phasing out three phase

[Bill Schafer]

I have a question as to wether or not I can parallel two three phase
transformers with a common tie on a double ended switchboard arrangement.

We have two identical dry indoor transformers 13.8kV - 480/277V with primary
sources from same utility line. In order to do service on each tansformer
the tie breaker will be required to close, parallel the switchgear (xfmrs),
and open one transformer and asscociated main breaker without disruption.

I have heard this can be accomplished if each of the three phases voltage
angles are with 15 degrees of each other. I am not planning on performing
this task until I am educated enough on this. Can anyone help or provide
some additional information pertaining to the subject.

Thanks in advance.

 

[Don Kelly]

I have a question as to wether or not I can parallel two three phase
transformers with a common tie on a double ended switchboard arrangement.

We have two identical dry indoor transformers 13.8kV - 480/277V with primary
sources from same utility line. In order to do service on each tansformer
the tie breaker will be required to close, parallel the switchgear (xfmrs),
and open one transformer and asscociated main breaker without disruption.

I have heard this can be accomplished if each of the three phases voltage
angles are with 15 degrees of each other. I am not planning on performing
this task until I am educated enough on this. Can anyone help or provide
some additional information pertaining to the subject.

Thanks in advance.

------
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.

 

[SQLit]

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 relaying as well. Double-ended
substations can be tricky, but there are established techniques for
dealing with the problem.

Regards,
Chris

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.

 

[Bill Schafer]

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

 

[Don Kelly]

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

 

[Steve Alexanderson]

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.