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 an earth
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 Ū
3H_T
and Ū
3H_N
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”.
Figure 316: Typical example of variation of the bias voltage and differential voltage
with a change in the active generated load (the angle between the third harmonic
terminal and neutral voltage is 150°)
Calculating CB Open factor
One of the factors, though not major, governing the magnitude of the generated
third harmonic voltage is the generator terminal capacitance. If there is no
generator breaker, the capacitive coupling to earth is the same under all operating
1MRS759142 F
Protection functions
REX640
Technical Manual
549
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