X'mer

This question has been answered before (a number of times).

Please see the previous threads on CR4 regarding "Neutral Earthing Resistors" and "Neutral Grounding Resistors".

I think NGR is for Natural Gas Robot or Next Generation Recycling or it is Nigerian

Errr''' No….. I think is some other thing.

NGR mean Neutral Grounding Resistor.

Neutral Grounding Resistors are designed to provide safety to power distribution systems by limiting ground fault current to reasonable levels. It is solidly grounded the three phase system directly to earth. Earth faults can cause high ground fault current typically 15,000A to 40,000A and cause excessive damage to transformers, generators, motors, wiring, and equipments. By using a Neutral Grounding Resistor between neutral and ground limits fault current to a safe level about 25 to 400 amps while still allowing sufficient current flow to operate fault clearing relays

'How to select the NGR rating?'

To be done by qualified electrical consultants. Pay them and they will do it for you.

"Why using this more the 6.6kV transformer?"

Have you see any NGR for Low Voltage transformer.??.

No i can't see any NGR for low voltage transformers..

Well, electricity is quite complex system. There is a relationship between the Voltage and the Current. For low voltage equipment, like transformer, the current will be very high and of course, the voltage is low. When there is a fault in the system, the high current will flow to the earth with the low voltage. Of course, when over current occurs, normal MCCB or ACB can break off these faults.

In medium or high voltage system, generally from 3,3KV to 275KV system, usually the current will be low, but when the faults occur, it is the high voltage that needs to be limited flowing to the ground. That why, the Neutral Grounding Resistor/Reactor is used to limits the faults level to a safe level, where it does not cause danger to our equipment that grounded to the same earth.

All the electrical equipment in the substation or in the plant is earthed to the ground or the same earthing point. If we don't limit the faults level, the high voltage will flow to the earth and back to our equipments thru the earth system. Those who touch the equipment body will get electrical shock and these faults level will damages our equipments.

That is why for medium or high voltage system, NGR will be install to limit the faults level.

Neutral of transformer can be gorunded solidly earthed OR with Neutral Grounded with Resistance.

Typical shunt calculations fro 5 MAV 11/6.6 kv transfomer neutral are as :_

Transformer rating = 5 MVA

Voltage ratio = 11/6.6 KV

Vector Group = Dyn11 (6.6 KV ground through Resistor)

During Earth fault voltage between Neutral & Earth = 6.6/√3 = 3.81 KV

Earth Fault current will be limited to = 5 x 10³ /(√3 x 6.6) = 437.38Amp.

N.G.R. value in ohm = V / I = (3.81 x 10³) / 437.38 = 8.71 Ohm

Value of N.G.R. is 437.38 Amp, 8.71 Ohm., 10 Sec

NGR are inserted On Higher voltage to restrict earth fault current

Thank you

I have a transformer with reatings 31.5MVA,33/6.6kv.but the NGR rating is 250A,15ohm,30sec....pls tell me how the NGR ratings is selected as 250A,snce according to your equation I didnt get it.?Thank you

Thank you

NGR is neutral grounding resistor.

The typical calculation detail is in traling mail for your ready refrerance

It depends on the down steram application to have solidly earthe dneutral or to add grounding ressitance in series with neutral earthing.