transformer for connecting 34.5 to 12.47 kV 1

[stevenal]

All,

I'm looking to specify an 8 MW transformer for connecting two four wire distribution systems that are in phase. This can be accomplished with an auto-connection, but will this be the most cost effective way to go? I know that's commonly the case at transmission voltages, but unsure at the lower voltages.
It will have a deenergized tap changer on the 34.5 side, and an LTC on the 12.47. Power might flow in either direction.

Full system unbalanced loading must be accommodated as discussed in thread238-514665. From that thread it appears the load unbalance might be handled by either a tertiary or a five limb core. Shall I leave that choice to the manufacturer as suggested by davidbeach, or are there other considerations?

Thanks.

 

[davidbeach]

I'd say talk to your top two or three transformer vendors and see what they suggest; they may take different approaches from each other. Then ask each about the alternatives proposed by the others. If, for instance, vendor A says they could easily do what vendor B proposes but it would cost a bit more (and vice versa) then you're looking a process differences that favor one solution over the other. But if one has serious reservations about what another is proposing then it needs a bit more digging.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[prc]

Theoretically, you can save material by going for an auto connection. However, the savings will decrease as the voltage ratio increases. It is almost 1:3 here(limit around 1:2), and you require both DETC and OLTC. Hence, I will suggest a two-winding transformer. We have already discussed stabilising tertiary in the earlier post.

 

[cranky108]

At the same time look at what this will do to fault currents for both systems, as this might drive you to a solution that is different than you first expect.

 

[stevenal]

So far I have recommendations from two manufacturers. Both agree a stabilizing tertiary is needed. One manufacturer says the auto will be more efficient, lighter, and smaller. The other says the auto will have lower impedance and less capability to handle short circuits. I question the impedance after consulting C57.12.10, though. I find 7.5% for the two winding and 7.184% for the auto. Not a huge difference.

I have also heard the voltage ratio might be a problem. Our supplier serves our 69kV system with a 230 kV auto, so there must be no hard limit here.

I'm still waiting to hear from manufacturer three. In the meantime, I'm considering using specs that allow for either configuration.

 

[stevenal]

Manufacturer three said wye-wye and the first manufacturer that previously seemed to recommend the auto now agrees. Looks like that’s what we’ll specify.

 

[prc]

Yes. That is what I had suggested from my manufacturing background.