Transformer Protection Part 8
This type of 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. Fig.1 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 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 2.
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| Fig. 1 Restricted earth fault protection : (a) neutral earthed within the protected zone (b) neutral not earthed within the protected zone |
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.
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| Fig. 2 Differential and restricted earth fault protection of a star/delta transformer |
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