X/R ratio

[MineGuy]

Hi,
I came across a short circuit study done by engineers of
Schd. Electric. It was contracted by our company to them.I read it and came across few things. They assumes 3 Phase X/R ratio of utility to be same as single Phase for X/R ratio. I know three phase ratio is used for calculating the asymmetrical component of contribution but what is the relevance of single phase X/R and how we can assume same as equal.

My other question, Single Phase to ground fault can only be greater than Three phase fault in case if fault occurs on the stator terminals of a solidly grounded generator.
These Sch, Electric guys had also Single line to ground fault current greater than Three phase on MCC bus. I have yet to simulate my self actually in SKM as I use ETAP but I asked this question here to get the concept.
Thanks

 

[mpparent]

Many times, the X/R ratios are not available and a conservative assumption is that the 3 phase and 1 phase X/R's are equal.

As to your second comment, the SLG fault current can also be higher at the secondary side of a delta-wye transformer. I can't comment of the SLG to LLL fault on your MCC. There's too little info.

Mike

 

[MineGuy]

Thanks Mapparent. I dont mean to spoil your weekend but when ever you have a moment, you can drop a word.


My question still stands what is the relevance of single phase X/R.

Could you advise me an example of a delta/Star secondary fault and then Three phase being smaller than Single phase.

When you asked about MCC, as far my knowledge is concerned MCC has nothing but starters. So how three phase current can be greater than Single Phase. Do you think that that particular case I came across refers to fault on motor directly and that is the reason because of a fault on motor, the three phase current might be greater than single phase.
Thanks

 

[dpc]

It's the zero sequence X/R ratio, not single phase X/R. It's used, perhaps, to try to estimate the maximum dc offset for a SLG fault.

You seem to be confusing "single-phase" and "line-to-ground". Faults can be either phase-to-phase or line-to-ground. I suppose either of these could be called a "single-phase fault".

At the secondary of a delta-wye core form transformer, the line-to-ground fault current can be higher than the three-phase fault because the transformer's zero sequence impedance can be less than the positive sequence impedance.

In the real world, the higher zero sequence impedance for cables and other conductors means that the line-to-ground fault current seldom exceeds, or even equals the three-phase fault current. But it is possible for faults very close to the transformer.

 

[jwilson3]

I'd like to add to DPC's comment, which is correct. In the case of a bolted single line to ground fault on the secondary of a delta-wye transformer, which equals
3xE/(Z1+Z2+Z0), Z0 only includes the impedance from the transformer to the fault. Primary side Z0 isn't reflected thru to the secondary as with a wye-wye transformer. If
Z1>Z0 then E/Z1 > 3xE/(Z1+Z2+Z0). E/Z1 is the 3phase value and the assumption is that Z1=Z2.

 

[jghrist]

Wouldn't the dc offset for a SLG fault be a function of the fault X/R ratio, not the zero-sequence impedance X/R ratio? The fault impedance is Z1+Z2+Z3.

 

[dpc]

For a single line-to-ground fault, the sequence impedance would be Z0+Z1+Z2+3Zf. The utility source impedance is a part of this overall impedance.

 

[jwilson3]

Response to dpc,
The utlity source impedance would be a component of Z1+Z2, but not Z0 as there is no current flowing in the ground loop on the primary side since the transformer is delta on the primary. In case of a bolted fault Zf would be zero.

 

[dpc]

jwilson,

Yes, you're right in this case. The zero sequence impedance can vary dramatically depending on the fault location. I should have been more explicit.

 

[cuky2000]

Regarding the first question, here are a few comments regarding the X/R ratio:
* The system reactance(X) and resistance (R) values are obtained on single-phase base.
* Typical range for utility X/R ratio is from 15 to 30.
* The X/R ratio is highest for generating units and transformers and lower for transmission lines.
* The positive sequence impedance X/R ratio increases with the system nominal voltage.
* In general, the positive to zero sequence impedance ratios are close for LV, MV and HV up to 138 kV and above 2 for highest system voltages.
* For installation with large inductive load, substation with large transformers, or close to generating plant, the X/R ratio is high.
* Protective devices are tested for a particular X/R ratio. If the actual X/R is exceeded additional analysis is require even if the available SC is less than the protective device rating.

http://cuky2000.250free.com/Protective_Device_LV_MF1.gif

Regarding your second question, enclose is a diagram to determine the highest SC current without doing the actual calculation with only the sequence impedance at the fault location.

 

[jghrist]

Doh! Where did I ever get Z3??

 

[dpc]

Maybe you assumed a four-phase system

 

[MineGuy]

Thanks Everybody for your valuable contribution. I was wondering, if any body who is into simulation could advise me about modeling a hoist for mining application in ETAP.
I would appreciate a word.

Thanks