Application Notes P54x/EN AP/La4
MiCOM P543, P544, P545 & P546
(AP) 6-29
AP
Setting of reactance lines Z5 and Z6 will depend on how far from the relay location the power
oscillations are to be detected. Normally, there is only one point where the system is to be
initially split and that point will be determined by system studies. For that reason, the Out of
Step protection must be enabled at that location and disabled on all others. To detect the
Out of step conditions, the Z5’-Z5 and Z6’-Z6 setting must be set to comfortably encompass
the total system impedance ZT, as shown in Figure 7. Typical setting could be:
Z5 = Z5’ = 1/
2 x 2 ZT = ZT
The Z6 and Z6’ setting is not of great importance and could be set to Z6 = Z6’ = 1.1 x Z5
2.3.17.3 Delta t, R5 and R6 setting determination (Distance option only)
The R5
MIN
and R6
MAX
settings determined above are only limit values, the actual R5 and R6
need to be determine in relation to the ‘Delta t’ timer.
Predictive OST setting:
For the ‘Predictive OST’ setting it is important to:
Set R6 (and R6’) equal to R6
MAX
Set R5 as close as practical to R6
MAX
The aim of pushing the R5 setting to the right is to detect the fast oscillation as soon as
possible to gain sufficient time to operate the breaker before the two source voltages are in
opposite direction. The only restriction would be the limitation of the ‘Delta t’ minimum time
delay of 30 ms and the speed of oscillation. Set ‘Delta t’ so that the following condition is
satisfied:
‘Delta t’ does not expire after positive sequence impedance has passed the R6-R5
region
For this setting, knowledge of the accurate rate of change
of swing impedance when
crossing the R6-R5 region is essential and therefore must be based on system studies.
Assumption that the rate of change of the positive sequence impedance during crossing the
R6-R5 region is average rate of change for the whole swing cycle is wrong and could easily
lead to incorrect ‘Predictive OST’ operation.
Note: For the fault, the R6-R5 region will be passed faster than 25 ms,
therefore even very fast oscillations of 7 Hz will not be mistaken with
the fault condition and ‘Predictive OST’ will not operate.
OST setting:
For the ‘OST’ setting option the precise setting of blinders and ‘Delta t’ is not necessary.
This is based on the fact that:
The wider the ∆R region and the shorter the ∆t setting, any oscillation will be successfully
detected. The only condition is that the fault impedance must pass through the ∆R region
faster than ∆t setting.
Therefore, for the ‘OST’ setting assume that θ = 120° and set:
R5 = R5’ = R5
MIN
= ZT/3.46
R6 = R6’ = R6
MAX
Delta t = 30 ms
The point is that ‘Delta t’ always expires, therefore the above setting will secure the detection
of a wide range of oscillations, starting from very slow oscillations caused by recoverable
swings up to the fastest oscillation of 7 Hz. It should be noted that any fault impedance will
pass the R6-R5 region faster than the minimum settable ‘Delta t’ time of 30 ms.
Predictive OST or OST setting:
As per ‘Predictive OST’ above.
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