Paralleling power transformers 4

[wa6lax]

I need to momentarily parallel two 1500 kva transformers manually, in order to maintain continuity of power to load. My client who is also a ee suggests we might get circulating current causing breaker trips and possibly harmonics. Does anyone have experience with this?

 

[cuky2000]

If the transformers voltage rating and impedance are the same or withing small difference the circulating current may not be signficant high to trip the breaker.

Shearch in this site for paralleling transfomer. There is good discussion in this subject.

 

[mpparent]

I hope you are braced for a closed transition switching operation.

Mike

 

[dpc]

As mentioned, if the transformer ratings are matched, circulating currents should not be a major concern. However, as Mike notes, while the two transformers are paralleled, the available fault current downstream will nearly double. Most designs do not provide for this amount of fault current.

If you must parallel, you should maintain the two units in parallel for as brief a period of time as possible, and be cognizant of the inherent dangers involved. This is not a trivial concern especially when arc-flash energy is factored in.

 

[rbulsara]

If these are two separate utility feeders, your utitliy co. will have a say in this..they would like to know the back up protections and do not want two sources to remain in parallel more than 500ms. (0.5 sec). Do not try to sneak this under there nose and in that case manual switch over will not work.

You can do manual closed transition, for extended period, if the two transformers are fed from a single source, such as the generator or a single utility.

 

[wfowfo]

rbulsara makes a good point. Even if the voltage ratings and impedances are identical, if the transformers are fed from different sources there can be significant current.

You refer to paralleling manually. Assuming you are un-paralleling with the same "manual" device, can it break this current?

 

[ruggedscot]

This is a proceedure that you really need to be careful with and understand the implications of doing.

as it has been mentioned the availiable fault current will increase when running with two transformers in parallel, this is very concerning and you should be looking to run like this for as short a time as possible.

Also as you are increasing the availiable fault enery you have to be aware that if you do get this wrong and make a mess then there is a very good chance that things will go pearshaped. Your panels may not be rated sufficiently to deal with this extra fault current.

Phasing and sequence...... This is another risk with 3ph systems although your two sources may be in the right sequence, you may find however the phasing may be out. This would mean that you would close on and have a fault right there and you would be at the mercy of the grid protection.

Your two sources are they referenced ? if not then you may find that frequency differences would cause trouble, the system would try to sync lock up through your 'breakers.

Power companies may get very jittery about this as it may develop issues with their own network and cause problems to them.

How critical is your process ? What are you supporting and can it survive a momentary dip ?

Have worked on a system where we retro fitted a UPS to a bus panel and then carried out the transition so that no power was lost. This can be undertaken by some specalist companies that deal with this sort of proceedure - Aggreko ?

rugged

 

[SMB1]

Hi

Don’t forget to check the maximum short circuit of your 20KV bus.

Thanks