Transformer Circulating Current

[vtpower]

Hello All,
I am currently setting up a three phase distribution step-down bank as part of a voltage conversion. This will be three 250KVA Single phase transformers connected ungrounded Y - Delta. My question is, if the impedances of the transformers do not match, I have been told you will only need to de-rate the transformer if they are connected delta-delta, otherwise circultating current will not flow. Is this true, or will I still have circulating current and will I still need to derate the bank in the ungrounded Y - delta connection?

The reason for the ungrounded Y is to not create a grounding bank.

 

[jwilson3]

With a delta-delta connection, there exists an unbalanced voltage on the secondary that results in a 'net' voltage around the secondary delta 'loop'. Normally the three phase voltages would 'sum' to zero. This 'drives' the circulating current, which is supported by the delta connected primary -- a 'loop' exists for circulating currents on the primary also.

Since secondary circulating currents are in-phase(zero sequence) and, since no zero sequence currents can flow in the primary of the ungrounded wye to support secondary circulating currents, they are blocked.

I haven't worded it well, but I agree with the advice you received.

 

[dpc]

Look at it this way - current (other than exciting current) cannot flow in one winding of a transformer unless it can flow in the other. So if one winding cannot pass zero sequence current, neither can the other side.

 

[jwilson3]

DPC,
That's a good way to put it.

 

[mc5w]

In a wye delta or delta wye transformer 3rd harmonic currents would still be able to circulate in the delta winding. How much depends on the loads and so forth.

A 3-wire delta primary 3-wire delta secondary transformer can have circulating fundamental current if there is a relatively heavy single phase load on the secondary. This is because of the excess voltage drop in one of the transformers which partially simulates a turn-to-turn short but not as much as with a 4-wire delta secondary.

What happens in a 3-wire delta primary 4-wire delta secondary is that any load that uses the secondary center tap simulates a turn-to-turn short to greater extent than with phase to phase single phase loading.

A secondary load on a delta delta trasnformer that is rich in 3rd harmonics will also mess up the transformer.

In other words, delta-delta is a bad transformer under some situations.

 

[itsmoked]

So if you want to generate a neutral from your delta generator so you can run a bunch of single phase loads a delta-y is okay?

 

[jwilson3]

mc5w,
I respectfully disagree with your first sentence, if the wye is ungrounded. Read what "dpc" said, in essence, if current can't flow in the primary, it can't flow in the secondary. Circulating currents in the delta are all in phase. That means the wye side currents would be in phase, which can't happen with an ungrounded wye. You can't have 3 inphase currents flowing to the neutral with nowhere to go. Zero sequence currents are blocked, so there can be no circulating current on the delta side.