Three single phase pole tops in a 3 Ph bank

[ffont]

There is an application for a GrdY-GrdY bank using 3 single phase transformers 14.4kV-120/240 into a 24.9kVGrdY-208GrdY bank.

2 of the units are 100kVA, with an 1.8% Iz and both will have their 120V LV winding paralleled; the third transformer is a 200kVA, same voltages, 2.2% Iz but only 1 of the 120V LV winding is used for making the third leg of the secondary Y, while the other 120 LV winding is feeding a separate single phase load at 120V.

For voltage unbalance I need to compare %Iz on each of the 3 legs. Do I need to compare the 1.8% Iz value of the two 100kVA units with the half winding %Iz of the 200kVA part of the 3 ph bank? There will be time when the 1 ph load is very low.... but at other time the 1 ph load could be higher than 50kVA.

Given that the full winding %Iz of the 200kVA is 2.2%, what would be the half winding %Iz to use for comparaison?

 

[prc]

It depends on how the LV windings are placed with respect to HV- axially or radially.Most cases, % Z will be 2.2 % with each LV.

 

[waross]

It gets a little complicated.
With a short circuit on one secondary winding, the impedance of that winding will be double the impedance of both low voltage windings together. However, the impedance of the primary winding will be the same.
The 200 KVA transformer at 2.2% will be 1.1% on a 100 KVA base.
However if you consider only using one winding that will increase the impedance somewhat. You may accidentally have quite a good balance.
For a rigorous solution you need to know the impedances of both primary and secondary windings. I don't know but possibly someone else does.
Don't forget that the %impedance is only valid for short circuit calculations. For voltage drop, use the % regulation of the transformer. This will be lower than the % impedance and your voltage drop will actually be less than that calculated using % impedance.
The reason is that the inductive reactance is greater than the resistance. At normal loads and power factors the reactive drop is almost at right angles to the load current and the lesser resistive voltage drop predominates.

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

 

[prc]

Let me simplify: Impedance of HV-LV1 = HV-LV2 = 2.2( or slightly less up to 1.9 %) on 100 KVA base. Impedance of HV- LV1+LV2 =2.2 %on 200 KVA base.

 

[ffont]

These transformers are wound core, shell type ie the HV and LV windings are wound concentrically on the same bobbin.

In this case the HV winding is sandwiched between the 2 - 120V windings. This is a very common design practice to obtain a low % Iz for these lower distribution kVA sizes.

The inner LV winding has a smaller resistance than the outer LV since the number of turns is the same, but the diam of the mean turn is lower for the inner LV.

I am told that the outer LV has a noticeably higher reactance than that of the inner LV in a GrdY transformer because of different spacing between the coils (ie graded insulation for a GrdY unit vs fully insulated for a unit installed line to line).

I understand of course that the %Iz of each half winding will be calculated at 1/2 the nameplate kVA and needs to be doubled to be put on the nameplate kVA basis.