4160 volt question

[ashtrak]

Could someone help me with a schematic for utilizing a three phase bank of transformers with one primary phase missing.
We recently lost one phase of our 4160 volt 4wire 60~ overhead line.We were able to refeed a lot of our single phase loads by retapping the primary of single phase transformers to a good phase.
However We have a bank of Three 25kva xfmrs that are single phaseing now.
I remember that two transformers in a three phase bank can be wired for three phase at (I think) 57% efficiency with one primary phase missing.
The reference book I have (Linemen manual) is missing this section, I can't read it clearly enough to get a good feel on the diegram and explanation.
We have 15kv aerial across the railroad and I need to cut a section out of the bad phase to get a cable type in order to order splice kits.
The three phase bank feeds a service to our catenary maint. car (wayside power)and i'ts critical that that rail vehicle is ready to go in an emergency.

Thanks

 

[winnie]

What is your _secondary_ configuration and voltage?

-Jon

 

[jim dungar]

It depends on the primary and secondary configurations, (i.e. delta-wye or gnd wye - gnd wye).

 

[ashtrak]

4160 question

4160 question

I guess we have a Y-Y as we have 4 wires on the primary of the three phase bank and 4 wires on the secondary.
4160 3~ 4 wire/ 120/240 3~ 4 wire.

 

[RICK NAPIER]

A secondary of 120/240 would be a delta.

 

[ashtrak]

4160 answer

4160 answer

Sorry/correct

 

[GUNNING]

wow... I'd say pull up a lawn chair pop a beer and wait for the fireworks. OR ask someone on the job that knows. There are never any dumb questions. Just say you are having trouble identifying the configuration on the transformer bank and need some guidance. Normal people like to help.

 

[ashtrak]

4160 answer

4160 answer

Can't see the configuration due to the location of the platform.
Can't get on the platform until we get more power off from other feeders.
Which can't be done until night.
Configuration/or what it was thought to be was relayed to me via people on the ground at that location.
Thought someone might have some schematics available so when we got our ducks in a row we could make some decisions based on different confiqurations.
we'll get the proper reference material and make a decision based on the facts eventually.

Thank you

 

[winnie]

Well, you have a total of 4 possibilities for the transformer bank when you have all three phases: delta-delta, delta-wye, wye-delta, and wye-wye.

The only common transformer connection used which requires only 2 supply phases (plus neutral) and provides 3 phases on the secondary side is the 'open-wye to open-delta' bank. This connection uses the same transformer voltages and connection as a full 'wye-delta' connection, just with one transformer missing.
See page 5 of http://www.jea.com/about/pub/downloads/contractor/OH-2-Transformers.pdf for an open-wye to open-delta schematic

It is _possible_ to do an open-wye to full-wye connection, however I don't belive that this is common because it would require _4_ transformers to work. I doubt that you will easily find a schematic for this connection.

If the transformer primary is _delta_, then it is not possible to derive a 3 phase secondary from 2 primary phases.

If the primary supply is a 4 wire solidly grounded supply, then the transformer primaries may still be connected delta. If this is the case, then even though the primary supply would be capable of supporting an 'open-wye open-delta' bank, your transformers would not be configurable to do this.

-Jon

 

[ashtrak]

4160

4160

Well, you have a total of 4 possibilities for the transformer bank when you have all three phases: delta-delta, delta-wye, wye-delta, and wye-wye.

The only common transformer connection used which requires only 2 supply phases (plus neutral) and provides 3 phases on the secondary side is the 'open-wye to open-delta' bank. This connection uses the same transformer voltages and connection as a full 'wye-delta' connection, just with one transformer missing.
See page 5 of http://www.jea.com/about/pub/downloads/contractor/OH-2-Transformers.pdf for an open-wye to open-delta schematic

It is _possible_ to do an open-wye to full-wye connection, however I don't belive that this is common because it would require _4_ transformers to work. I doubt that you will easily find a schematic for this connection.

If the transformer primary is _delta_, then it is not possible to derive a 3 phase secondary from 2 primary phases.

If the primary supply is a 4 wire solidly grounded supply, then the transformer primaries may still be connected delta. If this is the case, then even though the primary supply would be capable of supporting an 'open-wye open-delta' bank, your transformers would not be configurable to do this.

-Jon

Thank you for your explanation and the time it took.
It looks like the open-Wye to open -Delta is the way to go for us, page 5.
I found the similar schematic in the Electricians handbook.
We're going to do some rewiring when we can get to our xfmr platform and take some readings.
All kinds of thoughts are flying now, from renewing all three cables to going underground.

Thanks again

 

[charlie k.]

Ashtrak, I sure hope the fans I installed in the station didnt cause your problem.
I want to thank you and John for your help. You guys were great to work with.
Thank You.

Charlie

 

[ashtrak]

Fans vs 4160

Fans vs 4160

Ashtrak, I sure hope the fans I installed in the station didnt cause your problem.
I want to thank you and John for your help. You guys were great to work with.
Thank You.

Charlie

Hey Charlie

Great to hear from you.
Nope, it wasn't the fans they are working great.
We have some serious problems with our 4160 volt line and I came online asking questions without having all my info in order.
Hope the little credibility I had didn't all go out the window.
Anyway,we got our facts straight now,along with some of the info given by the guys here
we're gonna make a decision soon how to proceed.

Keep in touch Charlie

Dan