Directional Earth fault

Step 1: Operating signal & polarizing signal

Requirements are similar to directional overcurrent 

i.e.       need operating signal

            and polarising signal

Operating Signal

obtained from residual connection of line CT's

i.e.       I_op  =  3I_o

Polarising Signal

The use of either phase-neutral or phase-phase voltage as the reference becomes inappropriate for the comparison with residual current.

Most appropriate polarising signal is the residual voltage.

Residual Voltage

Notes :

1.         VT primary must be earthed.

2.         VT must be of the '5 limb' construction (or 3 x single phase units)

Step 2: Relay Characteristic Angle

0  -  Resistance earthed systems

45 (I lags V)  -  Distribution systems (solidly earthed)

60 (I lags V)  -  Transmission systems (solidly earthed)

Current Polarising

A solidly earthed, high fault level (low source impedance) system may result in a small value of residual voltage at the relaying point.  If residual voltage is too low to provide a reliable polarising signal then a current polarising signal may be used as an alternative.

The current polarising signal may be derived from a CT located in a suitable system neutral to earth connection.

e.g.