GE MiCOM P40 Agile Technical Manual page 322

Chapter 10 - Power Swing Functions P543i/P545i
the 'Predictive' options is that tripping will take a little longer so that the power oscillations may escalate further
after separation and the split parts may become separately unstable. An advantage, however, is that the decision
to split the system will always be valid even if the accurate system data and setting parameters cannot be
obtained.
The predictive setting options Pred. OST Trip and Pred. & OST Trip are recommended for systems
where Out-of-Step conditions could possibly occur, and where an early system split should minimise the phase
shift between generation sources. This should maximise the chances for the separated parts of the system to
stabilise as quickly as possible. Special care must be taken when these settings are used to ensure that the circuit
breakers at the different terminals do not open when the voltages at different ends are in anti-phase. This is
because most circuit breakers are not designed to break current at double the nominal voltage. Attempting to
break the current at double the nominal voltage could lead to flash-over and circuit breaker damage.
'Predictive' settings are designed to detect and trip for fast power oscillations. When predictive tripping is used
with a circuit breaker capable of operating in typically two-cycles, the two voltages angles may rapidly move in
opposite directions at the time of opening the circuit breaker. So, if you use the predictive settings you need to
apply settings that will ensure that the circuit breaker opening occurs well before the phase difference between
the different terminals approaches 180º. This means that accurate settings can only be determined by exhaustive
system studies.
The setting Pred. & OST Trip provides two stages of OST. If a power system oscillation is very fast, the
combination of ∆R (the difference between the Zone 5 and Zone 6 resistive reaches), and the Delta T settings, must
be set so that Pred. OST Trip operates. If the oscillation is slower, the condition for the predictive OST is not
met and so tripping is dictated by the OST condition being met. For the OST condition to be met, the resistive
component of the impedance must leave Zone 5 with opposite polarity compared with when it entered. If the
polarity is opposite when Zone 5 resets, OST will trip. If the polarity is the same when Zone 5 resets, OST will not
trip. This distinguishes between a slower non-recoverable oscillation and recoverable swings.
You should disable OST for applications on lines where unrecoverable power oscillations are not expected, or not
expected to be severe. This is likely to apply to strong interconnected systems operating with three-phase tripping.
4.4.1.1 DETERMINING THE LIMITS OF THE OST CHARACTERISTIC
The figure below shows the OST characteristic in conjunction with the system impedance, ZT, and particularly the
relationship with the resistive reach of the Zone 5 element.
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