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

Mechanism of Wye Delta Starter?

2 Answers   ABB,

---

how water and air molecules are conduct the heat?

1 Answers  

---

what is the difference between relay and a contactor.

5 Answers   Siemens,

---

Why current transformer is earthed??

2 Answers  

---

which is the best institute for PLC,SCADA,DCS in hyderabad

0 Answers  

---

---

application s to shunt coil

0 Answers  

---

how many earthing required for ups, what should be the earth voltages

9 Answers   Juniper Networks,

---

if there are six generators (3 gas CATT, one Jenbacher o1 MW each, and two nos Diesel 01 no 01 mw other 0.65mw EDGs ( say), 3 CAtt gas set have identical pole pitch,01mw EDG and Jenbacher gas Set have different pole pitch, how their power connection can be done to connect a common bus bar during installation. in short, how different pole pitched generators power connection can be done to connect a common busbar system through generators incomer ACBs.

0 Answers  

---

In case you don't have coal and salt for earthing, how can you achieve good temporary earthing?

1 Answers   Focus Energy, NHPC, ONGC,

---

# 4 hi friends i m from electrical branch and seeking for tech. + apti. both papers of hpcl plz get it available as soon as possible my email id is sumit_lig189@yahoo.co.in

0 Answers   HPCL,

---

what is V.L.F. testing of power cable? do you know the voltage ratings, like for what rating and size of cable what rating of V.L.F. testing is used?

0 Answers   Reliance,

---

What will be the current rating of Main switch for 80 kVAR panel

1 Answers  

---