fault current with neutral grounding transformer


Thread Starter


hi to all,

how can we get the fault current when we use a neutral grounding transformer with the ratio 11KV/110 V, loading resistor is 0.21 ohms?

with regards,
Responding to ch.thrimurthulu's Nov 19, 7:12pm query:

For some background info see thread # 1026227677 covering the application of a Neutral Grounding Transformer (NGT) for your application.

For the paramters you provided, ground-fault current is approx 3 A. To check adequacy of the resistor's kW-rating, as well as the distribution
transformer's kVA-rating, then the equivalent capacitance-to-ground must be known!

Let me know if additional info is required!

Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

FX Bambang Budi H

Dear ch.thrimurthulu,

Your can add 1. current transformer (CT) on the neutral side of the transformer, 2. protection relay to trip the neutral grounding transformer
when there is some ground fault on the electrical network.

hope this answer your question,
with regard,
FX Bambang
Reply to ch.thrimurthulu's query!
the turns ratio = 100(11kv/110). Reflecting 0.21 (loading resistor) to the primary = turns^2 * 0.21 = 2100 Ohms. (NGT is a "high resistace" neutral grounding method.) The fault current in this case will be equal to Vph/Rp = 3.02 Amps.

However as correctly indicated by Mr Corso you also need to have other data like NGT sec resistance, primary resistance values, equivalent capacitance-to-ground, transformer KVA rating, for a correct estimation. Hope you have got the answer to your query, else let me know, i will prepare a detailed explanation with sample values.

sumeet ,tata projects ltd.,patna

ok but by this way voltage became more across resistor when fault current occur. Why should not we use CT of higher number of turn ratio?
Sumeet, of course the current increases... it's zero without a fault. Is the meaning of your question, "Why not substitute a CT for the PT?"

Regards, Phil Corso