Taking Care Of The Circuit Breaker Soundness - ABB REG650 Technical Manual

Section 7
Impedance protection
7.4.7.4
170
value of slipsPerSecond (SLIPFREQ) is equal to the average slip-frequency of the
machine between the last two successive pole-slips.

Taking care of the circuit breaker soundness

Although out-of-step events are relatively rare, the out-of-step protection should take
care of the circuit breaker soundness. The electro-mechanical 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
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 to a user in order to minimize the
stress, of which the 2nd one is more advanced.
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 81 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 will open at almost exactly 0 degrees, as illustrated in
= 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 will decide when the trip command will be given. If
specified tBreaker > 0, for example tBreaker = 0.040 second, then automatically, the
TripAngle will be ignored and the second, more exact method applied.
Figure 82. The currents flowing are highest at rotor angle 180
1MRK 502 048-UEN A
Figure 82 for tBreaker
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