measured Z from point 1 to point 2 takes approximately 20 ms, due to Fourier filters.
The complex impedance then travels in 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 76 shows two pole-slips.
Figures like Figure 76 and Figure 78 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.
7.4.7.3 Maximum slip frequency
The maximum slip frequency fsMax in Hz where a pole-slip can be detected when
using a specific value of the setting (parameter) StartAngle (which determines the
width of the lens characteristic) is as follows. A parameter in this calculation routine
is the value of the minimum traverse time, traverseTimeMin. The minimum traverse
time determines the minimum time, the complex impedance Z(R, X) must travel
through the lens from one side to the other, 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
[ ]
≅
[ ]
⋅
°
[ ]
°
[
1000
1 000
180
. -
]]
IECEQUATION2319 V1 EN (Equation 40)
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 = 110degrees, fsMax =
7.777Hz. 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. 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
1MRK 502 048-UEN A Section 7
Impedance protection
169
Technical manual
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