Loss of 1 Phase on Single-Phase Ungrounded Delta Transformer

[pags920]

Good evening all,

First poster, but have been aware of this site for a long time. I am in need of help, trying to understand the outcome of a scenario.

I work for a utility company in Electric Distribution. One system I work with is a 4.8kV ungrounded delta system, serving single-phase customers and three-phase customers. I have a single-phase transformer serving residential, 4.8kV (primary) - 120/240V (secondary). For a ungrounded delta system, we need two phases, let's say A and B. If lose A, but B is still energized, due to upstream protection, what would my secondary voltage be, assuming ideal conditions? Would my voltage be cut in half? Just really trying to understand this so I can explain it intelligently. Thanks!

 

[gar]

210929-2329 EDT

pags920:

The output of your single phase transformer is 0 V.

But that is not the whole story.

Suppose line A is good, and line B is broken between the substation and you. Then the break in line B is an infinite impedance. This is the simple case where there are no other loads connected to B beyond your load point.

But in a real neighborhood this will not be the case. There will likely be a number of customers beside you beyond the break point, Some of these will be connected A to C, A to B, and B to C. The normal loads in some of these other connections will back feed into the broken part of B. Thus, depending upon the impedances of these back feeds to the broken part of line B there'll be some voltage on the broken part of line B. And it is likely this voltage will jump around a lot.

You want to immediately open your main breaker.

Some of the loads in other homes or buildings will be motors with bimetal automatic reset protective devices in them. So various motors will trip and reset producing random variation in the voltage on line B. I had this type of problem one time at my home.

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