Taking Care Of The Circuit Breaker - ABB REL650 Technical Manual

1MRK 506 382-UEN A
6.4.7.4
Line distance protection REL650 2.2 IEC
Technical manual
number of pole slips per second) is difficult to calculate. The easiest and most exact
method is to measure time between two successive pole slips. This means that, the
instantaneous slip-frequency is measured only after the 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

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 67. 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 66 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
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.
Section 6
Impedance protection
GUID-35B49D7D-80AF-4DB0-A3C5-0CA0E54A9CA1 v4
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