Transformer Vector Group (Which One?)

[UKEng]

Hi,
We are installing two new 4MVA transformers (11/3.3kv) to be connected in parallel to supply a switchboard feeding Motors. the 3.3kV switchboard will have a bus-section. The question is which transformer vector group should we use. Is there any preference to Yyn0 over Dyn11 etc. Could someone advise what are the criterea for selecting which vector groups to use and whether the primary's are star(wye) connected or Delta.
Thanks in advance.

Paul

 

[redtrumpet]

In Canada we would normally use a delta-wye with the neutral grounded and the secondary voltage lagging the primary by 30 degrees (ANSI Group 2 30 degree angular displacement). I believe this is a Dyn1 in your terminology.

It is desirable to ground the secondary neutral, either solid or resistance. It creates a ground current source for selective relaying of ground faults. It prevents high voltages due to neutral displacement (solid ground only). It prevents high transient voltages due to arcing ground faults.

The phase shift of the delta-wye connection must be considered when paralleling. I have a job where this is a real headache right now. The phase sequence on the primaries of both transformers must be the same, or you will have a 60 degree phase shift between transformer secondaries.

You could have a Y primary, but I believe it would have to be ungrounded (floating). The utility probably doesn't want you to be a ground source on their system, however briefly, in the event of a line fault. This connection would give a 0 degree phase shift from primary to secondary.

 

[UKEng]

What would be the reason for having a Y primary over a D primary, are there any benefits that you know of.

regards Paul

 

[redtrumpet]

There is no benefit. I shouldn't have even mentioned the primary Y winding as a possibility.

A grounded Y primary can be of advantage to a utility for several reasons, but in your case I believe the Y would have to be ungrounded and this is an unadvisable connection.

 

[redtrumpet]

To clarify, an ungrounded Y - grounded Y connection would no longer create a ground source on the secondary, thus defeating the purpose of grounding the neutral. This connection will also suppress the flow of third harmonic current and its odd multiples in the exciting current and give rise to harmonic voltages.

On a 4 wire multi-grounded neutral primary system, you could ground the primary Y winding and this would transfer the primary ground source to the secondary side. This is represented by a through-impedance in the zero-sequence network rather than an impedance to the zero-sequence ground bus. However, I have never seen this done at the industrial plant level. This also means your plant could transfer a ground source to the primary side, which may blind utility impedance relays. So, I doubt the utility would allow it in any case.

I approached this from the point of view of picking the secondary connection best suited to your motor bus, which I believe is the grounded Y secondary. Once this secondary connection is fixed, I think you are pretty much limited to a delta primary.

 

The only transformer you should consider is Dy. The actual vector group (Dy1 or Dy11) does not matter, just match other transformers that may already be at your plant.

You can directly earth the star points or you may wish to resistively earth them to limit the 3.3kV side earth fault current.

I have been told that for any given VA rating a Yy transformer costs less than a Dy transformer. I have not enquired from manufacturers so can not be sure if this is correct.

Contrary to what most text books and electrical analysis software packages say, a Yy transformer with the primary star point unearthed and the secondary star point earthed will most definately provide earth fault current on the secondary side, if the transformer is of the core type (most are). The zero sequence impedance should be about 50% (refer J&P Transformer book, Appendix 5) so the earth fault current will be low, nothing like the three-phase fault current.

 

[redtrumpet]

Insulation on a Y primary can be lower than on a delta primary because the windings only see line-to-neutral voltage and hence less insulation is required compared to a delta primary. This is why a Y primary can cost less than a delta primary.

I guess I'm one of those guys who has a textbook that says an ungrounded Y primary - grounded Y secondary doesn't create a secondary ground source.

 

[230842]

Besides all preceedings arguments, I'd like to point out the fact that once bought, transformers connections are tightly bound to high and low voltages suplies and/or disposals. One transformer is rated 11/3.3 kV after its group connection is fixed (Yy, Yd, Dy or Dd) Julian