Main-Tie-Main Arrangement with Tie Breaker Normally Closed

Your one sentence question lacks needed details so I will assume that it relates to distribution switchgear.

In most cases the manufacturer that you purchase the gear from will engineer the lineup and provide this information to you based on the particulars of your application requirement - Example: power generation, hospital critical versus non-critical loads, municipal critical versus non-critical loads, ect?

Forgive me for not providing a more direct answer as I am unclear on what your concept is of having the tie closed under normal conditions (assumed both feeders energized with means of disconnect closed).

In my local, we work with non-critical and critical loops (utility power sources), each with many separately derived sources.

When one of the two feeders fail, the breaker or means of disconnect on the failed feeder opens and the tie closes to provide power to the portion of load served by the failed feeder by means of the remaining energized feeder.

If you are using two separately derived feeders you cannot operate under normal conditions (both feeds energized) with the tie closed. In fact, here, it is mechanically impossible to operate with the tie closed with both power sources energized and means of disconnect closed.

Am I missing something obvious?

Yes it is possible to operate a M-T-M substation with the tie normally closed. The utility for which I work does it all the time in certain of our regions. Even with diverse sources for the two ends of the station, it is possible to connect them because phasing of our systems (including phase shifts through the transformer windings) are carefully planned.

Several items need to be included in the protection and control:

1. The OLTCs of the transformers must have paralleling circuitry so that they share the load and control the voltage properly.

2. There must be reverse-power flow functionality in the transformer relays so that some system problem (possibly outside the substation) doesn't result in power flows in through one transformer and out through the other.

3. There must be bus fault protection that will isolate the two halves of the bus at the tie for a fault on one side. That way you can still power at least some of your feeders if a fault takes out part of your bus.

There may be some other considerations of which I am unaware, since I am not a protection engineer. However, I am sure of the above.

I completely agree with you PeterT. On the power generation/distribution (primary switchyard and sub-stations) end I have seen this also.

Based on the simplicity of the question I had assumed (i know, when you assume you become the first three) that the OP was asking from a user distribution standpoint.

In any case, I would still like to hear from the OP as to his particular application.

This is for 2400 V switchgear. I have two feed lines at 13.8 KV. I have two 7.7 Mva , 13.8 KV- 2.4 KV, OA/FA transformers and M-T-M switchgear. Two Feeder lines are tied at 13.8KV side.

Thanks for the comments.