Electrical Isolation Resistance in Photovoltaic Arrays – it matters

One of the most common faults in PV systems are DC ground faults, and it is estimated 50% of all PV ground faults go undetected. In this article we will show why it is important to detect and locate such faults in the early stage – before the electrical isolation resistance (Riso) value becomes critically low. Ground faults are undesirable because they lead to financial loss while also being a safety hazard. Normally ground faults do not occur spontaneously, but rather they manifest over time as the electrical insulation of the PV array degrades. It is however possible to locate and mitigate the faults before they manifest in the PV system and impact the return on investment.
A DC ground fault is the condition where electrical current is flowing through the equipment grounding conductor. For this reason, it is essential to understand all components of the PV array, and not just focus on the circuit found inside the individual solar panels.

First we have the Equipment Grounding Conductor (EGC) which is the the conductive path, that provides a ground fault current path, and connects metal parts of the PV equipment to the grounded conductor. Then we have the Grounding Electrode Conductor (GEC) which is connecting system equipment to the grounding electrode. Last we have the Grounding Electrode. This is a Grounded Conductor which is a metal spear driven into the soil to create to actual grounding.

When the PV system is installed Riso is typically more than 40 MΩ, and it means that there is a very high barrier for current leakages. But over time Riso can go down substantially and cause current to flow in the equipment grounding conductor. This condition is a ground fault by definition.

The material damage causing the ground fault is normally invisible to the naked eye, when the PV system is observed in the field. The cost of troubleshooting ground faults can therefore become substantial, if the technician is relying on voltage based testing principles. Such testing methods do not allow early stage detection and localization of faults. The aim is to detect and find ground faults at 3 MΩ before they manifest as safety hazards and lost revenue, which typically happens when Riso goes below 1 MΩ.

By using the Z200 PV Analyzer ground faults can be detected based on impedance testing. This method is specifically developed for troubleshooting PV arrays and it is very gentle on the system components. The limit for locating a ground fault is at 3 MOhm, so the instrument allows the technician to find faults long before safety is compromised and well before revenue is lost. Finding ground faults is just one of many troubleshooting features build into the Z200 PV Analyzer, and the instrument was designed specifically for solar O&M professionals.

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