When 27/59THN is reduced to function as only a third harmonic neutral
point undervoltage protection, it is necessary to block the function during
start-up and shutdown of generator and also when there is no sufficient
voltage.
Calculating Beta value
The setting Beta gives the proportion of the third harmonic voltage in the neutral point of
the generator to be used as bias quantity. Beta must be set so that there is no risk of trip
during the normal, non-faulted operation of the generator. If Beta is set high, this limits the
portion of the stator winding covered by the protection. In most cases, the default setting
“3.00” gives an acceptable sensitivity for a ground fault near the neutral point of the stator
winding. However, to assure the best performance, measurements during the normal
operation of the generator are to be made during commissioning.
1. The value of the Beta setting must be set to “1.00”.
2. Loading of the generator is done at 5 to 10 different load points and the third harmonic
differential and bias voltage are measured. Both quantities can be obtained from the
Monitored data view of the function.
3. A graph indicating differential and bias voltages as a functions of the load on the
generator must be plotted.
4. Based on the graph, such value of the Beta setting must be selected that the bias
voltage, even in the worst condition, is at least 30% to 50% higher than the differential
voltage.
The angle between the third harmonic voltage phasors
and
is 150°, and
with the Beta setting value "1.0", protection guarantees a stability margin of 25%. This
requires the value of Beta to be increased so as to increase the stability of protection. The
recommended value of the Beta setting is at least “1.2”.
Section 4 1MAC059074-MB A
Protection functions
432 615 series ANSI
Technical Manual
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