4
Description of
operation
The relay incorporates two identical stages
which operate as independent units. Both stages
of the synchro-check relay has two parallel func-
tions: a synchro-check function and a voltage-
check function.
The synchro-check relay can be used for two
different operating conditions, the most typical
of which is where both sides of the circuit
breaker to be closed are live. Then synchronism
is always checked before the circuit breaker is
given the permission to close. The other situa-
tion is where one or both sides of the circuit
breaker to be closed are dead and, consequently,
frequency and phase difference cannot be meas-
ured. In this case the relay checks the energiz-
ing direction. The user is able to define the volt-
age range within which the measured voltage is
determined to be "live" and "dead".
The purpose of the synchro-check function is
to find the instant when the voltages on both
sides of the circuit breaker are in synchronism.
The conditions for synchronism are met when
the voltages on both sides of the circuit breaker
have the same frequency, are in phase and are of
such a magnitude that the concerned busbars
or lines can be regarded as live.
When the frequency, phase angle and voltage
conditions are fulfilled, the duration of the syn-
chronizing conditions is checked so as to en-
sure that they will still be met when the con-
tacts of the circuit breaker close. This duration
is determined on the basis of the frequency and
phase difference measured. Depending on the
circuit breaker and the closing system, the de-
lay from the moment the closing signal is given
until the circuit breaker finally closes is about
50 - 250 ms. The CB operate time selected tells
the relay for how long, at least, the conditions
have to persist.
The voltage-check function checks the energiz-
ing direction. Energizing is defined as the situ-
ation where a dead network part is connected
to an energized section of the network. The con-
ditions of the network sections to be controlled
by the circuit breaker, i.e. which side has to be
live and which side dead, are determined by set-
ting. A situation where both sides are dead is
possible as well.
When the energizing direction corresponds to
the settings, the situation has to be constant for
a certain time before the close signal is permit-
ted. The purpose of this operate time (dead time)
is to make sure that the dead side remains
deenergized and that the situation is not due to
a temporary interference. Should the conditions
not persist for the specified operate time, the
operate time is reset and the procedure is started
again when allowed by the conditions. Not un-
til the required energizing situation has been
constant throughout the set operate time, cir-
cuit-breaker closing is permitted.
