ABB insulation monitors keep solar PV plants safe and operational

Borje Axelsson

In solar PV plants, where operational continuity is important, installing an ABB insulation monitoring device can prevent faults and reduce maintenance

A unique characteristic of solar PV generating systems is that as long as the PV panels are exposed to the sun, they cannot be switched off. In addition, since PV modules are installed outdoors and exposed to the elements, critical attention must be paid to keep these generating systems – big and small – as safe as possible to prevent electrical shock and equipment damage.

In any electrical power system, ground (or earth) faults are a common fault type. Among short circuit faults, 98 percent are phase to ground. The purpose of grounding these systems is to prevent unwanted currents from flowing (particularly through people) causing personal injury or death, and potentially damaging equipment.

Lightning, natural and man-made ground faults and line surges can cause high voltages to exist in otherwise low-voltage systems. Proper grounding, along with overcurrent protection, limits the possible damage from a ground fault.

Well grounded
Local regulations and habits may vary widely from country to country, but the international standard IEC 60364 recognizes three main families of grounding arrangement: TN, TT, IT (T stands for Terra, meaning ground or earth; N is Neutral; I is isolation).

TN and TT are grounded systems; IT is an ungrounded system.

In a TN-configured system, the star point or neutral point of the transformer is earthed, and a PE (protective earth) wire connects all loads with the earth potential. In case of a ground fault, a TN configured system provides a high peak current to trip a circuit breaker or a fuse.

In a TT-configured system, the star point is also earthed, and all loads are locally earthed. In case of a ground fault, a TT system produces only a small earth leakage current due to the high loop resistance between the earth points. A residual current device (RCD) is the right protection here.

In an IT-configured system, the star point is not earthed, and the first ground fault will not trip the system’s protective devices, and it continues operating with the first fault present. A second earth fault could potentially lead to a short circuit between two lines and trip a circuit breaker or a fuse – so the first fault has to be detected.

Unique challenges
In DC systems such as solar PV plants, both grounded and ungrounded systems can be found. Here as elsewhere, the ungrounded systems need insulation monitoring, but large PV systems provide some unique challenges, relating to high system voltage and high leakage capacitance.

Solar panels need to have adequate insulation between their electricity-conducting components and the module’s frame or, in the case of a frameless panel, the outside world. Badly positioned panel cells, material weakness, or poor lamination may all result in inadequate insulation resistance. Current leakage can occur at the edges of the solar panel, or through the back sheet.

The global standard IEC 61557-8 specifies the requirements for insulation monitoring devices, and two of the most important factors it considers relate to the insulation being monitored: RF (insulation resistance) and Ce (leakage capacitance, from each conductor to earth).

Fault detection
Discharge current is generated mainly by the leakage capacitance of PV modules to the earth, which is affected by the type of modules and their positions, or environmental conditions like rain and humidity, as well as other components of the system (ie, inverters, lightning protection devices).

Capacitance rises with the size of the solar installation since the area covered by solar cells over the ground acts as a big capacitor. This means the more PV panels that are installed, the higher the potential incidence of leakage capacitance, creating problems of electromagnetic interference (EMI), personnel safety, issues of power quality and system losses.

ABB delivers a complete range of products for various power systems, both AC and DC, including insulation monitoring devices that measure industrial circuits and detect faults, emitting an alarm as soon as they occur.

In solar PV plants, where operational continuity is important, installing an ABB insulation monitoring device can prevent faults and reduce maintenance and downtime, protecting the investment. ABB products cover solar PV systems up to 1,000 volts (V) DC and 2,000 microfarads (μF).

About the author

Borje Axelsson

Working with electrification products towards consultants and the Marine industry. I'm in ABB since 30+ years, with different roles in various functions. Outside work, I play guitar in a rock band and banjo&mandolin in a country band.
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