Earth Fault Current Calcs

[dfdt]

Ungrounded 11kV system is grounded through a neutral earthing transformer(NET) with impedance of 63 ohms/phase and neutral earthing resistor(NER)of 21 Ohms which yields an earth fault current of 151 Amps if NET impedance is added to NER i.e.
3xVp-n/(63+3x21) but if I consider resultant impedance of NET and NER then i get earth fault current of 213 Amps i.e. 3xVp-n/(sqr root of [63x63+63x63]).
Please advice if method is right.
Please note to make it simple i have ignored source impedances.

 

[waross]

That is an approximation.
You have assumed that the impedance is completely inductive and ignored the resistance of the grounding transformer. There will be a small error. For an exact solution you must know the X/R ratio of the grounding transformer.
Normally the zero sequence impedance is three times the positive sequence impedance as determined by testing.
This is true if three distribution transformers connected wye:delta are used for grounding.
I don't know if this holds true for a transformer intended to be used for zero sequence current.
The question may be;
"Is the impedance of a grounding transformer reported as the positive sequence impedance or as the zero sequence impedance."
I don't know.
This will make a 3:1 difference to one term of your equation.
Others??

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dfdt]

Thanks Waross for quick response. The grounding trf is zigzag type with zero sequence impedance of 63 ohms per phase

 

[waross]

A close approximation:
Root([transformer impedance]²+[NER²])
or Root (63²+21²)
If the grounding transformer is tested for impedance voltage with the same procedure as a distribution transformer, then the zero sequence impedance will be three times the listed impedance.
That would make the equation Root ([63 x 3]² + 21²)
Hopefully one of the utility Gurus will post in on this point.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[prc]

213 A is the correct value,ignoring resistance of grounding transformer.Normally grounding transformer impedance will never be so high and usually (V/root3) divided by neutral grounding resistor (ohms) will give the fault current as the effect of grounding transformer impedance will be negligible when vectorially added to R of grounding resistor.

When we say 63 ohms (X) as impedance of grounding tarnsformer, it is the phase value and the value between the zig and zag windings in a phase and hence is the zero sequence impedance.It is the value ( X/3) that we will get if V is applied between shorted line terminals of grounding transformer and its neutral.