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Anonymous Poster #1

Primary Feed-Phase Loss

07/26/2017 10:07 AM

Recently we had a phase loss in our primary feed to our bldg. transformer. The transformer is delta - wye. The primary is side 7200 volts, the secondary being 480/277 volts. What voltages could possibly back feed through the lost phase in our transformer? Our 2000A main breaker to our main switch gear had no phase protection. We are definitely having this taken care of. We lost several coils 120v and 480v in some our motor control equipment.
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PeterT
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#1

Re: Primary Feed-Phase Loss

07/26/2017 3:24 PM

Here's a paper that does a detailed analysis for various transformer winding configurations. It considers both balanced and unbalanced loads.

Open phase transformer conditions

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Ivanov327950
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Re: Primary Feed-Phase Loss

07/27/2017 2:41 AM

What voltages could possibly back feed through the lost phase in our transformer?

It is a 3 phase voltage source, which should not be dependent on the load. If you want to know the exact voltage drop in the transformer you will need the short circuit impedance. Moreover, as there is no load in the lost phase, the voltage should be the one from the generator.

The only difference you should be getting between 2 and 3 phase is the lack of current in the lost phase, which leads to increased currents and unbalanced system.

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gutmonarch
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Re: Primary Feed-Phase Loss

07/27/2017 8:58 PM

Siemens has actually a nice product on protection. Please check this one out .
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