Reverse-connected step-up transformer

It has taken me quite a while to figure out what you want, I'm not sure I have it now.

You want a 3 phase supply with a neutral output. A neutral output requires a WYE configuration so with 110 volts (3 phase ) on the primary's the WYE connected secondary voltage would become 692.8 volts phase to Phase and 400 volts phase to neutral.

In order to get your 400 volts phase to Phase WYE connected you would have to excite the 110 volt (now primary) windings with 63.36 volts. (you have 110)

If you connect the 400 volt windings in Delta, (no neutral) then you would have your 400 volts phase to phase BUT as I stated NO NEUTRAL.

Any other primary winding configuration that I can conceive, makes the condition worse.

Using the transformers backwards is no problem as long as you stay within the design voltage ranges of the transformers, However

You used the term potential transformer in your question,

The word potential (in American Engineering) refers to a voltage metering transformer. This type of transformer is for voltage metering and is not designed for power. It has limited current capability.

Snakers.

yes, i agree your comments..

Thanks for your open-mindedness. You don't seem to understand what I want. All I want is to obtain a 400v, 3-phase and neutral supply (400V, phase to phase and 240v, phase to neutral) from an existing 11kv switchboard via the only way I find feasible, the potential transformers.

Your comment on potential transformers is highly appreciated. That is my major plight. I know they are instrument transformers and are not designed for power. I also know the winding configuration to adopt to achieve my aim. My concern is that I'm not sure if a damage to the PTs would not result when step-down transformers (400/110v) are reverse-connected to the PTs' secondary(110v) to obtain the 400v, 3-phase and neutral supply needed in view of the PTs' limited current capacity as you have noted. Can the PTs drive the step-down transformers without getting damaged? That is the issue at stake.

Perhaps one of our MV experts could give an opinion on safety issues,I suspect this will take the form of DONT. However:-

Your potential transformer should have a rating stated on it.This could take the form of a letter, W=12.5VA,X=25VA,M=35VA,Y=75VA,Z=200VA.This defines the load,'burden' in vt terminology, which the transformer will supply while remaining within specified accuracy and allowable temperature rise.(and at certain power factors)

Take into account any loads already connected and add these to the load your 110v/440v transformer imposes.

Be absolutely certain that the isolation,ie winding separation/interwinding shield provided by the potential transformers will ensure that even under fault conditions,eg burnt windings, there is NO possibility of galvanic connection to the 11kv.

sWsssddhat I am saying is, there is no configuration that I know of, whereby you can connect three 110 to 400 volt transformers to a 110 volt Wye conected source and derive 231 / 400 volts, 3 phase, 4 wire.

Running the transformers backwards is no proble, They don't know or care which way the current is flowing.

Using the potential transformer wont make any difference as long as you keep within the current and thermal limits of the windings. It will no longer meet its accuracy class rating.

OK, what am I missing here? Disregarding all the 11KV info.

If he has 110V 3-phase power available and wants to reverse-power up three 400/110 single phase transformers to get 400V line to line and a 231V line to neutral secondary, I don't see the problem. Why can't he connect the three transformers 110 side Delta and the 400V side as Wye. With 110V on the primary, he should get a secondary with 400V line to line and 231V line to neutral (line times the square root of three). Granted he will have no current carrying capability, but the voltages he wants should be there.

Because according to my math, 110 volts on the 110 volt Delta connected primary will give 400 volts on each leg to neutral and 692.8 volts phase to phase on the WYE side. In this instance you would multiply by square root of 3 instead of dividing by it. Remember you are stepping up.

Check it out, I'm pretty dam sure I am right.

Meester Snakers: You should have noticed that I retracted my comment immediately after I sent it because I realized I made an error.

Disregard my previous comment, my brain jumped time......again. Sorry.

Kyoto.. Sorry . I did not see your second comment until after I had filed my comment. Ive tried all kinds of buck and boost (autotransformer) connections but the ratios just don't work out. Adding 3 ganged variacs might solve his problem. Since his available power is limited by the potential transformer, this may be a viable solution.

It would give him 400volts phase to phase and 230 volts phase to ground.

Snakers.

Yeah, and it sounded like he wanted to use what he has on hand. If he has to spend money, he can get what will do the job.

I'm new to CR4, can I ask if the initiator of a thread,like this one ,usually doesn't make any comments on the replies or report back after trying the suggestions,or do they blow themselves up after following the advice and we should consult a medium to see how they fared ?

oldeng:

Very good question, and the corect response with no editorializing is (E). All of the above. Welcome to CR4.

Gnome1