Power Transformers In Parallel

[mbrooke]

I started reading this thread last month before there were so many replies, and one of the first questions you asked was

I didn't know the answer at the time, and it seemed like the original people replying also didn't know.

Since then, I've been putting some transmission lines into Open Street Map, and I've noticed that often a substation has two transmission lines feeding from different directions, with one transformer connected to each. Presumably these transformers are operated in parallel for reliability reasons if one transmission line goes down. And, it's simpler to use two transformers than multiple smaller ones.

So, that would seem to be the simple answer. which I think has by now been strongly implied by the additional responses since the last time I was here.

It is also correct to say

It really depends on the common practice for the utility and area you're working in. In rural areas, there can be just one transformer in each substation, a pretty small one depending on the load served.

Yup.

Typically the primaries are in parallel with either a center 115kv breaker, one breaker on each incoming line, or both.

The secondaries are typically run normally open with auto throw over for the loss of the other unit.


One advantage to looping in and out is that if extensive damage took place in a right of way simultaneously taking out multiple lines, power can still be supplied to the station from the other direction. It also makes relaying easier in theory in that you do not have to take a half dozen or more taps into account which can make relaying so much more complex. Ie, if a hard fault took place in the transformer's differential zone of protection, it is possible that zone 1 MHO at one or both end of the line will trip in addition to the 87 protection at the trafo. You could delay zone 1, but that can open stability problems and other issues.