Utility Impedance and Load Fkiw

[corvalan]

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.

 

[jghrist]

If you include the transformer, then you need to start with the minimum primary voltage from the utility to determine the minimum voltage at the load. The utility Thevenin impedance would not include the transformer. If you know the minimum primary voltage, the Thevenin impedance at the primary, and the transformer impedance and X/R ratio, it would be more accurate to include the transformer in SKM. This assumes that you are the only customer on the transformer, otherwise the load of other customers would contribute to voltage drop through the transformer.

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.

 

[waross]

Another reason to include the transformer impedance:
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
--------------------
"Why not the best?"
Jimmy Carter

 

[cranky108]

It is normal to exclude the utility impedance in arc-flash studies. But for motor start studies, you want to include it.

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.

 

[davidbeach]

The real world truth is that the utility voltage isn't what you think it is. Raise the utility voltage ahead of that source impedance to the point that you get the right low-side/service voltage during the load-flow case. This will also have a tendency to push up the fault current values and the arc-flash energy.

 

[bacon4life]

The utility impedance can very depending on how the utility has switched their system on a given day. Even the utility owned transformer is subject to being changed out with a different impedance transformer. Excluding the utility impedance from the SKM model may be appropriate only if you have done other calculations to verify that a change to the utility source impedance or utility voltage would have a negligible impact on the overall results.

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%.

 

[corvalan]

I think I was not very clear in my question because I am getting good answers but not in the issue in question.

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.

 

[HamburgerHelper]

You can have loads that are constant power, constant current, and constant impedance. I don't have SKM in front of me but in PSS/E, which is designed to analyze power flow, loads are defined this way. Utility impedance will determine how strong of a connection you have to the grid and how your voltages may drop as you add load and how you define the load will determine how the load reacts to the changing voltage. How much that matters for your study, I don't know.

 

[davidbeach]

I would include the impedance and then adjust the voltage on the utility side of the impedance so that the power flow gives the right answer. For instance, if you think you have a 12.47kV primary service, the utility may well be supplying something more like 13.0 or 13.2kV.

 

[jghrist]

I would not include impedance beyond the point where other customers are connected. If you do, the voltage response to your plant load will not correctly reflect the other loads.

 

[dpc]

I would include the Thevenin equivalent impedance for the utility source - wherever you have it in your model. The utility voltage can be adjusted to match what is actually being seen. Not sure why it would not be included, except for some NEC voltage drop calculations. In EasyPower, by contrast, there is no option to exclude the utility impedance in power flow, although the voltage can be adjusted. It's going to come down what you are trying to accomplish with the power flow calculation. "All models are wrong - some models are useful."

 

[krisys]

corvalan (Electrical)(OP),

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.

 

[juleselec]

In most cases, there is no reason to include the Thevenin source impedance of a utility grid as long as the assumption holds that either 1) the grid is far larger relative to the connected consumer (or generator), i.e. short circuit ratio (SCR) is high, and/or 2) the utility has access to a reasonable amount of voltage control in the vicinity of the connection point, e.g. reactive compensation, OLTCs, etc. That is to say, your connection point is relatively "stiff".

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.