Can External Power System be two Thevenin equivalents

[judy]

I studied a substation which was connected to the power grid in two voltage levels. In order to separate the substation from the grid, I will need two different voltage level equivalent Thevenin circuits connected to the substation for study. How can I make one external power grid into two Thevenin equvalent circuits and connect them to 230KV and 6.9kV buses of the substation individually? (ETAP can't fault on two bus simutaneously. I have tried open the substation from grid and run the short circuit study individually on the 230kV and 6.9kV and got two kVA and X/R. But this method is not correct.)

 

[gar]

190201-1234 EST

judy:

I do not understand your question.

When the equivalent circuit for each source is reflected to its secondary and I parallel these two secondaries (is that what you want to do?), then I have two voltages and their internal impedances connected in series, hopefully in series opposition. The voltage difference, if any, and the sum of the internal impedances will determine the amount of circulating current.

These two parallel equivalent circuits in turn can be converted into a single voltage source, and single series internal impedance equivalent circuit.

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[judy]

Attached is the target substation

Attached is the target substation

I simplified the substation to make my question more understandable. Two power grids in the drawings are connected to 230kV and 6.9kV bus individually. But how to get these two equivalent power grids' KVA and X/R from one big power grid network?

 

[jim dungar]

But how to get these two equivalent power grids' KVA and X/R from one big power grid network?

Are you familiar with the Per-Unit system, where all values are referenced to a base set of values (such as power and voltage)?

 

[judy]

Jim, the model I created is based on p.u. Although all the elements are referenced to p.u., the model still has two connections at different bus to the power system.

 

[jim dungar]

Jim, the model I created is based on p.u. Although all the elements are referenced to p.u., the model still has two connections at different bus to the power system.

Just like any other system with paralleled feeders or co-generation.

 

[gar]

190201-2016 EST

judy:

Replace power grid 2 with a 6.9 kV power source and the series internal impedance of it.
Replace T1 with a 6.9 kV power source and the internal impedance of T1 looking toward T1.

Now you have two 6.9 kV power sources. Each has its own internal impedance. Parallel these two and now you have a single 6.9 kV source with an internal impedance equal to the two separate internal impedances paralleled.

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