Transfomer placement

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steve66

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The application I'm thinking of is a 480V generator at a hospital that provides emergency power for the 120/208V critical branch.

The transformer could be placed on the load side of the transfer switch, and then it would always be in use. However, if the transformer fails, the critical branch would loose both normal and emergency power.

The other option, provide two transformers on the line side of the transfer switch. One for the normal source, and one for the emergency source. The potential problem I see, one transformer just sits cold for weeks, and then suddenly has to run when normal power is lost.

Steve
 
Re: Transfomer placement

Steve, if the transformer is sized properly the odds of it failing is far less than the transfer switch itself failing.

Roger
 
Re: Transfomer placement

what about the inrush of the transformer on startup? could that also cause a problem on the generator?
 
Re: Transfomer placement

I agree that there will be magnetic inrush current when the voltage will be applied to the primary of the transformer, but in transformer design there are means to supress those transients,allowing a sustained operation of the system. Such transients current and voltages are encountered even when sudden (heavy load) is thrown off or added to the system in order to establish the voltage level. Given the above mentioned condition though the Emergency transformer will remain ideal for a long duration of time, but will give more flexibility and will be able to cater the system demand during and whenever emergency arises. I beleive that second opyion is a better choice.
 
Re: Transfomer placement

Steve I would go with your first option.

If you do install two transformers on the line side of the transfer switch you have raised the costs considerably for what IMO is of little benefit.

You will have added the cost of purchase and installation of a second transformer and even more costly you have now doubled the size of the transfer switch.

I do not do Hospital work, I do a lot of buildings with emergency systems, not once have I seen a design that was set up in the way you describe in option 2.

I agree with Roger the chances of transformer failure are much less than other failures.

I would put the transfer switch at the top of the list for failures. During that last large blackout most problems buildings with emergency systems had was failure to transfer.

I would not worry about the transformer, if as Roger said is properly sized.
 
Re: Transfomer placement

I think I agree the odds of a transformer failing are small, however, due to rennovations and remodeling, and limiting the outage durations for the above work, it looks like we will have a system with a transformer on each feed to the transfer switch. That's why I was asking if anyone could think of any problems with that layout.

In addition, the client doesn't seem to like the idea of a possible transformer failure taking out part of the critical branch. And sometimes you have to give the client what they want even if you don't think it's really the most cost effective thing to do.

Steve
 
Re: Transfomer placement

Put the transfer switch on the critical feeder 120/208. The generator will supply only the priority system. Use a transformer if generator voltage is different.
 
Re: Transfomer placement

Steve, if the emergency transformer went south during an outage, the critical branch would still be lost.

Of course for a back up you could put in a manual transfer switch ahead of the two transformers. This would be getting closer to "Failsafe"

Roger
 
Re: Transfomer placement

is the generator only for the 120V critical branch circuits, if so why not get the generator at 208/120 and eliminate the transformer?
 
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