Neutral Ground Resistor Calculation for Transformer

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 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

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. sy stem. 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 usi ng the additional transformer having its neutral grounded. When ground fault occurs in HRG system, the monitoring systems gives alarm and the plant pla nt 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 current you would like to limit with the Neutral Grounding Resistor for your system. For LRG, it is 25A 2 5A 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 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 200 A at least for 10 seconds. Hope, it helps. - MS