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

Section 7
Impedance protection
7.4.7.4
184
PickupAngle = 120° → fsMax = 25 ⋅ 0.333 = 8.333 Hz
PickupAngle = 130° → fsMax = 25 ⋅ 0.277 = 6.944 Hz
The minimum value of fsMax is 6.994 Hz. When PickupAngle = 110degrees, fsMax =
7.777Hz. This implies, that the default PickupAngle = 110 degrees covers 90% of cases as,
the typical final slip frequency is between 2 - 5Hz. In practice, however, before the slip
frequency for example, 7.777 Hz is reached, at least three pole-slips have occurred. The
exact instantaneous slip-frequency expressed in Hz (corresponding to 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 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
Figure 82. 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 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.
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
1MRK 502 048-UUS A
Figure 81 illustrates
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