On the insulation resistance in high-power free-field grid-connected photovoltaic plants

In this paper, the authors discuss the crucial aspects of the insulation resistance testing of PV installations. This test verifies the integrity of wiring systems, and can be used to detect or prevent damages to wiring and ground faults. The insulation resistance test measures the resistance between ungrounded circuits and ground, under the application of high-voltage. Baseline insulation resistance can be compared to measurements over time to assess degradation of PV arrays or conductors. This test is worldwide accepted as a crucial element of commissioning, and acceptance, and is governed by several technical standards. In this paper, the authors critically discuss technical issues related to the insulation resistance test, which can be challenging in large PV systems, and the correct application of standards. In addition, a case study is presented, which includes the results of insulation measurements performed on a 4.9 MWp free-field grid-connected PV installation, located in Southern Italy, whose operation was affected by continuous insulation failures.