Utility Information

[tin2779]

Hi,

Utility provided me with a Three phase fault level, zero sequence impedance and 3 Io for a new transformer connection. Why this three with ground fault current.
People from utility can put a word.

Thanks

 

[davidbeach]

Could you rephrase the question? I'm not really sure what you are asking; but it looks like you should be glad that they didn't give you just the 3-phase fault value.

 

[tin2779]

Utility guys are very hard to deal with.

My question is, I asked for ground fault current and three phase fault current....

I understand the three Phase but instead of Io they provided 3Io. We are in the preliminary stage of our design and need to set our Equipment withstand(Breaker) capability and grid design.

They also did not mention if this half cycle, or 3-5 cycle value.

Could you advise how to interpret this information.

Thanks

 

[BJC]

3I0 is the ground fault.
If you do a symetrical fault fault calculation the standard termination for zero sequence current is 3I0 represented by three identical vectors.
The fault current is represented by adding a set of three identical vectors at 120degrees rotating clockwise to a set of three identical vectors at 129 degrees rotating counter clockwise and adding three IO vectors.

 

[tin2779]

Thanks BJC.

Is this rms half cycle, 5 cycle or 30 cycle value ?

I need half cycle for my ground grid design and equipment withstand capability.

Thanks

 

[mls1]

Regarding if the data is 1/2 cycle or 3-5 cycle, it's both. Utility data is considered stiff for analysis purposes so the ultimate source on the other side of the equivalent impedence is an infinite bus.

In what form did they provide the fault current? From what you wrote it looks like you don't have sufficient information to determine the X/R ratio. If the Zero sequence impedance is in X and R you have it for SLG faults but you'll still need it for 3-ph faults if you are to calculate the assymetric fault currents.

 

[SilverArc]

It is a good point actually, I never wondered utility information is what cycle value.
I think most of the utilities use PSEE like packages for running load flow or short circuit information. and When
a customer asks for a short cicuit level...what value of current they provide ?

If they provide half cycle, technically they should not provide X/R because a half cycle already has accounted for that value.


I guess this was just an obersvation...people who work in utilities will comment better..
Cheers !

 

[rbulsara]

Read BJC's response one more time. Those values provided by utility co are "symmetrical rms" values. You figure asymmetry based on X/R ratio if you know it or assume a reasonable one such as 12 to 15.

Equipment rated for symmetrical fault levels account for asymmetry per ANSI/IEEE or other applicable standards or some multipliers. Refer to its full spec or you need to contact the manufacurer's technical support.