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 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.
The earth fault relay is placed in the residual part of the current transformers shown in the figure below. This relay protects the delta or unearthed star winding of the power transformer against the fault current. The connection of earth fault relay with the star or delta winding of the transformer is shown in the figure below.
The current transformers are placed on both sides of the protective zone. The secondary terminal of the current transformer is connected in parallel with the relay. The output of the current transformer is equal to the zero sequence current flows in the line. The zero sequence current is absent for the external fault and for the internal fault it becomes twice the value of fault current.
Working with Earth Fault Protection System
The star connected side is protected by a restricted earth fault protection shown in the figure below.
Let F1 be the external fault occurs in the network. This fault induces the I1 and I2 current which flows in the secondary of the CTs. The resultant current of the I1 and I2 becomes zero. When the fault occurs inside the protective zone say F2, then only current I2 exist, and I1 is neglected. The current I2 passes through the earth fault relay. The earth fault relay operates only for the internal fault of the protective zone.
The earth fault relay must be very sensitive for sensing the fault. The relay is designed for sensing the fault current which is 15% more than that of the rated winding current. Such settings protect a restricted portion of the winding. Hence the name restricted earth fault protection.
The stabilising current is connected in series with the relay for avoiding the magnetising inrush current.