Earth fault is the unintended fault between the live conductor and the earth. It also occurs, because of the insulation breakdown. When the fault occurs the short-circuit currents flow through the system, and this current is returned through the earth or through any electrical equipment. This fault current damaged the equipment of the power system and also interrupted the continuity of the supply.
The earth fault can be dispersed by using the restricted earth fault protection scheme. The earth fault protection scheme consists the earth fault relay, which gives the tripping command to the circuit breaker and hence restricted the fault current.
Earth fault relay connected in the residual part of the line CTs (current transformer) provides protection against earth fault on the delta or unearthed star-connected windings of the power transformer. The connection with restricted earth fault protection for star-connected and delta connected windings are shown in the figure below
The current transformer is fitted in each connection which is to be protected and secondary of the CTs are connected in parallel to a relay. When the system is in the protected zone, the output of the current transformers is equal to the zero sequence current in the line. When the short circuit current flow through the system the external fault zero system current is absent and the internal fault zero sequence current is twice the total fault current.
Working of Earth Fault Protection System
The star connected side is protected by a restricted earth fault protection shown in the figure below.
When there is an earth fault outside the protected zone, i.e. at F1, it causes the currents I2 and I1 in CT secondaries as shown in the figure below. The resultant current in the earth fault relay is negligible. When the earth fault is within the protected zone (i.e., F2), only current I2 exist, I1 being negligible.Thus the current I2 flow through the earth fault relay. Thus restricted earth fault relay does not operate for earth fault beyond the protected zone of the transformer.
The relay has to be very sensitive for sensing the earth fault, and it would operate for spurious signal, external faults and switch surges. The relay is set as per practice, so as to operate for earth fault current of the order of 15 % of rated winding current. Such settings protect a restricted portion of the winding. Hence the name restricted earth fault protection.
The stabilising resistor is connected in series with the relay for avoiding the magnetising inrush current and also the saturation of the current transformer.