P54x/EN AP/La4 Application Notes
(AP) 6-26
MiCOM P543, P544, P545 & P546
AP
To resume, if area 1<area 2, the system will stay in synchronism. This swing is usually
called a recoverable power swing. If, on contrary, the system passes point G with a further
increase in angle difference between sending and receiving ends, the system drifts out of
synchronism and becomes unstable. This will happen if the initial power transfer Po was set
too high in Figure 6, so that the area 1 is greater than area 2. This power swing is not
recoverable
and is usually called out of step or out of synchronism or pole slip condition.
After this, only system separation and re-synchronizing of the machines can restore normal
system operation.
In Figure 6, the point G is shown at approximately 120° deg, but it is not true in
all cases. If,
for example the pre-fault transmitted power (Po) was too high and if the fault clearance was
slow, the area 1 will be greater so for the system to recover the angle θ would be close to 90
deg. On contrarily, if the pre-fault transmitted power Po was low and fault clearance fast, the
area 1 will be small, so that based on area comparison, the angle θ could go closer to 180
deg and the system will still remain stable.
The actual angle difference at which system will become unstable could only be determined
by a particular system studies, but for the purpose of settings recommendation where ‘OST’
setting is selected, the typical angle beyond which system will not recover is assumed to be
120 deg.
2.3.17.1 Setting option recommendation (Distance option only)
The rel
ay provides 4 different setting options:
1. Disabled
2. Predictive OST
3. OST
4. Predictive OST or OST
Set Option 1 on all lines except the line where tripping due to unrecoverable power
oscillations is required or for the system where power oscillations are not severe - mainly in
well interconnected systems operating with 3 phase tripping.
Setting Option 2 (and 4) is the best setting option from the system point of view, perhaps not
being widely used in the past. Some utilities prefer an early system split to minimize the
angle shift between ends and maximize the chances for the remaining two halves to stabilize
as quickly as possible. Special care must be taken when this method is applied to ensure
that the actual circuit breaker opening does not occur when the internal voltages at two ends
are in anti phase. This is due to the fact that most breakers are not designed to interrupt at
double nominal voltage and any attempt to break at that point would lead to flash over and
possible circuit breaker damage. The fact is that setting Option 2 (and 4) will be mainly
applied do detect and trip fast power oscillations. When this is coupled with a typical 2 cycle
circuit breaker operating time, the two voltages angles may rapidly move in opposite
directions at the time of opening the circuit breaker. Therefore, if this setting option is
chosen, the above facts must be taken into account so that the actual CB opening must
occur well before the angle difference between two ends approaches 180 degrees. On that
basis, accurate settings have to be determined based on exhaustive system studies.
Setting Option 3 is the most commonly used approach. Once the Out of Step conditions are
detected, the OST command will split the system at pre-determined points. The slight
disadvantages of this method in comparison to Option 2 (and 4) is that the power oscillation
will escalate further, thus causing more difficulties for the split parts to remain stable but the
advantage is that the timing of the circuit breaker operation (‘tripping angle’) is easily
controlled and the decision to split the system will be correct even if errors were made in the
system data and setting parameters. This extra security is achieved by measuring and
confirming the change of polarity of the resistive part of positive sequence impedance on
zone 5 exit (reset).
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