ABB RELION REX640 Technical Manual page 549

Protection and control

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1MRS759142 F

is no risk of trip during the normal, non-faulted operation of the generator. If

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.

The angle between the third harmonic voltage phasors Ū

and with the

25%. This requires the value of

protection. The recommended value of the

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

Technical Manual

Beta setting must be set to "1.00".

The value of the

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.

A graph indicating differential and bias voltages as a functions of the load on

the generator must be plotted.

Based on the graph, such value of the

bias voltage, even in the worst condition, is at least 30% to 50% higher than the

differential voltage.

Beta setting value "1.0", protection guarantees a stability margin of

Beta setting must be selected that the

Beta to be increased so as to increase the stability of

Beta setting is at least "1.2".

Protection functions