Three Winding Transformer Factory Test Missing X-Y Results 1

[SSLA]

Folks:

I have a three winding transformer, DYY, and need to do some load flow calculations. The T&D book and Blackburn show how to convert the factory test results on winding pairs H-X, H-Y and X-Y into an equivalent Y configuration that can then be analysed. But the factory test report shows only tests for windings H-X and H-Y.

Are there any rules of thumb that can be applied for estimating the X-Y impedance when none is given?

Does the lack of information suggest that the transformer is to be applied only in situations where the secondary and tertiary windings can never operate in parallel?

 

[prc]

If you give the rating of trf (all 3 circuits ),whether auto or two winding,voltage of H,X and Y ,impedance of HX and HY on correct MVA base ,I can reasonably accurately guess XY impedance .I dont think there is any formulae to arrive at XY value fromn other two values

 

[SSLA]

prc:

Thanks for the offer to help! Here is the info:
Connection: D-Y-Y
Rated KV: 220.00-14.00-14.00
Rated MVA: 60-30-30
Z1 H-X=0.4840+j17.6100 % on 30.00 MVA 220.00-14.00 kV
Z1 H-Y=0.4780+j17.9100 % on 30.00 MVA 220.00-14.00 kV

Core form build in 1967. Not an auto-transformer: it has three separate windings. Off-load tap changer on H (220.00, 214.50, 209.00, 203.50, 198.00). Underload tapchagers on the X and Y (+/- 2.1 kV in 24 steps) but my analysis will be on nominal taps.

Once again, thanks for any assistance you may be able to offer!

While doing research this morning I bumped in to a what might be a rough rule of thumb: page 100 of the Whse T&D Book seems to indicate that when specifying three winding transformers ZHL = 1.10*(ZMH+ZML) or in our terms Zhy = 1.10 *(Zhx+Zxy). Given Zhx and Zhy, one could solve for Zxy. Do you know if this guideline might be useful in this case? Would it agree with your estimate?

 

[prc]

SSLA ,
Your case seems a special case and there are some thumb rules for estimation.

T& D book case is for winding radial arrangement of core- L-M-H ,In your case the windings must have been arranged two LVs axially split with one above the other .For what duty these transformers are used ?Is there a tap changer for this trf.If so which winding anf how much variation?

With the guessed winding arrangement the impedances will be ,
H- (X+ Y ) = H-X or H-Y /0.88 on 60 MVA base -ie both windings equally loaded case.
X-Y = 2.1 (H-X or H- Y ) on 30 MVA base
In your case above values will be 0.55+j 20 on 60 MVA base and 1.02 +j36.98 on 30 MVa base .
A transformer of above type can also be designed as X-H-Y radial arrangement .Then the no- load voltage of X will vary with the loading level of Y which most applications may not tolerate.

 

[SSLA]

prc:

Thanks for the help!

The transformers are intended for use at a substation, stepping down 230 kV transmission voltage to 13.8kV 3Ph-4W distribution. There are two transformers operating in parallel with each X winding connected to bus B and each Y winding connected to bus Q.

There is an off-load tap changer on the HV side of each transformer (220.00, 214.50, 209.00, 203.50, 198.00).

There are also underload tapchangers on the each X and Y windings: 14.0 kV +/- 2.1 kV in 24 steps.

Once again thanks very much.

 

[SSLA]

prc:

Test results for the sister transformer to the one we were talking appeared today. Once again the X-Y test is missing but the H-X and H-Y results are identical. This seems suspicious to me. I think it is a typo. Can the H-X and H-Y test results be identical? If so does it tell us anything about Zxy?

Here are the listed impedances:
Z1 H-X = 0.4780+j17.9100 % on 30 MVA 220kV
Z1 H-Y = 0.4780+j17.9100 % on 30.00 MVA 220kV

 

[prc]

SSLA:
To have exact values for H-X and H-Y values of impedance for these types of transformers is quite normal and in fact it should be like that if the windings of X&Y were manufactured properly.You can counter check these values at site by using Doble's Insulation analyser or by applying low volatages on HV by shorting X or Y .% reactance will be applied voltage / current.Values will be approximate only due to low voltages and current employed.