P54x/EN OP/La4 Operation
(OP) 5-204
MiCOM P543, P544, P545 & P546
OP
5.8 Synchronism check functions (P544/P546)
5.8.1 Overview
Two
stages of system synchronism check supervision are provided for each circuit breaker.
When required, they control the manual closing and/or auto-reclosing of the associated
circuit breaker. CB1 CS1 and CB1 CS2 supervise CB1, while CB2 CS1 and CB2 CS2
supervise CB2.
The functionality of the first two stages (CB1 CS1 and CB2 CS1) is the same for each, but
each circuit breaker has individual settings.
The functionality of the second two stages (CB1 CS2 and CB2 CS2) is the same for each,
with each circuit breaker having individual settings, and the functionality is similar to the first
stages, but the second stages have an additional “adaptive” setting.
The synchronism check function in P544/P546 relays can be set to provide appropriate
synchronism check supervision of circuit breaker closing for either synchronous or
asynchronous systems.
5.8.2 Synchronous systems and asynchrono
us systems/system split
Systems in which the frequency difference (“slip frequency”) between the voltages on either
side of an open circuit breaker is practically zero are described as “synchronous”. Such
systems are typically interconnected by other circuits in parallel with the open circuit breaker,
which help to maintain synchronism even while the circuit breaker is open.
Systems which are electrically separated when a specific circuit breaker is open do not have
parallel connections of sufficiently low impedance to maintain synchronism, and in the
absence of any power flow between them the frequencies can drift apart, giving a significant
slip frequency. Such systems are described as “asynchronous” or “split”, and are recognised
by a measured slip frequency greater than the limiting slip frequency setting for synchronous
systems.
5.8.3 Synchronism check functions provided i
n the P544/P546
Two independently settable synchronism check functions are provided for each circuit
breaker controlled by the relay. CB1 CS1 and/or CB1 CS2 can be applied to supervise
closing of circuit breaker CB1. CB2 CS1 and/or CB2 CS2 can be applied to supervise
closing of circuit breaker CB2.
CB1 CS1 and CB2 CS1 are designed to be applied for synchronism check on synchronous
systems, while CB1 CS2 and CB2 CS2 provide additional features which may be required
for synchronism check on asynchronous systems. In situations where it is possible for the
voltages on either side of a circuit breaker to be either synchronous or asynchronous
depending on plant connections elsewhere on the system, both CBx CS1 and CBx CS2 can
be enabled, to provide a permissive close signal if either set of permitted closing conditions
is satisfied.
Each synchronism check function, as well as having the basic maximum phase angle
difference and slip frequency settings, can also be set to inhibit circuit breaker closing if
selected “blocking” conditions such as overvoltage, undervoltage or excessive voltage
magnitude difference are detected. In addition, CB1 CS2 and CB2 CS2 each require the
phase angle difference to be decreasing in magnitude to permit circuit breaker closing, and
each has an optional “Adaptive” closing feature to issue the permissive close signal when
the predicted phase angle difference immediately prior to the instant of circuit breaker main
contacts closing (i.e. after CB Close time) is as close as practicable to zero.
Slip frequency can be defined as the difference between the voltage signals on either side of
the circuit breaker, and represents a measure of the rate of change of phase between the
two signals.
Having two system synchronism check stages available for each circuit breaker allows the
circuit breaker closing to be enabled under different system conditions (for example, low slip
/ moderate phase angle, or moderate slip / small phase angle).
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