Delta Star Transformer

The secondary output will drop too. Possibly some of the circuit protective devices will operate.

It is more likely that two phases will go, and not one, on the primary side. If that happens, the secondary output will drop to zero.

Please see the following thread, it may help.

http://cr4.globalspec.com/thread/40007

-MS

I had a Delta wound transformer supporting our 220-3 phase secondary system and the utility transformer blew a link up on the pole. Our 3 phase machinery slowed down noticeably, some popped off and we quickly shut all the 220v-3 phase down before the rest could overload. We lost a 220 single phase on the boiler circulator because no one remembered that it was on 220.

On a Delta wound transformer each leg only carries 57.7% on the current on each leg. It tends to balance out the loads on each leg. However, when you lose a phase you now have an "open Delta" and the system can't support the full load.

Another time we lost one of the three 100A fuses at the disconnect protecting a Delta wound Transformer with the same resulting slowdown of machinery. Fortunately we had learned our lesson from the first incident and saved everything that time.