Vector group

[jasl]

I have to make a check to find the suitable vector group for a main transformer. Whether Dy11 or Dy0 ? What studies would you experts recommend? Also, could someone say any added advantage of using a Dy0 transformer other than using it for appropriate phase displacement?

 

[cuky2000]

The phase shift of the proposed transformer output should match the existing phase angle from difference sources. Probably a three line diagram showing the system phase shifting is very helpful in large systems with different power sources and transformations. Typical example could be a power plant with step-up transformers connected to the utility, auxiliary transformers connected to generator, starting transformer and distribution feeders.

 

[busbar]

 
“Dy0” does not exist—it’s typically Dy1 øø Dy11. Zero phase shift is not possible in a [Δ]-Y transformer. See J&P Transformer Book.

 

[mc5w]

You can get zero phase shift if the transformer is Dzz, delta primary zig-zag secondary.

Are the existing transformers at other points for the same transformation Yd? Cleveland Electric Illuminating Company has a lot of 1 MVA single phase transformers that are 6,600-volt primary and 4,400 volt secondary. The primary system is 11,500 volts ungrounded and the transformers are connected 3 wire wye primary delta secondary and the secondary is 4,400 volts ungrounded. They also have some 3 voltage transformers that are 33,000 volts delta, 4,400 volts secondary, and 6,600Y11,500 tertiary with a knife switch between neutral and ground. They also have some transformers for this voltage that connect to the 138,000 volts - the older ones are 80KVwye138KV to 11,500 delta ( back when a lot of power came FROM the mills ) and the newer ones 138KV delta to 6,600Y11,500 with the secondary neutral floating.

Measuring the phase shift at the proposed point could be invalid since current flow through transmission lines also causes phase shift.

If you have other transformers elsewhere that do the same transformation hook up some potential transformers as close to the transformer as is reasonable and measure the phase shift. It will NOT be exactly 30 degrees or 90 degrees because of transformer series reactance.

I take it that documentation for existing transformers is spotty. There is also a book titled Underground Power Transmission that is at our local library that has a section on the kinds of phase shifts that you can get in 3 phase transformers.

If you can, you might be able to take an existing transformer off line and use some 12 volt transformers to inject a low test voltage and measure the phase shift that you get. Without any ( substantial ) load there will be no oddball phase shift due to reactance.

Mike Cole, mc5w@earthlink.net