Operation P54x/EN OP/La4
MiCOM P543, P544, P545 & P546
(OP) 5-49
OP
1.20.1.2 Operating principle (Distance option only)
The Out of Step detection algorithm is based on measuring the speed of positive sequence
impedance passing through the set ∆Z region. As soon as measured positive sequence
impedance touches the outer polygon, a timer is started.
If the disturbance takes less than 25 ms from entering zone 6 to entering zone 5, the relay
will consider this to be a power system fault and not an out of step trip condition. The timer
of 25 ms is a fixed timer in the logic and not user accessible. During a power system fault,
the speed of impedance change from a load to a fault is fast, but the relay may operate
slower for marginal faults close to a zone boundary, particularly for high resistive faults inside
the zone operating characteristic and close to the Z5 boundary. Therefore, the fixed time of
25 ms is implemented to provide sufficient time for a distance element to operate and
therefore to distinguish between a fault and an extremely fast power system oscillation.
If the disturbance takes more than 25 ms but less than DeltaT set time from entering Zone 6
to entering Zone 5, this will be seen as a very fast oscillation. Therefore, the relay will trip if
setting option 2 or 4 was selected. The minimum DeltaT setting is 30 ms, allowing 5 ms
margin to the fixed 25 ms timer.
If the disturbance takes longer than the DeltaT setting time to enter Zone 5 after entering
Zone 6 then it is considered as a slow power oscillation. On entering Z5, the relay will record
the polarity of the resistive part of the positive sequence impedance. Two scenarios are
possible:
1. If the resistive part of the positive sequence impedance leaves Z5 with the same
polarity as previously recorded on entering Zone 5, it is deemed a recoverable swing.
No tripping will be issued.
2. If the resistive part of the positive sequence impedance has the opposite polarity when
exiting Zone 5 to that of the recorded polarity on Zone 5 entering, an Out of Step
condition is recognized, followed by the tripping if setting option 3 or 4 was selected. It
should be noted that in the case when the DeltaT timer did not expire and setting
option 3 is selected, the Out of Step condition will also be detected, followed by OST
operation.
As the tripping mode for the detected Out of Step condition is always 3 ph trip, the ‘Predictive
OST’ and OST DDB signals are mapped to the 3ph tripping in the default PSL. Also, Out of
Step operation will block auto-reclose function. The Out of Step tripping time delay TOST is
also available to delay the OST tripping command until the angle between internal voltages
between two ends are at 240 deg closing towards 360 deg. This is to limit the voltage stress
across the circuit breaker. In the case of a fault occurring during the swing condition, the out
of step tripping function will be blocked.
The Out of Step algorithm is completely independent from the distance elements and setting
free power swing detection function. The load blinder does not have any effect on the OST
characteristics. For the Out of Step operation, the minimum positive sequence current of
5%In must be present.
The Out of Step algorithm is given in Figure 28.
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