Utility Impedance and Load Fkiw
Utility Impedance and Load Fkiw
(OP)
In SKM software, the Utility contribution is modeled considering the nominal voltage delivered by the Utility and a Thevinin Impedance.
For load flow calculations, SKM has an option to include or not to include this impedance.
If we include it, the voltage drop from this Utility Thevenin Impedance could be substantial, (I.e. 2% to 3.5% or so).
Add to this voltage drop, the voltage drop of the transformer and we start with a high voltage drop (from the nominal secondary voltage) at the secondary of the service transformer.
Due to this fact, I do not include the transformer impedance in my low flow calculations.
Do you include it? If you do, why?
I have a customer that insists that the Utility impedance should be included but gives little explanation why.
For load flow calculations, SKM has an option to include or not to include this impedance.
If we include it, the voltage drop from this Utility Thevenin Impedance could be substantial, (I.e. 2% to 3.5% or so).
Add to this voltage drop, the voltage drop of the transformer and we start with a high voltage drop (from the nominal secondary voltage) at the secondary of the service transformer.
Due to this fact, I do not include the transformer impedance in my low flow calculations.
Do you include it? If you do, why?
I have a customer that insists that the Utility impedance should be included but gives little explanation why.






RE: Utility Impedance and Load Fkiw
If you know the minimum utility voltage at the secondary and the utility Thevenin impedance at the secondary, then it would be appropriate to not include the transformer in SKM. The minimum secondary voltage would already account for voltage drop through the transformer.
Either assumption would work. Neither will be completely accurate because you don't really know what the utility voltage is, only an assumption.
RE: Utility Impedance and Load Fkiw
The customer may not always be right but he is always the customer.
By the way, in addition to jghrist's comments:
The transformer impedance is applicable under the condition of a short circuit at the transformer secondary.
For voltage drop under load, with typical but unknown loads use the transformer regulation to calculate the transformer voltage drop.
For known loads use the load power factor plus the transformer X:R ratio or the transformer inductive reactance plus the transformer impedance to calculate the transformer voltage drop.
Ignoring the utility impedance will yield a nice low voltage drop figure but it will not produce an accurate forecast of the voltage dip under motor starting conditions nor will it show the lowest voltage that voltage sensitive equipment may be subject to.
Bill
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"Why not the best?"
Jimmy Carter
RE: Utility Impedance and Load Fkiw
This is the reason that as a utility engineer, I don't just give out system numbers to customers without having some idea what they want it for.
The lower voltage makes since because it would be when the system is at its weakest, and when you would have the greatest voltage drop.
The transformer impedance as it should not change (as a minimum), while the power system can change, be switched around, or operated differently.
RE: Utility Impedance and Load Fkiw
RE: Utility Impedance and Load Fkiw
anecdote- A customer with several very large motors was having occasional low voltage nuisance trips. The original power flow study they had performed included the utility feeder impedance but excluded the utility substation transformer impedance. This resulted in overestimating the steady state voltage by about 1.5%.
RE: Utility Impedance and Load Fkiw
The specific question is the following:
When performing a load flow study (not a short circuit study or arc flash study) in SKM (and I assume isnthe same in all othe software) do you choose to include the Utility impedance or not?
I do not. But I would like to hear from someone that does and the reason for this inclusion.
Thanks.
RE: Utility Impedance and Load Fkiw
RE: Utility Impedance and Load Fkiw
RE: Utility Impedance and Load Fkiw
RE: Utility Impedance and Load Fkiw
RE: Utility Impedance and Load Fkiw
It all depends on what is your bottom line?
How big the size of your network?
What is the scope of your study battery limit etc.?
All these power system study issues are like rubber band. Sometimes, the clients have tendency to stretch the issue beyond the practicality.
Utility connection has the Short circuit level, but for the purpose of calculation, the voltage is considered to be rigid as a normal practice. Further based on the source side modeling some voltage drops you might see.
David,
If you change the Utility voltage, I guess there could be some problems/confusion in the software generated reports. You may need to apply some corrections for the proper interpretations.
So to summarize, we should make such studies as simple as possible, unless some major generation is involved or the distributed generation (DG) are involved.
RE: Utility Impedance and Load Fkiw
However, the source impedance is worth modelling explicitly when you are connecting something big to a weak part of the utility network and it doesn't have much voltage support (e.g. large consumer at the end of a long radial distribution line). In this case, the connected consumer (or generator) can have significant influence on the voltage at the connection point.