3-phase transformer question 2

[GOTWW]

For a Delta-Wye connected transformer, will non-balanced primary voltage be transformed through the transformer proportionally, or does the core-form construction re-balance the line?

 

[Mstrvb19]

Unfortunately, no. An unbalanced primary will still lead to an unbalanced secondary.

 

[peebee]

Yup. Think of a Dy as about the same thing as three single-phase transformers. All the primaries wired together in delta and all the secondaries connected in wye.

Then it becomes pretty clear that a line-to-line imbalance on the primary will translate nearly directly to a line-to-neutral imbalance on the secondary.

 

[GOTWW]

For 3-single phase units, sure, since each phase is magnetically isolated. However for 3-phase core form transformers the three transformers share a 3, 4 or 5 legged wound cores, it would be intuitive that there is some amount of magnetic flux cross-coupling on the return paths that would influence the balance on the secondaries. what am I missing here?

 

[Mstrvb19]

There is flux cross coupling in certain legs, but the important thing to note is that the secondary windings are laid on the same leg of the magnetic circuit as the primary windings (I am speaking in a general nature here-i I am certainly not an xfmr designer, so there may be other designs than the ones I am aware of,but the principle of the design remains the same). The only flux in these legs is the flux developed by the dv/dt of the primary winding, so the secondary windings never 'see' the flux in those areas of the core where flux paths 'mix'. This feature is intentional, since the flux induced by each primary winding is 120 degrees shifted from the other two, any cross coupling that would induce mutual inductance will decrease the voltage transfer (i.e if I put a winding on the "flux return" leg, I would not get much of any induced voltage on the secondary winding at all, since the flux from each phase cancels out (just like the currents in a wye connected system cancel out, leaving little current, if any, to flow on the nuetral leg).

 

[GOTWW]

Great!, I appreciate the analogy that I can understand. Now reiterating what you have said, for a four legged transformer, the residual flux in the forth leg would represent the imbalance in the primary phase voltages. Therefore hypothetically, If one would attach a fourth winding set on this core with adaptive loading such as maximum power transfer; (i.e. 50% of the unwanted voltage imbalance power could be extracted as useful energy. Take this energy, run it through a solid-state inverter, such as most of the 50% voltage imbalance power is canceled out for 3-phase regulation. Much faster than the three tap chanter bottles, less real estate also.

 

[Mstrvb19]

GOTWW, What size/service transformer are we talking about? Removing the core and adding a winding would be an arduous task, and if the system is large, the cost of the inverter would be quite high as well. There are many products/solutions for this, but it is tough to tell without going through the network and looking at each issue. Any extra info would be good.

 

[alehman]

This article has an in depth discussion of various three phase transformer configurations.

http://basler.com/php/download.php?file=practical.pdf&id=204&type=res