Ground Fault Current

[pithpal]

Hi,
Plz advise a rough guess, any possible number which could probably match the exact figure. What will be the value of ground fault current on a 44 KV line where the substation transformer is star grounded. I know this might be a stupid question but I need a rough idea.
Thanks

 

[davidbeach]

Like most questions of this nature, the answer is "It depends". There is no where near enough information given to even come close to an answer.

 

[Dumbo2929]

I would agree that this is not even close to enough information. You could use the infinite source method on the primary side and calculate for a fault on the secondary side of teh transformer making several assumptions.

 

[mpparent]

pithpal,

So...where is the fault?

Mike

 

[mykh]

It really depends on power system configuration. You may try SC MVA method for short circuit analysis described and implemented at

http://www.arcadvisor.com

to get an estimate of ground fault current value on the 44kV line.

Apparently, ground fault current value will depend on type of grounding - the higher grounding resistance the lower ground fault current. I said "an estimate" because you'd have to have most accurate system data if you are expecting accurate ground fault current values.

If you have approximate input data, you'd have to consider your input data error and make an adjustment for the data error at each calculation step. Apparently, commercial short circuit analysis software don't necessary consider input data tolerances. Neglecting input data tolerances is very shaky business and can impact results that arise from measurements and/or calculations in surprising ways.

You should also be cautious of disclaimers saying that company doesn't take responsibility, expressed or implied warranty for accuracy of produced results, disclaimers stating that actual measures could be different from the given results, software assuming certain amount of load, power factor, operating on typical input values, and nevertheless producing results with three or more digits after decimal point.

 

[rbulsara]

Assume a low short circuit MVA of 100 and high of 500. This will give you the lower and higher limits...rest is anybody's guess. This assumes solidly grounded system, and fault on secondary side of the substation xfmr.

 

[jghrist]

If you know the source transformer size and impedance, the maximum ground fault current can be calculated by:

I = kVA/[kV·Zpu·sqrt(3)]

For a 10 MVA transformer with 8% impedance:

I = 10000/[44·0.08·1.732] = 1640A

This assumes a stiff system (low impedance) on the high side of the source transformer.

 

[mykh]

jghrist's calculations make much more sence then previous contributor deliberate estimate of 100 to 500 SC MVA for lower and higher limits and stating that rest is anybody's guess.

 

[rbulsara]

mykh:

I beleive jghrist was only providing an example for calculation. The calculation method of jghrist is correct for a given transformer unit and for a fault on its secondary side. In most cases the L-G current is close to or little more than a 3 phase fault current. The limits I suggested are for 3 phase symmetrical fault MVA.


The 1640A amounts to 125MVA (for 3 phase), very much in line with low end of what I suggested (Its not me, its the normal range found in the USA at MV)

And why would you assume a 10MVA tx? What if it is a 20MVA? or 5MVA? The fault MVA will double for 20MVA and cut in half for a 5MVA, in that example

Guess is a guess. Those limits are still valid, and normally quoted by utilties in USA for distributioin system at medium voltages.

It also depends what you need this information for. For relay setting you need more accurate data. As for the equipment (switchgear etc) rating goes, 500MVA is the minimum SCCR rating required by utilities in the USA and I beleive that is the minimum most manufaturers build to.

I would further clarify, 500MVA for 13.8kV-22kV systems. But one can easily look up manufactures catalogs for 44kV class equipment.

 

[mykh]

I find that showing computational example is more helpfull than assuming 100 to 500 range of possible fault currents. What if trasformer is rated 500kVA 6.5% impedance with infinite short circuit current on primary available? The available short circuit MVA would be around 50 SC MVA which substantially lower than indicated minimum of 100 MVA. I agree that the 100 to 500 SCMVA gives rough estimate although I think prospective fault current beyond the 100 to 500 SCMVA limits are still possible. I also agree any practical application calls for more data and require more detailed analysis.

 

[rbulsara]

mykh:

You are missing the point...

The fault you are calculating is based on a transformer rating, assuming "Infinite" bus on the primary side. If you are going to assume a "infinite" bus then there is no need to seek information for the available SCC!!!!!! (on the primary side).

Infinity is infinitely larger than 100 or 500MVA. If you want to use any realistic data that will lie between those values, which is quiet a narrow band than infinite.

 

[mykh]

rbulsara,

my point is that ground fault SC MVA may very well be beyond 100 to 500 SC MVA range.

Ground fault SC MVA value on trasformer secondary is limited only by the transformer positive sequence impedances provided the transformer is connected to infinite buss.

I think 100 to 500 SC MVA estimate sounds rather like

a rough guess, any possible number which could probably match the exact figure

 

[davidbeach]

Come on guys, the question, as asked, can not be answered. The OP doesn't bother to include relevant information such as source impedance, grounding impedance, location on line, other sources, etc., etc. The only answer the available information supports is that the ground fault current will probably be greater than zero.

 

[mykh]

I agree!

Thanks, David

 

[rbulsara]

Well, the original question itself asked for a "rough guess" and he got it!!! He original poster apparentlu does not know the transformer size and hence he asked for a guess.

I am not sure what mykh is talking about.The 100-500MVA is realistic available SCC on 44kV lines.

The question does not even say 44kV is primary or secondary voltage. And if one knew the transformer size, there wille no need to guess.