Taking Care Of The Circuit Breaker - ABB RES670 Technical Manual

1MRK 511 408-UUS A
7.2.7.4
Phasor measurement unit RES670 2.2 ANSI
Technical manual
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 = 110 degrees, fsMax =
7.777 Hz. 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. In other words, if we consider a linear increase of frequency from 50
Hz to 57.777 Hz, at least three pole-slips will occur (in fact: (57.777 - 50) / 2 = 3.889).
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

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 48. 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 47 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.
Section 7
Impedance protection
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