Transformer Current Distribution 1

[Cerkit]

Hi,

I am looking at a star-delta (HV/LV) transformer configuration.

Please can you correct me if wrong on the following:

For a phase-phase fault external to the transformer on the LV side (delta side) with the source being on the HV side, the current distribution in the windings should be as I have drawn attached.

Thanks

 

[Cerkit]

Hi. But I am referring to the Star windings, my modelling shows no current in one phase in the star winding.

 

[AndreyIli]

Hi, I think your model is wrong, because it is classic scenario, when double phase current on LV side appears in all tree phase on HV side in Y/D transformer. I think good way for calculating is transform phase currents in symmetrical components, turn them depending on windings group and then calculate phase currents on HV side.

 

[waross]

Are you modeling a grounded wye or a floating wye?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

Sorry for the delay.
The attached sketch of 12-04-2019 2:54:00 PM it seems to me correct except the transformer impedance has to be 11.5% instead of 10%.

 

[waross]

Hi 7anoter4;
My experience has been with single phase loading of the delta secondary of a grounded wye:delta bank and with generator delta windings., not with line to line faults on a delta.
I have had to evaluate the capacity of wye delta transformer banks with combined three phase loads and single phase loads on one phase (Four wire delta, 240:120/240 Volts).
There is also a similarity with the single phase loading of a delta generator winding.
In both cases single phase loading causes equal currents to flow in all windings of the delta.
If a line to line fault causes a phase angle shift that results in different current distribution than heavy single phase loading, then I may be in error, but not for heavy loading.
Remember that the load current through the unfaulted phases will be at 50% lagging in one transformer and at 50% leading through the other transformer.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

Thank you waross for your comment. You’re right. What is difficult in this calculation it is the fact we don't know the actual impedance of the primary separate from secondary but a bulk impedance which facilitate us to calculate the outgoing current only. Usually we don't need more. The inner windings’ current it is not important actually.

 

[waross]

Hello 7anoter4;
I am more concerned with the current split than with the actual currents.
For single phase loading on a delta, the current split is 1:1. not 2:1.
Paradoxically, The load in kilo Watts will split in the ratio of 50% 25% and 25% while the load in KVA will be 50%, 50% and 50%.
That is; with a 100 kW single phase load on one side of a delta, each transformer will produce 50 KVA while the kW loading will be 50%, 25% and 25%.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Cerkit]

Hi,

On the same theme. If I have a R-G fault in the attached is my current distribution in the CTs correct?

Thanks

 

[Cerkit]

Apologies the attached above was not very clear I have attached a more clear drawing. Is my assessment correct? If not can you please give some guidance on where I am going wrong?

Thanks