Getting the Source Impedance from Utility

[Murloc]

For some reason, when I try to ask for the source impedance from the utility company(Entergy), they keep telling me they cannot give the info because of liability reasons.

However, i need to know that info in order to get the lower bounds of my arc flash calculations.

the sometimes provide - maximum phase to phase short circuit fault, however a minimum is needed for proper protection.
** a min value could be used to calculated Zsource.

Ibf-min = Isc-min = FLA/(Zsource+Zxfmr)*100

Log(Ia-min) = 0.662*Log(Ibf-min)+0.966*V+0.000526*G+0.5588*V*Log(Ibf-min)+(-0.00304*G*Log(Ibf-min)+K)

How do you guys typically deal with this issue?

 

[David Castor]

You have to make a pest of yourself and find someone in Protection who knows what arc flash is and has access to their computer model. By this point, most utilities have people who understand the issue and will provide the actual fault current along with appropriate disclaimers. If you start asking questions about relay settings and fuse settings you are more likely to get to the right person. As a last resort, you can make some reasonable assumptions based on your transformer size and impedance.

 

[Murloc]

You have to make a pest of yourself and find someone in Protection who knows what arc flash is and has access to their computer model. By this point, most utilities have people who understand the issue and will provide the actual fault current along with appropriate disclaimers. If you start asking questions about relay settings and fuse settings you are more likely to get to the right person. As a last resort, you can make some reasonable assumptions based on your transformer size and impedance.

yeah it does seem, it is rather tricky to get info that's not just infinite bus. guess I'll have to be persistent

 

[pv_n00b]

You can ask for the 3L and SLG available fault current and the X1/R1 and X0/R0 ratios. Same information as the voltage and impedance but it seems like it's more available.

 

[gar]

230130-1656 EST

Merlon:

Measure the source impedance.

Measure no load source voltage. It would also be useful if you could get reference phase information from somewhere that is invariant when you load the transformer. Load the transformer with a known resistive load of some reasonable value. Obtain this loaded voltage, and calculate your source impedance. And if possible measure the phase shift.

.

 

[JoeStillman]

230130-1656 EST

Merlon:

Measure the source impedance.

Measure no load source voltage. It would also be useful if you could get reference phase information from somewhere that is invariant when you load the transformer. Load the transformer with a known resistive load of some reasonable value. Obtain this loaded voltage, and calculate your source impedance. And if possible measure the phase shift.

.

Have you done this before? I'm intrigued.

 

[jim dungar]

230130-1656 EST

Merlon:

Measure the source impedance.

Measure no load source voltage. It would also be useful if you could get reference phase information from somewhere that is invariant when you load the transformer. Load the transformer with a known resistive load of some reasonable value. Obtain this loaded voltage, and calculate your source impedance. And if possible measure the phase shift.

This is not likely going to be possible on a utility owned transformer and distribution network where you do not have control of the voltage nor other loading.

 

[gar]

230131-1148 EST

Your comments indicate that you do not have a good understanding of electrical circuit theory.

Sometime later I will come back and discuss this with you. It is very simple electrical theory.

.

 

[ggunn]

yeah it does seem, it is rather tricky to get info that's not just infinite bus. guess I'll have to be persistent

Infinite bus refers to the available fault current (AFC) to the primary, and it is not related to the impedance of the transformer. If you know the impedance and kVA of the transformer you can calculate the AFC on the secondary, and for a utility transformer the infinite bus model is usually fine.

 

[Michsora]

Murloc, we have never had this problem, although it sometimes takes time. Since the Utility source is critical for modeling the as built condition of the system under study, estimating the contribution would go against standard. I suggest to ask the owner who is their contact with the Utility, write an email requesting the information and CC the owner along. If the utility is charging the owner for power, then the owner has the right to request the data. Another option is to leave it in the hands of the owner to get the data. Typically when I submit a bid, the utility information is part of the requirements to be supplied from the owner, and the schedule is linked to when the data is supplied.

 

[ggunn]

Murloc, we have never had this problem, although it sometimes takes time. Since the Utility source is critical for modeling the as built condition of the system under study, estimating the contribution would go against standard. I suggest to ask the owner who is their contact with the Utility, write an email requesting the information and CC the owner along. If the utility is charging the owner for power, then the owner has the right to request the data. Another option is to leave it in the hands of the owner to get the data. Typically when I submit a bid, the utility information is part of the requirements to be supplied from the owner, and the schedule is linked to when the data is supplied.

FWIW, I have never dealt with a utility who refused to give me the kVA and either the %Z or the secondary AFC of one of their transformers. Any difficulty I have had was in finding the right person to talk to who could get me the information.

 

[Hv&Lv]

I don’t understand why it’s not given out.
I have someone call at least 3-4 times a month to get our data.
It’s so easy…

I go to the model, Input their wire, and *poof* there’s the value.

 

[gar]

230131-1710 EST

To continue my discussion.

You have a utility power source, various transformers, and distribution lines that feed you main panel. As a good approximation this can be replaced by an equivalent circuit of a voltage source feeding a series, internal to the transformer, inductor and resistance. This is the source to whatever your load is.

When your load is infinite ( no load ), then the voltage you read at your main panel is equal to the open circuit voltage of the source. So for discussion purposes assume a value of 120 V. We assume this voltage remains sufficiently constant during our tests. Observations during our testing will allow us to pick the results use.

Note: you do not need to turn off loads on the system during your experiments, you just need them to be stable ( unchanging ). Effectively they are just part of your power source impedance ( basically being in parallel with the source impedance ).

Now apply a known resistive load to your circuit, basically a known current load ( you can measure this current ), and measure the change in load voltage. The difference between this voltage, and the open circuit voltage is the voltage across the internal resistance of the source assuming negligible internal inductance. If there was series inductance of a significant amount compared to the internal resistance, then there will be a significant error in this means of measuring internal resistance.

You get an answer to this by using two different load resistances. In many cases you will be able to ignore internal inductance.

If you believe internal inductance is big problem, then do a second loading with a load resistance that is much lower, and compare the results.

If inductance is a major problem, then additional techniques are required to determine the magnitude of the inductance.

.

 

[David Castor]

It's a fairly common complaint from engineers doing arc-flash studies. Traditionally, requests for information on available fault current were driven by determining the required equipment short circuit ratings, so utilities routinely provided extremely conservative, often unrealistically high current values to make sure the customer bought equipment with sufficient rating to cover possible transformer upgrades in the future.

But with arc-flash, using "conservative" values or the infinite bus approach is not actually always conservative. Utilities are concerned about liability that might develop if there was an arc-flash injury on the customer's site. In general, most utilities understand the issue by now and will provide the data with the understanding that it is subject to change and they assume no liability, blah, blah, blah.