Answer Posted / vrishali ajagaonkar

CRITERIA-IN ORDER TO PROTECT THE STATOR CORE FROM DAMAGE
DURING INTERNAL EARTH FAULT IT IS NECESSARY TO LIMIT THE
STATOR EARTH FAULT CURRENT AL LOW AS POSSIBLE.MAY BE UPTO
10AMP FOR GENERATOR.
BASIS FOR SELECTION OF NGTR-
IF GENERATOR IS CONNECTED TO THE SYSTEM THROUGH GENERATOR
TRANSFORMER THEN GENERATOR CAN BE TREATED AS AN ISOLATED
SYSTEM WHICH IS NOT INFLUENCED BY THE DISTRIBUTION EARTHING
SYSTEM. IN THIS CASE YOU CAN SELECT EARTH FAULT CURRENT AS
LOW AS POSSIBLE, BUT IF WE USE NGR THEN IN ORDER TO LIMIT
THE LOW EARTH FAULT CURRENT SIZE OF THE NGR WILL BE
INCREASED.DUE TO BIG SIZE OF NGR WE REQUIRED MORE SPACE &
ALSO ITS TOO COSTLY.

V = IR MEANS IF V CONSTANT I IS INVERSELY PROPORTIONAL TO R
EXP:-TO LIMIT 10AMP CURRENT , V =11KV/ROOT 3

R = 11/ROOT 3/10*1000=635.10 OHM ( VERY HIGH VALUE)

FOR OVERCOME ON THIS, GENERAL PRACTICE TO USE A NGTR SO THAT
WE CAN LIMIT LOW EARTH CURRENT WITHOUT ANY EXTRA COST.
EXP:- NGTR 11/240V
TO LIMIT 10AMP CURRENT
CURRENT REFLECT ON SECONDARY SIDE OF NGTR WILL BE
= (11000/ROOT 3/240)*10=264.6 AMP.
RESISTOR VALUE ON SECONDARY SIDE R =240/264.6 =0.9 OHM (
APPROXIMATELY) WHICH IS MUCH LOWER THAN 635.10 OHM.


BASIS FOR SELECTION OF NGR-
IF GENERATOR IS DIRECTLY CONNECTED TO THE SYSTEM THEN WE CAN
NOT LIMIT THE EARTH FAULT CURRENT UP TO 10 AMP. WE HAVE TO
SELECT THE EARTH FAULT CURRENT VALUE ABOVE THE RESIDUAL
CAPACITIVE CHARGING CURRENT OF SYSTEM.ALSO WE NEED TO CHECK
THE POSSIBILITY OF RELAY SETTING OPERATION WITH CT RATIO.
FOR EXMP- IF CT RATIO IS 2000/1AMP & EARTH FAULT LIMIT
CURRENT IS 100AMP RELAY SELECTED WITH 10 % SETTING.
THEN THAT RELAY WILL SEE ONLY 200AMP EARTH FAULT CURRENT IT
WILL NOT OPERATE ON 100AMP CURRENT.

ALSO WE CAN NOT KEEP GENERATOR SOLIDLY GROUNDED BECAUSE OF
HIGH EARTH FAULT CURRENT STATOR CAN DAMAGE SAME WILL HAPPEN
WITH HT MOTOR IF IT IS CONNECTED TO THIS SYSTEM.
SO FOR THIS CASE WE NEED TO SELECT THE NGR WITH PROPER EARTH
FAULT CURRENT ( ABOVE THE RESIDUAL CAPACITIVE CHARGING
CURRENT OF SYSTEM) TO AVOID UNWANTED TRIPPING.

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