Reversal of Transformer Phasing from Dyn11 to Dyn1

[SMA7875]

Hi,

Come across a situation where an old 33/11 Dyn11 transformer has to be replaced. The old transformer was connected R-Y-B to U-V-W bushings on 33kV side and R-Y-B to u-v-w on the 11kV side. The new transformer orientation is such that it is very difficult to connect R-Y-B to U-V-W bushings on 33kV but to W-U-V due to base size and clearances etc. We cannot change the 3kV bus bars due to cost. We can change cables on 11kV side as they are replaced. I am a bit confused whether this connection will work. I have been told that by doing this I will change the vector group to Dyn1 instead of 11 but I don't know how it works. Normally there are links on LV side to convert Dyn11 to Dyn1 but not on this transformer (its an emergency replacement). I thought if we change the 11kV cables according to 33kV side (R-Y-B to w-u-v instead of v-u-w) it will be in phase with existing but there is definitely something I don't know. Phasing has never been my strongest area. Your expert opinion requested. I am in UK.

 

[Marmite]

Connecting WUV as you describe should work as it retains the phase sequence. It's commonly called a "roll" in the UK. Do the same roll on both HV and LV sides. The way a Dy11 transformer changes to a Dy1 or vice versa is when you cross any two phases which changes the phase sequence, not when you roll them.
Regards
Marmite

 

[SMA7875]

Thanks for your answer Marmite,

Sincere apologies for a mistake in original post. When we rotate the transformer physically, only U and W bushings are swapped whereas V remains in the middle. This would mean a swap of only two phases or a transformation to Dy1 instead of Dy11. And I dont think we can do anything on the LV side to bring it back to Dy11 as we cant chnage the winding connections. So unfortunately this is not a roll anymore. The only option would be to ratate the transfomer in such a way that U-V-W can be connected to R-Y-B as before. Or am I missing something?

 

[dpc]

faq238-1154

 

[SMA7875]

dpc,

Thanks. I found the FAQ a few hours ago and went through it. It really helped me understanding the pahsor references, vector group conversion and roation.

All,

The above FAQ has a reference to Basler Electric Company paper (

http://www.basler.com/downloads/3phXfmrs.pdf).

I was trying to understand figure 11 for Dy11 transforer on page 13 but got very confused. The H and x representation looks OK but the UVW doesn't look right to me. Looks like V and W vectors need to be swapped unless I am missing something.

 

[sibeen]

SMA7875, here's something I did a year or so ago on the same question.