Friday, July 1, 2011

Frame Leakage Protection

Transformer Protection : Part 4
Frame Leakage Protection
       This protection is nothing but the method of providing earth fault protection to the transformer. This protection can be provided to the metal clad switchgear.
       The arrangement is shown in the Fig. 1.
Fig. 1  Frame leakage protection

       The metal clad switchgear is lightly insulated from the earth.
       The frame of the switch i.e. enclosure is grounded. This is done through a primary of current transformer in between.
       The concrete foundation of switchgear and the other equipments are lightly insulated from the ground. The resistance of these equipments with earth is about 12 ohms.
       When there is an earth fault, then fault current leaks from the frame and passes through the earth connection provided. Thus the primary of C.T. senses the current due to which current passes through the sensitive earth fault relay. This operates the relat.
       Such a protection is provided only for small transformers. For the large transformers, the differential protection is enough to sense and operate for the earth fault.

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