Earthing Resistor Calculation

iT IS ISMILAR TO THE GROUND RESITOR CALCULATION FOR TRANSFOMER

THE TYPICAL EXAMPLE WAS ANSWERED EARLIER FOR TRANSFORMER DT.19-06-2009

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

Typical shunt calculations fro 5 MAV 11/6.6 kv transformer 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

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

Can you please mention the basis of above statement?

It may be noted that,437.38 A is the nominal current of the 5 MVA Transformer on 6.6 kV side.The fault current would be much higher.

Is it advisable to allow the nominal current to flow through the Neutral of the Transformer for 10 seconds under Earth fault condition?

There are primarily three types of grounding system which are:

(1) Solid grounding – The neutral point of the system is grounded without any resistance. If the ground fault occurs, high ground current passes through the fault. Its use is very common in low voltage system, where line to neutral voltage is used for single phase loads.

(2) Low Resistance grounding (LRG) - This is used for limiting the ground fault current to minimize the impact of the fault current to the system. In this case, the system trips for the ground fault. In this system, the use of line to neutral (single phase) is prohibited. The ground fault current is limited to in the rage from 25A to 600A.

(3) High Resistance Grounding (HRG) - It is used where service continuity is vital, such as process plant motors. With HRG, the neutral is grounded through a high resistance so that very small current flows to the ground if ground fault occurs. In the case of ground fault of one phase, the faulty phase goes to the ground potential but the system doesn't trip. This system must have a ground fault monitoring system. The use of line to neutral (single phase) is prohibited (NEC, 250.36(3)) in HRG system, however, phase to neutral is used with using the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant operators start the standby motor and stop the faulty one for the maintenance. This way, the process plant is not interrupted. The ground fault current is limited to 10A or less.

There are other two types such as Corner Grounding (for Delta system) and Ungrounded system but they are not commonly used.

Now, return to your question. The important parameter that is missing here is amount of current you would like to limit with the Neutral Grounding Resistor for your system. For LRG, it is 25A to 600A, while for HRG, it is less than 10A. I assumed your system is LRG and you would like to limit it to 200A (one of the common values in use for LRG).

The line to neutral voltage is 11,000/1.732 = 6,350V

The resistance value is 6,350/200 = 31.75 ohm or 32 ohm.

You have to select the resistance so that it can withstand 200A at least for 10 seconds.

Hope, it helps.

- MS

hi msamad,

we have a transformer Wye Delta 11, 420 / 33,25 kV, 125 MVA.

and we use it with the additional transformer having its neutral grounded system. max 600 A

my question 1 is what shall we use for neutral ground overcurrent coordination, inverse time or definite time ?

we use definite time at this transformer too but all faults very turbulent, busbars strong swings. have u got any idea for this ?

P.S. we always use definite time for neutral ground overcurrent coordination at low resistance grounding system.

question 2: can it (two transformer, 125 MVA) work parallel ? We try it but scared. 2 bundle conductors coherent and extremely noise.

sorry for bad english,

thanks ahead your answers

best regards

hello all master

we have trouble in in ground fault

Transformer rating 2 MV

voltage rating 0,4KV / 20 KV

Vector group Ynd5

no NGR

i want to ask all master in this forum about my design, every month i have ground fault in my sistem and the problem is all low voltage damage due this problem like deepsea, deif and other equiptment.

i want to ask

1. should i use NGR ?

2. went i use NGR how to calculate the NGR because went i calculate base on master nascon wrote it not imagin on my mind.

thank u

need ur sugestion