Answer / engr. l. o. osuji

Hi questioner,
Earthing T/F or T/Fs are not required in distubution S/S.
For example, here in Nigeria the distribution volage for
the T/F is either 11/0.415kV or 33/0.415kV detla-star.
However, in Transmission S/S where there are 132/33kV star-
delta connected T/F or T/Fs earthing T/F or T/Fs are
necessary. The earthing T/F is connected on the 33kV side
of the main T/F while the star-point of earhing T/F is
connected to ground to provide an artificial star-point for
the main T/F. This so because 33kV distribution ground-
fault current must flow back to the main T/F. Earth fault
protection for the main T/F must sense this current and
isolate the main T/F otherwise the earthing T/F will roast
i.e. burn.

Answer / engr. l. o. osuji

Further to my answer,note that a ground fault at the
distribution S/S must be sensed fast by the relay so as to
trip the disribution cb before the cb for the 132/33kV in
the Transmission S/S is trip by its relay as a back-up.

Answer / deep

in delta connected t/f winding has not neutral point to connect to ground and beacuse LG,LLG fault is sensed by zero sequence current so providing earting on delta side we use earthing t/f.

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