How do we measure earth fault current in Generator Transformer(20/420kv),Statio

How do we measure earth fault current in Generator
Transformer(20/420kv),Station Transformer(20/11 kv) and Unit
Transformers(20/11kv) for our plant system, since these
don't have any Neutral earth Fault relay connected to their
Neutral Earth?
But for Unit Service transformers(11/.433 Kv) we have an
Earth Fault Relay connected to its neutral earth.Why is such so?

Question Posted / shyamal raghav

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How do we measure earth fault current in Generator Transformer(20/420kv),Station Transformer(20/11 ..

Answer / a.p.nanjappa

I assume your generator rating is around 250-315MW.

GENERATOR TRANSFORMER IS YN-d1 or d11, 420kv power
transformer is provided with neutral bushing and solidly
grounded to earth system (U,V,W-N). YOU CAN MEASURE THE
EARTH CURRENT WITH CLAMP ON METER.

GENERATOR 20KV OUTPUT IS CONNECTED WITH lv side of
generator transformer by bus duct. there is a voltage
transformer, 2 winding y/y or three winding VT Y/y-
opendelts. This open delts voltage indicate the fault in
20kv system.

Genrator is provided with 90% and 100 % earth fault
ptortection.
Generator neutral is connected with generator neutral
grounding transformer. Generator neutral transformer
primary is connected to genrator neutral point and other N
is connected to earth. at this point ,You can measure earth
current by clamp on meter.
I assume you have a Generator circuit breaker.

Generator bus is 20kv. This sw.gr feed stn tranformer -
20/11kv system.This station 11kv feed the auxilary
transformers. Vector group of this stn transformer is D/d.

Also unit transformer is 20/11kv. this 11kv bus feed motor
and auxiliayr transformers. vector group is also D/d.

your medium voltage operates as isolated ground system

Unit service transformer is 11/.433 , thereforn vector
group is D/yn. your low voltage feeder need a nuetral.
therefore neutral is brought out. three phase 4 wire, and
230v-n .

please send the rating of transformer. we can explain you
more.