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Utility Power Source line impeadance calculatons

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tcmtech

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I am working with a customer on a project and we had a odd problem possibly come up.
What is the most realistic way to get a reasonable measurement or estimates of a utility power transformer secondary impedance? (customer side)

We are working with a 25 KVA 120/240 volt AC transformer with approximately 80 feet of 3/0 aluminum conductor going from one side of the transformer to the source and back again. 40 feet each line.

We are working with a 30 KW grid tie inverter and are getting it certified next week. I realize the secondary becoming the primary in this application will be responsible for some of it but still we would like to be able to factor in the impedance to the equation.

Being I work with Grid tie inverters of lower powers I have not needed to ever factor active line impedance into a system design. I simply have not had a GTI capacity that could push enough power back to get the line voltages to raise more than 2% at most at my farm.

But for certification and production references we need to be able to calculate what may happen with a large GTI hooked up to undersized wiring of long lengths with an undersized power transformer. That way we can simulate the condition and make possible control program adjustments to limit current and voltage if the GTI recognizes a problem like this while feeding back.
 

Hero999

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Contact the power company, maybe they may be able to help you?

They'll probably charge you though so maybe not.

I've never done this before, I would have thought that providing the current doesn't exceed the rating of the wires and transformer, it doesn't matter. Therefore you should limit the power you put into the grid to 25kW.

You probably know all of this, sorry I couldn't be more helpful.
 

tcmtech

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Its Okay. Unfortunately I have never worked with utility line men or service installation people that could do much more than basic math.

The one guy I am working with on this project is a micro processor and small digital control type electronics wizard but thus tends to over analyze the power side of things. As a digital control person he is used to having to include every variable in an equation in order to make something work.
I am an analog power person. I dont worry about the little stuff until it actually becomes a problem.

He raised an issue I have yet to actually need to deal with but still I would like to know if there is a simple way to get a rough estimate of the possible impedance of the system on our side of the power transformer.

Some how he came up with a .1 ohm equivalent but I have yet to fully understand the way he came up with it.

If he is correct there should be about a 10 volt rise on the system at 100 amps while pushing some 24 KW back at 240 volts.
 
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