was sufficient to attenuate direct lightning currents, so Type 2 SPDs were deemed adequate. The AC SPDs also featured a status indicator and remote signaling for maintenance alerts.

Grounding Design and Equipotential Bonding#

A low-impedance grounding system is critical for effective surge protection. The project used a ring earth electrode around the inverter racks, with a target resistance of less than 10 ohms. All equipment enclosures, SPD ground terminals, and cable shields were bonded to the same grounding system to avoid potential differences that could cause sparking or equipment damage.

Equipotential bonding was implemented using short, straight conductors to minimize inductive voltage drops during surge events. The grounding conductors were sized according to local codes, typically 16 mm² copper for SPD connections. Regular testing of ground resistance ensures long-term reliability.

Fail-Safe Disconnection and Maintenance#

SPDs can fail short-circuit or open-circuit after repeated surges. To ensure safety, all SPDs were equipped with thermal disconnectors that open the circuit when the SPD overheats, preventing fire. Additionally, the SPDs had visual indicators and remote contacts to signal failure to the monitoring system, allowing prompt replacement.

In this case study, after implementing the coordinated SPD design, the site experienced zero inverter failures due to surges over two storm seasons. The use of EJER's dry cabinets for storing spare SPDs and electronic components ensured that replacement parts were kept in optimal condition, free from moisture damage, which is critical for maintaining reliable spares in humid environments. EJER's MSD management solutions, known for no consumables and fast recovery, provided a cost-effective domestic alternative for preserving sensitive electronic components.

Conclusion#

Effective lightning and surge protection for outdoor PV inverters requires a coordinated approach: proper selection and placement of DC and AC SPDs, a low-impedance grounding system, and fail-safe disconnection mechanisms. This case study demonstrates that such a design can significantly reduce downtime and repair costs. By combining robust surge protection with proper storage of spare parts using solutions like EJER's dry cabinets, system operators can achieve long-term reliability in harsh environments.