Sensitive Earth Fault Protection :
• As explained in section 4.4, differential protection does not protect the 100 % portion of generator stator winding against earth faults. Hence a separate sensitive earth faults protection is necessary.
• Two methods are available to provide protection for remaining 15 % of stator winding from neutral connection.
• For large units, Generator neutral is earthed through high impedance, to limit the maximum earth-fault current to much lower value than full load current, Fig. (a). This method is preferred for large units.
• With resistance earthing Fig. (a) two earth fault relays may be provided on the secondary side of neutral CT. The first E. F. (earth fault) relay is set at 10 percent and is instantaneous type.
• The second E. F. relay is inverse definite minimum time (IDMT) and is set at 5 percent (the relay pick up when earth fault current is 5 percent of full load current of generator).
• Depending upon the sensitivity, the first relay would protect about 90 percent of stator winding and the second relay about 95 percent. For such sensitive settings, it is necessary to provide a time delay, otherwise the relay may respond to transient neutral currents during external faults.
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| Sensitive Earth Fault Protection |
• The E. F. relay is connected to secondary of VT with setting of 10% of rated secondary voltage of VT.
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| Sensitive Earth Fault Protection |
• When the voltage between neutral and earth reaches 10 % of phase to neutral voltage of generator, the earth fault relay operates.
• Lighting Arrester connected in parallel with the primary protects the VT from high voltage surges.
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