Description of Operation
GARD 8000 Distance Relay
RFL Electronics Inc.
9.20.4
ENERGIZING CHECK
The energizing check unit allows closing of the circuit breaker when the voltage is high (live) on one
side and low (dead) on the other. Two overvoltage elements with individual settings are used to
determine the live condition and the dead condition. These elements assert at 105% of set value and
resets at 100%.
Note that the “line voltage” refers to the side where the three phase VT used for the distance protection
is located. In some applications this may be on the bus, while a single phase VT used for synch check
is located on the line side.
9.20.5
SELECTION OF SYNCH CHECK
The recloser can be set to use the internal synch check element or an external synch check condition
via a digital input.
9.20.6
APPLICATION OF SYNCH CHECK
The synchronism check function is used to monitor the reconnection of the two parts of the circuit by
breaker closing. It verifies that the voltages on both sides of the breaker (BUS and LINE) are within
the magnitude, angle and frequency limits established in the settings.
Verification of synchronism is defined as the comparison of the voltage difference of two circuits with
different sources to be joined through an impedance (transmission line, feeder, etc.), or connected with
parallel circuits of defined impedances. The voltages on both sides of a breaker are compared before
allowing its closing so as to minimize possible internal damage due to a voltage difference in phase,
magnitude and/or angle. This is very important close to steam-powered power plants where an
unsynchronized closing of the line with considerable angle difference could cause serious damage to
the shaft of the turbine.
The difference in voltage level and phase angle at a given point in time is the result of the load between
sources connected through parallel circuits and the impedance of the elements that join them.
In interconnected systems, the angle difference between two sides of an open breaker is generally not
significant since their sources are joined remotely by other elements (equivalent or parallel circuits).
However, in islanded circuits, as in the case of an independent generator, the voltage angle, magnitude
and/or frequency difference can be considerable.
August 1, 2009
9-73
973.334.3100