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:
- Islanding may interfere with the automatic or manual re-establishment
of normal service for the neighboring customers [7].
- 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].
- 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.
- 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.