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I would like to have comments on the difference of phase sequence which could occur in transformer. Here I have 2 transformers of different make but of same rating, impedance and vector group (Ynd1). However the phase sequence of the two transformers are different whereby Transformer #1 has the phase sequence of R-Y-B and the Transformer #2 has the phase sequence of B-Y-R. I tried to put in form of drawing as shown below. Hope it could make things clearer ;)

Transformer #1:

From generator on left to transformer and grid on right:

IPB LV(r)---- [xmer lv (r)]-----[xmer HV(R)]--- grid (R)
IPB LV(y)---- [xmer lv (y)]-----[xmer HV(Y)]--- grid (Y)
IPB LV(b)---- [xmer lv (b)]-----[xmer HV(B)]--- grid (B)

Transformer #2:

From generator on left to transformer and grid on right:

IPB LV(b)---- [xmer lv (b)]-----[xmer HV(B)]--- grid (B)
IPB LV(y)---- [xmer lv (y)]-----[xmer HV(Y)]--- grid (Y)
IPB LV(r)---- [xmer lv (r)]-----[xmer HV(R)]--- grid (R)

I would like to know on whether these two transformers are interchangeable without any modification. Lets assume the connection to the IPB, height, weight and other structural matters does not have any problem. I believe that should the change is done, the synchronization of the generator to the grid could not be performed due to the different phase sequence. Appreciate any comments and views on the matter as different people have different opinion on the matter.

Sridaran... to eliminate the possibility of misinterpretation can you send drawing or sketch?

regards, Phil Corso (cepsicon [at] aol [dot] com)

Hi Phil,

Just sent you the drawings. Hope its useful ;)

After a few simple test, it is found that if the transformer with the different marking is installed, the vector group will changed from Ynd1 to Ynd11. The phase sequence remain the same as CW rotation. Can this transformer be synchronized to the grid and run parallel with my other Ynd1 transformers on the HV side?

Sd... the answer is NO!

For the Ynd1 xfmr, the lo-voltage (delta) winding's phase-to-centroid vector, LAGS the hi-voltage (wye) winding's phase-to-neutral vector, by 1x30 or 30 deg.

Conversely, for the Ynd11 xfmr, the lo-side LAGS the hi-side by 11x30 or 330 deg. (It can also be said lo-side LEADS hi-side by 360-30 or 30 deg.)

Regards, Phil Corso

I'm very sorry for the very late reply.

The transformer in question was finally commissioned successfully last year. The generator was also synchronized successfully without any problem. No modification was performed. Only precaution exercised was the LV board of the unit was not synchronized with the other unit's LV board.

It was later taken off service and replaced with a permanent transformer with the same winding configuration with the initial transformer. ;)

Sd… thanks for the update on your successful transformation!

Can you confirm that cable, and not bus-bar was used on both the LV-side and HV-side connections?

Regards, Phil Corso

Dear Phil,

The LV side was connected to isophase bus ducts. The HV side was connected via lead wires from HV bushing to the cable bushing.
None of the LV/HV phases were changed (i.e. swapped).