short circuit study - transformer data 2

[tonyflair]

The software that I'm using to perform a short circuit study asks me the following information when I insert a 2 winding transformer:

KVA/phase, Z+(pu), Zo(pu), configuration, grounding impedance (ohms).

The transformers that I'm considering are 3 phase banks (single phase transformers connected to the desired 3 phase config.).

For example I have three 500 kVA single phase transformers, each with an impedance of 6.04 % (0.85+j5.97) and they are connected delta-wye (600 V - 25 kV).

When I enter this data into the program that I'm using, is it correct to say that,

1.) KVA/phase is 500 kVA (obvious)
2.) Z+ = 0.0085 + j0.0597

I was under the impression that for delta-wye transformers, Zo is the zero-sequence impedance to ground, seen at the wye-side of the transformer.

Is it correct to assume that Z+ and Zo should be identical for transformers made up of single-phase banks ?


If I don't know the impedance of the ground.....will this pose a problem ? Is it usually a critical value ?

Thanks in advance

 

[jghrist]

You are correct. Zo seen from the wye side will equal Z+ for banks of single phase transformers. The ground impedance is zero for solidly grounded wye; for resistance grounded, enter the ohmic value of the grounding resistor.

 

[Mstrvb19]

jghrist is correct, Zo and Z+ are the same for the transformer, but experience with load flow software indicates that the 'Zo' the software is not the intrinsic 'Zo' of the transformer, but a separate entity- It is looking specifically for the grounding impedance-i.e. resistance grounded or reactor grounded. The software uses the positive sequence impedance in series (to ground) with the zero sequence impedance when it creates the zero sequence bus matrix. The zero sequence bus matrix is only used for calculating unbalanced load flows and fault analyses other than a three phase bolted fault, so if you do not need these analyses then this value is not important. However, some load flow programs require a value in the field in order to create the bus matrices regardless of what type of analysis you are doing, so you might want to check the manual on the software and see if this is a "required" field.
Also- most people don't necessarily know the ground impedance, especially places that use cable trays and other structures for grounding, but the software is not looking for the impedance of the return path to the transformer, so you only have to know what the impedance is from the transformer windings to whatever ground connection you have. You can then take some ground impedance into account when you put zero sequence impedances in for your cables and that will complete the zero sequence circuit.