width of the lens characteristic). A parameter in this calculation routine is the value of the
minimum traverse time,
traverseTimeMin
. The minimum traverse time is the minimum time that
the travel of the complex impedance Z(R, X) through the lens, from one side to the other, must last
in order to recognize that a pole-slip has occurred. The value of the internal constant
traverseTimeMin
is a function of the set
PickupAngle
.For values of
PickupAngle
<= 110°,
traverseTimeMin
= 50 ms. For values
PickupAngle
> 110°,
traverseTimeMin
= 40 ms. The expression
which relates the maximum slip frequency
fsMax
and the
traverseTimeMin
is as follows:
fsMax Hz
traverseTimeMin ms
PickupAngle
[ ]
≅
[ ]
⋅
°
[ ]
°
1000
1 000
180
. -
[[ ]
ANSIEQUATION2319 V1 EN-US (Equation 110)
The maximum slip frequency
fsMax
for
traverseTimeMin
= 50 ms is:
PickupAngle
= 90° →
fsMax
= 20 × 0.500 = 10.000 Hz
PickupAngle
= 100° →
fsMax
= 20 × 0.444 = 8.888 Hz
PickupAngle
= 110° →
fsMax
= 20 × 0.388 = 7.777 Hz (default 110°)
The maximum slip frequency
fsMax
for
traverseTimeMin
= 40 ms is:
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.
7.11.7.4 Taking care of the circuit breaker
GUID-35B49D7D-80AF-4DB0-A3C5-0CA0E54A9CA1 v4
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
206. 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.
Section 7 1MRK 502 066-UUS B
Impedance protection
384
Technical manual
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