phasing out three phase

------ You should have no problem. This has been done and is still done routinely in many places and with identical or nearly identical transformers you should have no circulating current problems (and the length of time that they will be in parallel will be small) . - provided that you make sure that you connect corresponding phases together (don't trust the labels) - This will require checking of the phases as phase A on one may not correspond to phase A on the other. You can check the voltages between corresponding phases on the secondary with a voltmeter- these should be small (unlikely 0 unless the loads are fully balanced). A competent inductrial electrician should be able to check this out. Then the next consideration is the load- so the thing to remember is that you will be carrying the full load on one transformer when the other is out of service- best to do such maintenance at a low load time of day.

With both transfomers energized, record 12 "expected" secondary-voltage measurements..

1X1-1X0, 1X2-1X0, 1X3-1X0 T1 ø-n 1X1-1X2, 1X2-1X3, 1X3-1X1 T1 ø-ø 2X1-2X0, 2X2-2X0, 2X3-2X0 T2 ø-n 2X1-2X2, 2X2-2X3, 2X3-2X1 T2 ø-ø

Then between secondaries..

1X1-2X1 1X2-2X2 1X3-2X3

An IEC 1010-1 600V category-IV meter is suggested. Healthy fault currents are likely present.

?s falke

The two earlier replies are well taken.

The available fault current will double when you parallel the two transformers, so you should review the interrupting capacity of feeder breakers.

If other facility personnel have a habit of changing things without informing everyone, you may want to install a sych. check relay across the tie breaker as insurance. This relay's permissive contact should be wired into the "close" circuit of the tie breaker so it couldn't be closed unless the phases were synched. Since the transformers are identical and supplied from the same source, you could set the relay up pretty tight.

You should review the ground fault relay>I have a question as to wether or not I can parallel two three phase

Everyone has given good ideas. Do you know if there is a control scheme in the main tie mains that prevents just what your doing? Most will have this hardwired and/or software controlled as well. What I have seen is that the MOC and TOC contacts are series and/or paralleled to prevent all of the breakers from closing. You will have to change all of the this wiring and or software to allow a close of the tie. Next you had better be talking to the utility folks. They can be of great assistance for doing this. Lastly it will be time for intensive and compressive training for the worker bees.

I wrote a main tie main scheme for a 15 kv system once. After 3-8 hour days of training the workers did as I had feared. They tried to do the process out of step. My safety circuit caught the mistake and took over for them. This was a live test with no load. Go very carefully into this situation.

One 'phasing circle' illustration is

--s falke

Note *fixed* URL above.

Suggest 13.8k?480V transfomers be set on like taps.

This exercise becomes especially interesting with two 20MVA transformers about

2 miles apart with 12kV secondaries. Even after check and recheck with a phasing voltmeter, closing the tie is "puckering" in its own right. For circulating current, it's kinda' weird seeing ½ MVAR going "nowhere" until taps are adjusted.

--s falke

Thanks all for the quick reponse!

I have not seen the job yet, information was handed to me from the facility electrician. The question was raised if I was able to tie the boards together and take transformer offline with no interuption to the building.

They are hiring us to perform de-energized maintenance tasks to each transformer as well as infrared thermography previous while energized. I will be able to review their tie scheme and take appropriate voltage measurements as suggested by s. falke.

If I find out that the transformer primary sources are not from the same utility line but phased correctly between both switchboards, would there still be a possibility of pulling this off?

Bill

arrangement.

---------------------- Yes. The key is making sure the voltages across the tie breaker are all small -You could even have a set of lights across the breaker contacts. - if out or dim- no problem -if bright -don't close the breaker. This is a variation of the old synchronising light scheme used for generators.

"Bill Schafer" wrote... ...

...

This is probably apparent, but phasing two 'fixed systems' that eventually originate from a common bus is different than phasing two generators, or a stiff bus and one generator that naturally drift with respect to one another until paralleling.

Aside - Another caution is that if one set voltages is solidly 52% 'out,' then they cannot be paralleled, for there is a 60° mismatch, like that of a wye?wye bank and a delta?wye bank.

--s falke

If the source is from the same bus, perform the phasing check once. Then recheck whenever a transformer is removed and returned to service, or other maintenance work is performed that could possibly affect phasing.

If planning to tie together two utility feeds, better check with them first. You may be subjecting your transformers to large circulating currents, possibly overloading them. With this case, since part of the circuit is outside of your control, perform the phasing check every time prior to closing a tie switch.