How to Size the Neutral Grounding Transformer


Thread Starter


I would like to know the calculation method to determine the actual capacity of grounding Transformer.

Transformer details for which NGT to be determined as follows

11 MVA,11/6.9 V, Delta-Delta Oil cooled.

NGT Shall be as follows
6.6 / (0.415/root3), 50Hz, 3 phase, Two winding, ONAN Earthing
Transformer with winding connection-
HT Side - Star
LT Side - Open Delta

Earthing Transformer connected on the LV side of 11 / 6.9kV (delta / delta) oil filled power transformer, shall be suitable for locating outdoor. Earthing Transformer shall be of star- open delta configuration. The star connected primary winding of the earthing transformer shall be directly connected to lower voltage terminals of the power transformer. The neutral point shall be brought through air bushing insulators and directly connected to earth.

Earthing Transformer with star - open delta configuration, shall be utilized for providing artificial neutral point for the 6.9 kV side. Open delta configuration shall permit the insertion of a resistor in order to adjust the zero sequence impedance.

Zero sequence impedance of the earthing transformer shall be subject to a tolerance of +20%, -0%. The earthing transformer shall carry the specified neutral fault current for the specified duration without exceeding the winding temperature by 2500C.

The earthing transformer with external resistor to limit earth fault current, shall be able to withstand dynamically and thermally the maximum earth fault current without external resistor for a period of 5 secs.

Thanks & Kind regards
Vashim Bhari
Responding to Vashim's 01-Apr-09 (10:32) query:

The starting point is to determine the 6.9kV system capacitance to ground.

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Regards, Phil Corso ([email protected])