ABB REG670 - Maximum Slip Frequency

the direction from the right to the left, and exits the lens on the opposite side. When the
complex impedance exits the lens on the side opposite to its entrance, the 1st pole-slip
has already occurred and more pole-slips can be expected if the generator is not
disconnected.
Figure 111 shows two pole-slips. Figures like Figure 111 and Figure
113 are always possible to draw by means of the analog output data from the pole-slip
function, and are of great help with eventual investigations of the performance of the
out-of-step function.
7.6.7.3 Maximum slip frequency
A pole-slip may be detected if it has a slip frequency lower than a maximum value
fsMax. The specific value of fsMax depends on the setting (parameter) StartAngle
(which determines the 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 StartAngle.For values of StartAngle <= 110°, traverseTimeMin = 50 ms. For
values StartAngle > 110°, traverseTimeMin = 40 ms. The expression which relates the
maximum slip frequency fsMax and the traverseTimeMin is as follows:
fsMax Hz
traverseTimeMin ms
StartAngle
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1000
1 000
180
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IECEQUATION2319 V1 EN (Equation 48)
The maximum slip frequency fsMax for traverseTimeMin = 50 ms is:
StartAngle
= 90° → fsMax = 20 × 0.500 = 10.000 Hz
StartAngle = 100° → fsMax = 20 × 0.444 = 8.888 Hz
StartAngle = 110° → fsMax = 20 × 0.388 = 7.777 Hz (default 110°)
The maximum slip frequency fsMax for traverseTimeMin = 40 ms is:
StartAngle
= 120° → fsMax = 25 × 0.333 = 8.333 Hz
StartAngle = 130° → fsMax = 25 × 0.277 = 6.944 Hz
The minimum value of fsMax is 6.994 Hz. When StartAngle = 110 degrees, fsMax =
7.777 Hz. This implies, that the default StartAngle = 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
Section 7 1MRK502052-UEN B
Impedance protection
268
Technical manual