not already disconnected after the first pole-slip. The measured value of
slipsPerSecond (SLIPFREQ) is equal to the average slip-frequency of the machine
between the last two successive pole-slips.
7.6.7.4 Taking care of the circuit breaker
Although out-of-step events are relatively rare, the out-of-step protection should take
care of the circuit breaker health. The electromechanical stress to which the breaker is
exposed shall be minimized. The maximum currents flowing under out-of-step
conditions can be even greater that those for a three-phase short circuit on generator
terminals; see Figure
117. The currents flowing are highest at rotor angle 180 degrees,
and smallest at 0 degrees, where relatively small currents flow. To open the circuit
breaker at 180 degrees, when not only the currents are highest, but the two internal
(that is, induced) voltages at both ends are in opposition, could be fatal for the circuit
breaker. There are two methods available in order to minimize the stress; the second
method is more advanced than the first one.
The first method
The circuit breaker is only allowed to break the current when the rotor angle has
become less than the set value TripAngle, on its way to 0 electrical degrees. A
recommended value for the setting TripAngle is 90 degrees or less, for example 60
degrees. Figure
116 illustrates the case with TripAngle = 90 degrees. The offset Mho
circle represents loci of the complex impedance Z(R, X) for which the rotor (power)
angle is 90 degrees. If the circuit breaker must not open before the rotor angle has
reached 90 degrees on its way towards 0 degrees, then it is clear that the circle delimits
the R – X plane into a “no trip” and a “trip” region. For TripAngle = 90 degrees, the
trip command will be issued at point 3 when the complex impedance Z(R, X) exits the
circle. By that time the relay logic had already ascertained the loss of step, and the
general decision to trip the generator has already been taken.
The second method
This method is more exact. If the break-time of the circuit breaker is known, (and
specified as the setting tBreaker) than it is possible to initiate a trip (break) command
almost exactly tBreaker milliseconds before the rotor (power) angle reaches 0
degrees, where the currents are at their minimum possible values. The breaker
contacts open at almost exactly 0 degrees, as illustrated in Figure
117 for tBreaker =
0.060 s. The point in time when the breaker opening process must be initiated is
estimated by solving on-line the so called “synchronizer” differential equation. Note
that if tBreaker is left on the initial (default) value, which is zero (0), then the
alternative setting TripAngle decides when the trip command is given. If specified
tBreaker > 0, for example tBreaker = 0.040 second, then automatically, the TripAngle
is ignored and the second, more exact method applied.
1MRK502052-UEN B Section 7
Impedance protection
269
Technical manual
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