Assessment of ROCPAD Relay for Islanding Detection in Distributed Generation

The paper presents a new approach for islanding detection in distributed generation (DG) using rate of change of phase angle difference (ROCPAD). The process starts with retrieving the voltage and current signals at the DG end and estimating the phasors (amplitude, phase, and frequency) using synchronous transformation based algorithm. The next step is to compute the phase angle difference and the ROCPAD for registering islanding detection. The proposed algorithm is compared with the widely used rate of change of frequency relays (ROCOF) and found working effectively in the situations where ROCOF fails. The proposed ROCPAD relay is tested on standard microgrid structures with possible variations in operating parameters and the results indicate that the ROCPAD relay can reliably detect islanding conditions in large microgrids.

I. Introduction

Islanding of a grid-connected DG occurs when a section of the utility system containing such generators is disconnected from the main utility, but the independent DGs continue to energize the utility lines in the isolated section. Unintended islanding is a concern primarily because it poses a hazard to utility and customer equipment, maintenance personnel and the general public. As more DG systems become part of the power grid, there is an increased safety hazard for personnel and an increased risk of damage to the power system. Despite the favorable aspects grid-connected DGs can provide to the distribution system, a critical demanding concern is islanding detection and prevention. Failure to trip islanded DG can lead to a number of problems for these resources and the connected loads, which includes power quality, safety and operation problems. Therefore, the current industry practice is to disconnect all distributed resources (DRs) immediately after the occurrence of islands [1], [2]. The disconnection is normally performed by a special protection scheme called islanding detection relays, which can be implemented using different techniques. However, during the design process of the islanding detection scheme, the detection of islanding conditions according to international standards [3]–[6], must be also considered.