Solar and BESS Surge Arrester Selection: MCOV, TOV and AC/DC Coordination

Renewable collector systems combine long outdoor cable runs, inverter switching, transformer interfaces and exposed equipment. Lightning impulses are only one part of the protection problem. Switching events, ground faults and inverter operating conditions can also create temporary overvoltage that a poorly selected arrester may not tolerate.

Start with the circuit, not the catalog title

Separate the AC and DC sides of the project. An AC metal-oxide arrester and a PV DC surge protective device are not interchangeable simply because their voltage numbers look similar. Confirm whether the device is intended for the inverter DC input, inverter AC output, medium-voltage collector circuit or transformer primary.

MCOV is the operating voltage limit that matters

Maximum continuous operating voltage must be higher than the highest continuous voltage the arrester will see, including the actual system tolerance and grounding arrangement. Selecting an MCOV too close to nominal voltage can create unnecessary leakage current and thermal stress. Selecting one excessively high can reduce protective margin. The correct value comes from the system voltage, neutral treatment, phase-to-ground voltage and manufacturer application data.

Do not ignore temporary overvoltage

Temporary overvoltage may last much longer than a lightning impulse. Review the expected duration and magnitude of line-to-ground faults, ferroresonance risk, inverter islanding behavior and transformer connection. The arrester’s TOV withstand curve should be checked against the project study, not replaced by a generic rule of thumb.

Coordinate the protection path

Protection works as a system. Verify lead length, grounding conductor routing, upstream backup protection and the distance between successive protection stages. Long conductors add inductive voltage during a surge, so a device with an impressive nominal discharge current can still underperform if the installation path is poorly arranged.

Questions to put in the project submittal

• Is the device rated for the correct AC or DC circuit?
• What are the continuous operating voltage and TOV withstand values?
• Which IEC or IEEE test classification applies?
• What is the short-circuit current rating and backup fuse requirement?
• How is end-of-life indication connected to the plant alarm system?
• Are the grounding and conductor lengths shown on the installation drawing?

For BESS sites, add the battery enclosure, PCS, transformer and collector feeder to the same protection review. A surge arrester protects a defined circuit; it does not compensate for an uncoordinated grounding design.