Applications - GE MLJ1000 Manual

2.2 APPLICATIONS

Synchronism
In general, synchronism check is intended primly for application where the two parts of a system to be
joined by the closure of a circuit breaker are interconnected at other points throughout the system.
Usually, performing synchronism check measurements is done with relatively long times in order to
make sure the voltages are synchronized. Nevertheless, this long timer, which can be on the order of
10 to 20 seconds, is not appropriate when both ends of the line are to be reclosed at high speed. If the
measurement time is lower, then the synchronism check can be done faster, although this means that
the reclosing could be done under no-synchronism conditions, with greater frequency slip than for ideal
condition.
It is essential to point out the intrinsic relation that invariably exists between time, frequency slip and
angle of closure, in such a way that, for constant slip, the following expression is carried out:
Where: D = angle of closure in degrees
S = Frequency slip in mHz
T = Total time in seconds
For applications where a preferred value for frequency slip does not exist, it is recommended to use
the maximum. This way the behavior of other synchronism relays which do not have this feature can
be reproduced.
If a preferred value for the voltage difference does not exist, it is recommended to use the maximum
rating; in doing so closure is permitted monitoring only conditions of phase, slip and time.
If the time of the circuit breaker closure is known, the maximum angle difference can be estimated by
applying the above expression; it will be obtained during closure of the circuit breaker.
For example, let us take: a setting of 30º for the angle, a setting of 167 ms for the time,
and that the time of operation of the circuit breaker to be 83 ms.
From the above equation we obtain the value of 1 Hz (360º/s) for frequency slip S. If we
had frequency slip greater than 1 Hz, we would not have close enable.
If we adjust the maximum frequency slip to 330 mHz, then:
and the movement in the difference of the angle during the circuit breaker closure will be:
20º+10º=30º, which is the fixed phase setting, and therefore the closure occurs at the
optimum moment, when the voltages in the line and bus are completely in phase.
GEK - 105560
D *
=
S
180
330 mHz ≅ 120º/s ⇒ 120º/s * 167 ms = 20º
83 ms * 120º/s = 10º
7
1000
* T