# Parallel Three Phase Transformers



## Tactical Sparky (Sep 6, 2015)

I am installing a new feed to an existing panel that cannot be powered down. To do this I need to parallel two three phase transformers temporarily. Besides matching impedance, voltage, phasing and phase rotation is there anything else I need to be aware of or do for a safe change over?

Thanks in advance for any help provided.


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## redblkblu (Mar 3, 2012)

Never mind


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## ELECTRICK2 (Feb 21, 2015)

Tactical Sparky said:


> I am installing a new feed to an existing panel that cannot be powered down. To do this I need to parallel two three phase transformers temporarily. Besides matching impedance, voltage, phasing and phase rotation is there anything else I need to be aware of or do for a safe change over?
> 
> Thanks in advance for any help provided.[/QUOTE.
> Just to be clear, you're plan is to double feed a panel, both sources live?


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## Tactical Sparky (Sep 6, 2015)

Yes they will both be live, temporarily, to prevent the outage. Once the new is energized then the old will be de-energized and removed.


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## ELECTRICK2 (Feb 21, 2015)

Tactical Sparky said:


> Yes they will both be live, temporarily, to prevent the outage. Once the new is energized then the old will be de-energized and removed.


Yikes, 2 dffererent sources, both live, tied together.
My questions to the customer would be why can't the panel be shut down? What are the critical loads?
Maybe you can temp feed critical loads?
Bottom line, if I had to do it as you describe, I'm pretty sure, no matter how many precautions were in place, a little bit of urine would be running down my leg every time I attached a live feeder to a live lug.
Does customer realize the $hailstorm that will hit him if things go sideways?


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## don_resqcapt19 (Jul 18, 2010)

Tactical Sparky said:


> I am installing a new feed to an existing *panel that cannot be powered down*. To do this I need to parallel two three phase transformers temporarily. Besides matching impedance, voltage, phasing and phase rotation is there anything else I need to be aware of or do for a safe change over?
> 
> Thanks in advance for any help provided.


There is no such thing.


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## Tactical Sparky (Sep 6, 2015)

Okay guys thanks for the help.


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## Southeast Power (Jan 18, 2009)

In theory,
You can pull this off.
I would do a small scale mock up and give it a try.

I would connect a three phase motor to the load side of a panel.
Make up two separate transformers and have them connected to back fed breakers across from each other on the panel bus..
Have one breaker closed.
Energize both transformers
Then, say a Hail Mary and close the second breaker with a 6' long stick.

It should be just fine.
Take amperage readings and check the voltage

Does anyone see a problem with this?


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## micromind (Aug 11, 2007)

When making final connections, measure voltage from the conductor to the lug before landing it. If it's within a few volts, then phasing, etc. are good. 

If it's more like line-to-line or line-to-neutral, then you'll be glad you checked it first.......lol.


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## telsa (May 22, 2015)

Crazy to do the connection at full power.

You want to synch the two first.

This exact same 'trick' is performed every day of the year -- by the Poco -- when a massive synchronous alternator is powered up and dropped into service.

This is ALWAYS done in stages.

The boiler is brought up to pressure.

The turbo-alternator is brought up close to speed.

Then it is phase matched at low power -- running as a motor.

(96% of the power from the steam, 4% from the outside -- the grid)

Then the power flow is reversed.

The key step is that when the connection is made every thing is synchronized.



I'd use a meter// 'scope to verify that the voltage _difference_ between the secondaries -- each leg in turn -- has been reduced to zero volts -- instantaneously.

( while powered up, yet before connection )

Then I'd have assurance that they were in lock step.

I would never trust to luck, prayers, hope or faith. 

This step is necessary because you can't trust that Trans #1 has it's A leg mated to the A leg of Trans #2, etc.

You do NOT want to find out that you're attempting to change the rotation of the 3-phase field. Such a 'plugging' would crush most motors. 

( Yes, your fuses should blow, first. )

The new transformer would have to have a fused disconnect so that if there is a screw up, the fuses eat the mistake.

&&&

That being said. I don't believe that the loads from such a lighting and appliance panel could not endure a shut down.


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## Bad Electrician (May 20, 2014)

We do this regularly, if properly set up with fused safety switches or circuit breakers it is feasible.

There are safe ways and dangerous ways what we do is safe and operational, no different than a bypass for a UPS.

One item is voltage difference, when you measure the voltage between the two sources there will be a difference in voltage the transformer that is loaded will have a lower voltage, due to voltage drop in the system.

When you set the system up at the final bypass CB prior to energizing measure across the CB or switch phase to phase A to A, A to B, C to A, B to B and C to C. The phase to phase voltage should be low 0 to 5-10 volts depending on loads on the system.

VERIFY, VERIFY, VERIFY.


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## micromind (Aug 11, 2007)

Bad Electrician said:


> When you set the system up at the final bypass CB prior to energizing measure across the CB or switch phase to phase A to A, A to B, C to A, B to B and C to C. The phase to phase voltage should be low 0 to 5-10 volts depending on loads on the system.


This is the most important step, by far.


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## Tactical Sparky (Sep 6, 2015)

Thanks everyone for more information. I know my posts are rather vague as far as what my exact steps would be so I will add some to clarify some of your questions.

Crazy to do the connection at full power.

Agreed, we will be doing all testing with both feeds energized but only original feed connected. Voltage difference, phasing and phase rotation as well. After all "checks" are complete then the new feed will be connected and energized. 

The new transformer would have to have a fused disconnect so that if there is a screw up, the fuses eat the mistake.

Agreed, both existing and new transformers indeed have fused disconnects on the secondary side. Both existing and new feeds will supply breakers that will be used to energize new and de-energize existing seconds later.

That being said. I don't believe that the loads from such a lighting and appliance panel could not endure a shut down.

Agreed, however the customer is not really concerned if we agree on this one or not.

When you set the system up at the final bypass CB prior to energizing measure across the CB or switch phase to phase A to A, A to B, C to A, B to B and C to C. The phase to phase voltage should be low 0 to 5-10 volts depending on loads on the system.

VERIFY, VERIFY, VERIFY. 

Agreed, this was what I was waiting for someone with numbers. I have heard numbers from 5-17 volts which was somewhat bothersome for me. The lower the better for sure but nearing 20 volts seemed too much.

The actually time they will both be energized will literally only be long enough to operate both breakers.

Again, thank you all for your knowledge it is much appreciated.


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## Southeast Power (Jan 18, 2009)

I've seen plenty of guys do this with generators, by hand, looking at only a couple of meters.


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## Bad Electrician (May 20, 2014)

jrannis said:


> I've seen plenty of guys do this with generators, by hand, looking at only a couple of meters.


Been there done that, Don't care how many times you have done that or how careful your set up is it still can be a tad worrisome.


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## pcmike (May 14, 2009)

Tactical Sparky said:


> I am installing a new feed to an existing panel that cannot be powered down. To do this I need to parallel two three phase transformers temporarily. Besides matching impedance, voltage, phasing and phase rotation is there anything else I need to be aware of or do for a safe change over?
> 
> Thanks in advance for any help provided.


Are these two xformers fed from the same source (utility) or from different generators?

If from different sources, I don't think you can do it safely enough to meet OSHA regs without some specialized equipment. You have to get the gens synchronized, in phase, and same voltage. Without synchronizers, you'll not likely to get it to work safely.

If from the same source, the the key thing to verify is the correct phases for each and every connection at the sources, xformer primary and secondaries, and at the load panel.

Still with all of that done, your sphincter pucker factor will be through the roof.

Make sure you have really good safety equipment on hand and used. Good luck.


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## Bad Electrician (May 20, 2014)

pcmike said:


> Are these two xformers fed from the same source (utility) or from different generators?
> 
> If from different sources, I don't think you can do it safely enough to meet OSHA regs without some specialized equipment. You have to get the gens synchronized, in phase, and same voltage. Without synchronizers, you'll not likely to get it to work safely.
> 
> ...


 
It is very possible to parallel generators manually a tad worrisome but doable. Much easier if you have a closed transition ATS, but we have done it manually. 

It can also be done safely with the right distribution equipment.

My question for the poster is what size transformers?


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## Southeast Power (Jan 18, 2009)

Bad Electrician said:


> Been there done that, Don't care how many times you have done that or how careful your set up is it still can be a tad worrisome.


Every time we sit there and watch that needle go around that synchro dial and it makes it around the 5or 10th time, someone is going to tell the "did it by hand" story.


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## Bad Electrician (May 20, 2014)

jrannis said:


> Every time we sit there and watch that needle go around that synchro dial and it makes it around the 5or 10th time, someone is going to tell the "did it by hand" story.


I have worked with old Coast Guard electricians that will tell stories of using light bulbs, I have worked on gear with bulbs for synchronizing


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## Big John (May 23, 2010)

Bad Electrician said:


> I have worked with old Coast Guard electricians that will tell stories of using light bulbs, I have worked on gear with bulbs for synchronizing


This stupid thing is from about 1910 and the synchro long ago gave-up-the-ghost. I replaced it, but prior to that the only choice you had was closing with the dark-lamp method. It took some getting used to.









The only thing I can add to OP is you must use multiple analog voltmeters. If measuring across any single phase at a time you can read zero with the other two phases grossly out of synch. 

If any two are crossing zero at the same time you are in theory okay to close, but shift your test setup over one phase to verify your third as well.


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## Tactical Sparky (Sep 6, 2015)

My question for the poster is what size transformers?

The transformers were supposed to be matching 75kva but when we arrived to the job that was not the case. The existing was a 15kva and the new was 75kva.

We were unable to changeover as planned because of the different transformers. The 15kva also had capacitors installed for noise reduction. The capacitors, _I think let me know your thoughts_, made the voltages unequal to the new transformer. Regardless of our test connections there was always 122 volts on two phases but 212 on one phase.

In the end we completed the changeover by de-energizing the existing long enough to energize the new feed. 

Thanks to everyone for their input on this subject.


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## hardworkingstiff (Jan 22, 2007)

Tactical Sparky said:


> In the end we completed the changeover by de-energizing the existing long enough to energize the new feed.


Which just goes to show that yes, it can be powered down.

Thanks for sharing. :thumbsup:


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