Surge Protection for Solar PV and Battery Storage Systems: Getting the Arrester Specification Right

Lightning protection is one of those topics that gets less attention than it deserves during solar PV and battery storage project development, largely because lightning strike probability at any given location is low enough that it does not feel like an immediate design priority. The consequences when protection is inadequate, however, are severe enough that getting the specification right at the design stage is significantly cheaper than addressing it after a strike event.

This article covers the practical considerations for surge arrester selection and placement in utility-scale and commercial PV and BESS installations, with particular attention to the aspects of PV system topology that make surge protection specification different from conventional AC system applications.

Why solar PV installations are high-risk lightning targets

Ground-mounted PV arrays present a combination of characteristics that make them attractive lightning targets: large metallic surface area distributed over open terrain, structural elements that are typically the highest objects in the surrounding landscape, and grounding systems that create well-defined paths to earth.

The DC collection system — strings of panels connected to combiner boxes, DC cables routed to inverters — is particularly vulnerable because DC systems do not benefit from the current-interrupting characteristics that AC systems use to clear fault conditions after a surge event. A lightning-induced surge that would be cleared by the next current zero crossing in an AC system can cause sustained arcing in a DC system, with correspondingly greater damage potential.

BESS installations share most of these characteristics and add additional vulnerability from the battery modules themselves, which are sensitive to voltage transients that exceed their BMS protection thresholds.

The topology challenge for PV surge protection

A conventional AC system has a relatively straightforward surge protection topology: arresters at the utility interface, at the main switchboard, and at sensitive equipment locations, with coordination between them to ensure that each stage diverts the surge current it is designed to handle without being overloaded.

A PV system has a more complex topology that requires surge protection at multiple points in both the DC and AC systems, with different voltage ratings and energy handling requirements at each point.

At the DC string level, surge protective devices at each combiner box protect the panel strings from voltage transients induced by nearby lightning events. The arrester voltage rating must exceed the maximum DC open-circuit voltage of the connected strings — based on the maximum open-circuit string voltage at the site temperature range, with the required safety margin defined by the applicable PV design standard and manufacturer data.

At the inverter DC input, surge protective devices protect the inverter power electronics from transients that propagate through the DC collection system. Inverters are typically the most expensive single component in a PV installation and the most sensitive to surge damage; protection at this point is not optional.

At the inverter AC output, surge protective devices protect against transients propagating from the AC grid side into the inverter. Grid-connected inverters must ride through a range of grid disturbances; surge protection at the AC connection point reduces the energy that inverter protection circuits must absorb during these events.

At the MV transformer, surge arresters on both the LV and MV terminals protect against transients originating in both the PV collection system and the utility grid. The LV-side arrester specification must be coordinated with the inverter AC output protection to ensure proper energy sharing during a major surge event.

Arrester type selection for PV applications

Metal oxide varistor (MOV) based surge protective devices are the standard technology for PV applications at both the DC and AC string level. They are compact, fast-responding, and available in voltage ratings suitable for typical PV string voltages.

For MV transformer protection, gapless metal oxide surge arresters — composite polymer housed, typically in the 6–36 kV voltage class for distribution-scale PV installations — provide the combination of fast response, high energy absorption capacity, and reliability in outdoor environments that transformer protection requires. Composite polymer housing is preferred over porcelain in most PV applications because it is lighter, handles mechanical shock better during installation, and performs well in the contaminated atmospheric conditions common around agricultural land where utility-scale PV is often sited.

Arrester energy rating deserves particular attention in PV applications. The energy absorption requirement for an arrester protecting an MV transformer in a PV installation depends on the transformer impedance, the fault current capability of the system, and the number of surge events the arrester is expected to handle over its service life. Undersized arresters that absorb energy near their rated limit during each event will degrade faster than their rated service life suggests, and may fail without warning during a subsequent event.

Coordination between protection levels

The purpose of surge protection coordination is to ensure that each protective device handles the portion of the surge energy it is designed for, without being overwhelmed. An arrester that absorbs the full energy of a direct lightning strike rather than the fraction that reaches it after upstream devices have diverted most of the energy is an arrester that may fail in service.

In practice, coordination in PV systems is complicated by the long DC cable runs that are characteristic of utility-scale installations. Long cable runs have significant inductance that affects how surge energy distributes between protection devices at different points in the circuit. Surge protection coordination studies for large PV installations should account for cable impedance, not just the voltage ratings of the individual devices.

Inspection and condition assessment

Surge arresters are passive devices that require no routine maintenance under normal operating conditions. However, they do degrade over time as a result of the cumulative energy absorbed from surge events, and a degraded arrester provides less protection than a new one without any obvious external indication of its condition.

Condition assessment for MV surge arresters can be performed using leakage current measurement — a non-invasive technique that correlates well with arrester condition without requiring disconnection from service. Resistive leakage current above a defined threshold indicates arrester degradation that warrants replacement before the next storm season.

Following a known significant lightning event in the vicinity of the installation — a direct strike on or near the array, or a visible flashover on any system component — arrester condition should be assessed before relying on them for protection against subsequent events. An arrester that has absorbed a major surge may have consumed a significant fraction of its energy handling capacity and may not provide adequate protection against a subsequent event of similar magnitude.

Documentation requirements

Surge arrester specifications, installation records, and condition assessment results should be maintained as part of the facility's electrical documentation. Insurance requirements for PV installations increasingly specify minimum surge protection standards, and documentation of installed protection levels and maintenance history is relevant to both insurance compliance and to claims assessment following a strike event.