Transformer w/ Hi-Side LTC to control Low-Side?

[FEinTX]

I need a transformer expert...

Recently, during commissioning of a new 230 MVA 345-34.5 KV transformer we came upon an interesting problem. The LTC controller (Beckwith) was wired to monitor the low-side voltage and thus regulate it to a pre-set value, even though the LTC is built on the high-side of the transformer. When the controller was set in Auto mode w/ the low-side voltage purposely outside the range (lower), the controller issued commands to change to raise the LTC one step, but the new voltage level actually went down.

After some thinking, this made sense. The 345 KV high-side is tied to directly to a very stout grid which is fixed at 345 KV--our relatively small power plant won't move this much. The Beckwith is wanting to raise the voltage at its input. Normally, this voltage is from the high side, so it will issue a command to raise the tap increasing the ratio between low and high side. We are inputting the low-side voltage, however, so now with a higher ratio and a high-side that is fixed at 345 KV, our low-side voltage goes down.

Now for questions:

(1) Someone suggested to us that we simply reverse the up and down outputs between the Beckwith controller and the LTC and our problem is solved. This sounded good until GE-Prolec (the transformer manufacturer) said not to do this. They cited problems w/ the design of the high-side coils saying flux densities would go beyond their design specs if we used the transformer this way. This makes no sense to me, but I'm not a transformer expert. Can anyone explain Prolec's concerns?

(2) Is there another way to make a transformer w/ a high-side LTC regulate the low-side? Basically, we want to keep the low-side set at 34.5 KV regardless of what is happening out on the grid.

Thanks for any help you can offer!

FEinTX

 

[dpc]

To raise the voltage on the low side you need to LOWER the tap (low voltage ratio) on the high side.

You should be able to regulate low side voltage with the high side LTC. I don't see why this is a problem for the LTC **assuming the transformer designer knew how it was going to be operated***. It is possible that with the high side tapped down too far in order to boost the secondary voltage, the core could saturate - transformer cores operate pretty close to saturation under normal conditions.

The intended application and voltage operating range should have specified when the transformer was ordered. If it was, then it's Prolec problem. If not, it may be your problem.

 

[tinfoil]

Don't most modern LTC controllers have 'primary side LTC / secondary side LTC' as a basic setting to be set by the user?

It is very common to have a high-side LTC, as the current levels are lower in your windings, and I thought that most mfrs would accomodate this.

Contact Beckwith and ask them your question:

http://www.beckwithelectric.com/contact-us.html