Figure 259: Earth-fault situation in an isolated network
Directional earth-fault protection in a compensated network
In compensated networks, the capacitive fault current and the inductive resonance
coil current compensate each other. The protection cannot be based on the
reactive current measurement, since the current of the compensation coil would
disturb the operation of the protection relays. In this case, the selectivity is
based on the measurement of the active current component. The magnitude of
this component is often small and must be increased by means of a parallel
resistor in the compensation equipment. When measuring the resistive part of the
residual current, the
Operation mode should be set to "Phase angle" and the relay
characteristic angle (RCA) should be set to 0 degrees. Alternatively, the
Operation
mode can be set to "IoCos".
illustrates a simplified equivalent circuit for a
compensated network with an earth fault in phase C.
1MRS759142 F
Protection functions
REX640
Technical Manual
461