Answer / vishal i vadodaria

As we know that in Y connection there are 3 phases [R,Y&B] & a neutral.

In neutral we provide a resistance & than we used to ground it.Such system called neutral ground resistance [NGR].

Why we use NGR?

AS we know ideally the sum of current Ir+Iy+Ib is zero. So in normal condition the current which flow through neutral of Y winding is very low.

In case of occurrence of phase to phase or phase to ground fault heavy unbalance current will be generated & that will flow through neutral. To restrict this current we provide a resistance in neutral.

If we do not provide resistance in neutral & fault occurs at that time a heavy current will flow & it cause excessive heating in stator winding & it may damage the winding. to prevent this we provided a resistance in neutral.

Answer / 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.

Answer / rohit

Gud answer

Answer / x

very gud explanation
ur exactly right

Answer / kl

ans 3. Very good. Thanks

How to design a Unit auxiliary transformer for a power house having a unit of 65 MW and Generating voltage of 11 KV and Secondary voltage of 433V

0 Answers  

the total iron closs in a transformer core at normal flux density was measured at 25hz and 5ohz and was found to be 250w and 800w respectively.the hysteresis loss at 50hz would be................?????

1 Answers   ABB,

salam,hi, i need to know what the spesification of 300amp msb...

0 Answers  

What is meant by turbo alternators?

0 Answers  

Does the electricity supply authority send its customers power (kW) or voltape (V)? Asked by Ivan Muliro

2 Answers  

what is form factor.and rms value..

1 Answers   IIM, Torrent Power,

1.25 kg of hydrogen are burnt with stiochiometric air, the mass of air in kg required is

0 Answers   Caterpillar,

wat is mean by HRC fuse... WHY we choose it...?

3 Answers  

whatare the disadvantages occur,if the form factor of transmission will not be equal to 1.11

0 Answers  

What is the function of electron gun assembly used in crt?

0 Answers  

what is the advantage of using 3 phase line rather than using 2 phase line?

2 Answers   Havells,

800a siemens vcb in our company can not be closed electrcally an mechanicaly cc and oc coils are found ok. why?

2 Answers