Preventing fire hazards and damage in a photovoltaic plant through ground-fault detector interrupter (GFDI)

ABB S800 High Performance Circuit Breaker protects photovoltaic solar systems from ground faults

The energy source in photovoltaic (PV) solar generation systems is the sun, which of course can’t be switched off. When daylight falls on a PV panel, it is converted into direct current (DC).

This DC is then converted through the inverter into alternating current (AC), which can be fed into our standard AC power grids. In addition, a growing number of PV panels today are being made with built-in microinverters, enabling them to produce AC power directly.

A PV power system must be properly grounded to ensure its safety. If proper grounding methods or hardware are not used, ground faults may occur and, among the numerous possible faults, they are reported to be one of the major reasons behind catastrophic failures resulting in electrical fires. That’s why ground-fault protection is a blind spot and a major safety concern in many PV solar plants.

Usually, a PV array has several exposed noncurrent carrying conducting parts that do not carry any current during normal operation (module frames, mounting racks, metal enclosures, distribution panels, the chassis of end-use appliances and power converters). There is a potential risk of electric shock hazard from these conductors when an electrical connection is established between the current carrying conductors and noncurrent carrying conductors due to a fault (e.g., corrosion, loss or melting of insulation, wire cutoff and wrong wiring). Therefore, all of these conductors are connected together to the ground or earth through a conductor (equipment grounding conductor – EGC).

USA National Electrical Code (NEC), Article 690.43, requires equipment grounding to protect people and animals from being electrocuted. Similarly, any accidental connection between a current carrying conductor and EGC/earth can cause significant current flow to the ground circuit (ground fault). For this reason, proper grounding is required for any electrical system to provide adequate personnel and system safety in case of one or multiple ground faults.

Wired in different ways
PV systems are wired in different ways to handle system grounding. The DC side of the system may be directly grounded (e.g., one pole connected to earth) or ungrounded, or the array may be grounded through a connection to the AC side ground (see Fig. 1).

Systems also may or may not have galvanic isolation between the DC and AC sides. These design factors influence a system’s fault tolerance and response to ground faults, and add complexity to properly implementing ground fault protection.

A ground-fault detector interrupter (GFDI) such as the new ABB S804U-PVS5 high performance circuit breaker (HPCB) is a safety device that is specially designed for PV solar arrays. A ground fault at the PV generator will trigger the GFDI, interrupting the leakage and preventing damage to the system.

For grounded systems
A GFDI is used in grounded PV systems, which means the manufacturers of PV modules recommend or require positive or negative grounding of the PV generator when using thin-film and back-contact PV modules.

This type of system is commonly used in the USA, although these requirements can basically come from any country in the world where thin-film PV modules are used. In the USA, the GFDI is also referred to as a GFCI – ground-fault current interrupter.

In utility scale projects, the GFDI device is normally installed in the central inverter between the negative and the grounding. GFDIs are also used close to the inverter in residential and commercial PV systems, mainly in the USA.

ABB’s S804U-PVS5 MCB has been tested according to UL489B at 1000V DC for GFDI applications. In the next future will be available up to 1500V (DC) to meet the growing demands.