Determining the Sufficiency of Standard Protective Relaying for Islanding Prevention in Grid-Connected PV Systems

Abstract

Recently there has been a resurgence of concern about islanding of grid-connected photovoltaic (PV) systems. This condition occurs when the PV system continues to energize a section of the grid after that section has been isolated from the main utility voltage source. Generally, islanding is undesirable for a number of reasons, and it is therefore important that PV systems incorporate methods to prevent islanding. However, PV systems are in general already required to be equipped with standard protective relays, namely over/undervoltage relays (OYR/UYR) and over/underfrequency rclays (OFR/UFR), which disconnect the PV system from the utility system in the event that either the magnitude or frequency of the PV system's terminal voltage deviates beyond certain thresholds. These relays will prevent islanding in the majority of cases. In fact, it has been argued that the probability of an islanding event that would not be detected by these relays is insignificant. The purpose of this paper is to investigate the probability of occurrence of the conditions which could lead to failure of the OVR/UVR and OFR/UFR to detect islanding using three methods, and thereby to determine whether the standard protective relays provide sufficient islanding protection.