ABB RELION REX640 Technical Manual page 524

Section 4
Protection functions
518
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
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".
GUID-F1F0F83A-D928-4FB6-B8CE-FC4FF4D94F38 V1 EN
Figure 283: 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 conditions.
1MRS759142 C
and Ū is 150°, and
3H_T 3H_N
REX640
Technical Manual