GE MiCOM P40 Technical Manual page 279

P446SV
Therefore, for the OST Trip setting, assume that θ = 120° and set:
OST R5 = OST R5' = R5min = Z
●
● OST R6 = OST R6' = R6max
Delta T = 30 ms
●
Delta T always expires. Therefore, the setting value given above will secure the detection of a wide range of
oscillations, starting from very slow oscillations (caused by recoverable swings) up to a fastest oscillation limit of
7Hz. Note that any fault impedance will pass the OST R6 – OST R5 region faster than the minimum settable Delta T
time of 30ms.
Predictive and OST setting
The recommendations for Pred. & OST Trip are the same as for Pred. OST Trip.
4.4.1.4 SETTING THE OST TIME DELAY
For either of the predictive OST settings, the OST time delay setting (Tost) must be set zero.
For the OST Trip setting, Tost would normally be set to zero, but if you want to operate the breaker at an angle
closer to 360º (when voltages are in phase) you could apply a time delay.
4.4.1.5 BLINDER ANGLE SETTING
Set Blinder Angle, α, the same as the total system impedance angle, Z
4.4.1.6 OST FOR SERIES COMPENSATED LINES
The maximum phase currents during an Out-of-Step condition rarely exceed 2In, which corresponds to the
minimum swing impedance passing through Zone 1. Since the Metal-Oxide Varistors (MOV) bypass level is
normally set between 2-3In, they will not operate during the power oscillations and therefore in the majority of
applications they will not make any impact on Out-of-Step operation.
In the worst case, power oscillations are triggered on fault clearance on a parallel line. Approximately twice the
load current starts flowing through the remaining circuit. This increases further and eventually exceeds the MOV
threshold. The OST R6 - OST R5 region is usually set far from Zone 1, therefore it is unlikely that the positive
sequence impedance's trajectory can traverse in and out of the set ∆R region due to the operation of MOVs. If
MOVs do operate in the ∆R region, a timer that has been initiated may reset and be reinitiated, or the impedance
may remain in the ∆R region for slightly longer. This is because resistive and capacitive components are added to
the measured impedance during MOV operation as shown in the figure below. This effect may have an impact on
the Delta T measurement if the Pred. OST Trip setting is used. If the recommendation to set R5min as close
as practically possible to R6max is followed, it is unlikely that the swing currents will exceed the MOV threshold in
the ∆R region. If a study shows that the MOVs could operate within the ∆R region, we recommend setting Pred.
& OST Trip operating mode to cover all eventualities.
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Chapter 10 - Power Swing Functions
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