Delta-wye to wye-delta reconfiguration?

[low1]

Good evening everyone.

I was hoping some of you may be able to help me with something. We have an excitation transformer that is in need of replacement. The existing transformer is a wye-delta 13.8kv to 630v. Somehow, the replacement was mistakenly ordered as a delta-wye. We have one group saying that they can just re-configure the windings to wye-delta, and it will work as intended. I'm not so sure that I agree. I've ran my own numbers and came up with a completely different secondary voltage. Is there something I am missing?

Thank you

 

[jghrist]

Each winding would have a turns ratio of 13800/363.7. Connect the primary at 7967 V Ø-n and the secondary would be 210 V not 630 V.

 

[low1]

That's what I come up with as well. I'm being reassured its fine but I don't believe it will work. I guess it'll come out in the testing. Thank you

 

[prc]

You have a Yd transformer with 13.8/0.63 kV. You ordered a Dy with 13.8/0.63kV.In case you change this second unit in to Yd by reconnecting the winding ends( that is not easy), the volatge ratio will be 23.9/0.364 kV (all LL voltages).So it will not work as intended.

 

[ScottyUK]

An HV Y connection is unusual on a generator excitation system: normally the generator star point is the only connection to earth, usually via a matching transformer and resistance. It is possible that the transformer you have may be usable for purpose, unless the HV star point was used as part of the earthing scheme. Do you have any detail of the rest of the system, i.e. the generator and its earthing package, the unit auxiliary transformer, etc. A single-line diagram would be helpful.

 

[low1]

Thank you.

Prc, reconnecting the winding ends is exactly what's going on. It's a dry-type transformer and all winding ends are exposed, so it's not that difficult to reconfigure it, and that's what's going on right now.

ScottyUK, the generator's star point is connected to ground via a neutral grounding transformer and a large resistor bank. The excitation system is an old toshiba static excitation system. This transformer feeds the rectifier bridge. the existing one is wye-delta. There is also a 13.8kv/115v sync transformer that feeds the AVR and provides the waveforms the firing pulses are synchronized to. I'm not sure how much more detail you would like, but just ask

 

[waross]

Can you float the secondary wye point and use the transformer as is?
If the original secondary was delta, you have no need of a secondary wye point. Just ignore it.
The rectifiers probably don't care what vector group they are fed from.

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

 

[low1]

For nominal voltages, using the transformer as is would work, however, this transformer is directly parallelled with a synchronizing transformer. It's my belief that by using the transformer as is, the phase relationship will be skewed. Both this transformer (we'll call it the bridge transformer) and the synchronizing transformer share a common high side. The bridge transformer goes directly to the rectifier bridge and the sync transformer goes into the AVR. The AVR uses the secondary of the sync transformer to determine when to fire the SCRs, so by having the bridge xfmr delta-wye and the sync transformer wye-delta, would the phase relationship not be out by 30 degrees? Or am I misunderstanding that?

 

[jghrist]

Standard US transformer connections have the high voltage leading the low voltage by 30°, regardless of whether it is delta-wye or wye-delta.

 

[ScottyUK]

Star-wound secondaries are common on AVR rectifiers. Westinghouse used it as a standard on many of their designs. The star point is left isolated and floating. You have the right angular displacement with both Yd and Dy which is what is required for timing of the thyristor firing pulses. It is unlikely that the AVR has sufficient intelligence to notice anything 'wrong' with the Dy transformer. You do need to match phase rotation though.

Curious: was the Yd transformer neutral point left floating on the original design?