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

Section 6
Impedance protection
6.3.6.4
138
second pole-slip, if the protected machine is 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.

Taking care of the circuit breaker safety

Although out-of-step events are relatively rare, the out-of-step protection should
take care of the circuit breaker safety. 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
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 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 74 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
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 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 75. The currents flowing are highest at rotor angle
1MRK 502 034-UEN -
Figure 75
Technical Manual