What is a series winding on a power transformer?

[wfowfo]

We are purchasing a 12/15/20 MVA transformer that is dual/dual (138/69 kv primary, 24.9/12.47 kv secondary).
The transformer had just be completed and the factory was doing the acceptance tests when they said they had some "high" readings on the "series" winding.
All the sales rep could tell me is that "due to the high currents" on a dual/dual, there is a series winding. I'm assuming it's some form of reactor coil.

Any idea what he's talking about?

 

[cuky2000]

Perhaps, the vendor is referring to one connection in series to obtain one of the dual voltage ratings.
The enclose link is a typical representation of one of the three windings configuration

http://www.hammondmfg.com/par_serA.gif

 

[wfowfo]

That was my first thought too. But he insisted that it was a separate winding.

 

[wfowfo]

Don't know how to edit my own post, so I'll add this here. When I asked the vendor if they would have to rewind the unit, he said (and I quote), "No, they put in a new one they had to be on the safe side".

By this I'm assuming it is independently wound from the main windings.

 

[electricpete]

If there were a LTC involved, there could be a series reactor that minimizes circulating current when in bridging positions. If not LTC, then this wouldn't apply.

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[electricpete]

If I were spending big $ for a new transformer, I'd ask the sales guy to let me talk to someone at the factory!

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[apowerengr]

wfowfo - does the transformer have an LTC? If so, many suppliers provide a series winding or RV (regulating voltage) winding. It's intent is to raise the voltage (and thus lower the current) the LTC sees in order to use an LTC with lower current carrying capacity.

20MVA at 12.47kV is over 900 amps. Waukesha Electric Systems, for example, might use an RV winding and their standard 600 amp UZD LTC assembly. Some others might just use a fully-rated LTC (such as a Reinhausen)and avoid the RV winding. As power transformers get larger, RV windings are used frequently as the commericially available LTC's are limited to 3000 amps or less.