Restricted Earth Fault Protection:
This type of Restricted Earth Fault Protection is provided to detect earth-faults within the protected zone of the transformer. A CT is fitted in each connection to the protected winding, and the CT secondary windings are connected in parallel to a relay.
Figure (6.8) illustrates the connections of Restricted Earth Fault Protection for star and delta windings. Ideally, the output of the CTs is proportional to the sum of the zero sequence currents in the line and neutral earth connection if the latter is within the protected zone. For internal earth faults this sum is equal to two times the total fault current, but for external faults zero-sequence currents are either absent or sum to zero in the line and neutral-earth connection.
A typical arrangement of differential and Restricted Earth Fault Protection of a star/delta transformer is shown in Fig (6.9).
The amount of winding protected against earth-fault is determined by the minimum primary current at which the earth-fault relay operates. When the neutral point of the star winding of a transformer is earthed through a resistor the amount of winding protected therefore varies according to the rating of the neutral earthing resistor and the relay setting.
Overcurrent and Earth-Fault (Unrestricted):
Overcurrent and earth-fault protection using IDMT relays is used mainly to protect the transformer against the effects of external short circuits and excess overloads. This protection however acts as a backup protection for the transformer with settings properly chosen. The current settings must be above the permitted sustained overload allowance and below the minimum short circuit current. Extremely inverse characteristic is the ideal as it closely resembles the thermal curve of transformer.
Also the time setting may have to be high in order to grade with other overcurrent relays on the system. The protection is located on the supply side of the transformer and is arranged to trip both the HV and LV circuit breakers.
Tank Leakage Protection:
If the transformer tank is lightly insulated from earth (an insulation resistance of 10 ohms) earth-fault protection can be achieved by connecting a relay to the secondary of a CT the primary of which is connected between the transformer tank and earth. Such an arrangement is usually necessary where banked transformers are provided with a single overall differential protection and it is difficult to find as to which transformer is faulty. This scheme is similar to the frame leakage bus bar protection scheme.