Introduction

DGs are working in parallel to with utility power grid to feed power to the load, the operation is known as a grid-connected mode [1]. In contrast, in case of utility failure, the DG unit continues to supply power to the remaining load, and the operation is known as the islanded mode [2].
This paper discusses this problem that occurs during the connection between a DG plant and the rest of the power grid system. This problem has been mentioned and discussed widely in the last few years. Islanding occurs when a portion of the distribution power system becomes electrically complete isolated from the rest of the power system, hence load is remains energized by DG connected to the isolated subsystem as shown in figure.1. The causes of islanding divided to intended and unintended. Intended islanding is performed due to the scheduled maintenance required for the main utility, whereas unintended islanding may occur at any time due to regular faults or other uncertainties in the power system [3]. Therefore, islanding detection is considered as an important problem so IEEE 1547 [4,5,6] describe DG interconnection, planned and unplanned power islanding, and other important operating considerations. Risks of unintended Islanding are:
  1. Islanding may interfere with the automatic or manual re-establishment of normal service for the neighboring customers [7].
  2. The frequency and voltage provided to the users in the islanded system can vary suddenly if the DGs do not provide regulation of frequency and voltage and do not have protection relaying to limit voltage and frequency excursions [8].
  3. The DGs in the islanding mode could be damaged when the rest of network is reconnected to the supply system. This is because the generators are likely not in synchronization with the system at the instantantly of reconnection. Such out-of-phase reclosing.
  4. Islanding may create hazards for utility grid line-workers or the public by causing a line to remain energized that may be assumed to be disconnected from all energy sources.