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Grounding riddle No.8 – Why yellow phase of secondary of PT is earthed?

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  • #195

      I was going through 11kV schemes.For every phase R,Y,B there is an individual Potential transformer for the outgoing feeder.

      In that I found that the secondary of PT of outgoing feeder is connected in star and the Yellow phase is earthed and not the star point(the neutral). The general philosophy is to earth the star point i.e. the neutral but I wonder why they are earthing the Yellow phase and not the neutral.

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    • #1419

        The neutral is normally grounded on three phase four wire circuits. An exception is sometimes found with wye-wye connected voltage transformers on generator leads. The VT secondary neutral may be provided for connection to phase-to-neutral loads with the ground being on a phase (usually Y). This reduces the possible occurrence of VT secondary phase-to-neutral (ground) faults that would appear to be phase-to-ground faults on the generator leads. The three phase, three wire circuit is in common use today for VT circuits in metal clad switchgear. It is common practice to ground one of the phases, usually Y. This arrangement is used when it is desirable to eliminate coordination problems between primary and secondary VT protective devices for phase-to-neutral (ground) short circuits or phase-to- ground insulation failures. Because each mentioned fault in secondary side appears as two phase faults in primary side.

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