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Neutral ground resistor Calculation for Transformer

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 10 /(3 x 6.6) = 437.38Amp.

N.G.R. value in ohm = V / I = (3.81 x 10) / 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

There are primarily three types of grounding system which are:

(1) Solid grounding The neutral point of the system is grounded withoutany resistance. If the ground fault occurs, high ground current passes

through the fault. Its use is very common in low voltage system, where lineto neutral voltage is used for single phase loads.

(2) Low Resistance grounding (LRG) - This is used for limiting the groundfault 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 ofline to neutral (single phase) is prohibited. The ground fault current is

limited to in the rage from 25A to 600A.

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(3) High Resistance Grounding (HRG) - It is used where service continuity isvital, such as process plant motors. With HRG, the neutral is groundedthrough 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 faultyphase 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 itsneutral 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 notinterrupted. The ground fault current is limited to 10A or less.

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

Now, return to your question. The important parameter that is missing

here is amount of currentyou would like to limit with the NeutralGrounding 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 wouldlike 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 for10 seconds.

Hope, it helps.

- MS

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