Synchronizing (2) feeders that are out of phase with each other 5

[bdn2004]

I work at a plant that is considering putting in a 4160V auto transfer scheme on their cooling water pumps. They want to do this with a main-tie-main-tie-main type configuration ( 3 - 4160V feeders). The problem is that one of the feeders is 60 degrees out of phase with the other two.

Is this possible? Are there more questions I need to ask?

 

[dpc]

A live transfer is not possible. You will need to briefly de-energize the system, allow the motors to coast down, then re-energize from the alternate source.

You should key interlock the main and tie breakers to makes sure someone does not get too creative.

 

[cranky108]

When using delta to wye transformers you will always see a phase shift. The phase shift can be + or - depending on the phase rotation entering the transformer primary.

 

[dpc]

I just noticed you mentioned an auto transfer scheme, so forget the key interlocks. You will want a time delay or an voltage relay to make sure the bus is dead before the transfer is completed.

 

[davidbeach]

If the phase shift is truly 60[°] and not 30[°], you can do some rewiring of the transformer that is 60[°] out to line it up with the other two transformers.

 

[dpc]

David may be correct, assuming that this would not screw up the phasing on some other portion of the system.

The 60 deg phase shift does seem odd. 30 degrees would be much more common.

 

[davidbeach]

60[°] is generally a miswired transformer, 30[°] is a delta-wye transformer. 60[°] could also be two delta-wye transformers in series.

Rewiring a delta-wye to change the phase shift by 60[°] will require swapping two leads on the primary and two leads on the secondary. Assuming an ANSI standard transformer, swapping A&B and a&b will put the low side 30[°] ahead of the high side instead of 30[°] behind the high side. Swapping one pair on the high side and a different pair on the low side will then also add or subtract 120[°].

 

[dpc]

Could be two delta-wye units with opposing phase shifts, I guess.

 

[davidbeach]

Yeah, that too.

 

[slavag]

Hi.
What do you mean auto transfer scheme?
I see this term several times and would like connect it to my understanding.
For me is: loss of one of infeed, trip of "bad" infeed ( in case of alternative supply), connect of supply.
Always break before make. I'm correct?
Regards.
Slava.
In case of bus with asinch. motors: Do you use system like to HBT motor bus?

 

[cranky108]

That's a good question, in that there are several schemes that can be called auto transfer schemes.
I know of the make-before-break used for source switching.
I know of the break-before-make used more as a auto-transfer for a loss of a source, or for source switching.
There is a scheme called fast transfer, where it is a break-before-make scheme, but with much shorter delays.

It is also possible to run the buses togather, or split, or with parallel sources.

 

[slavag]

Thanks Cranky108. Now it's more clear for me
For make before break scheme we use two terms:
1. Silent transfer.
2. Manual transfer.
Regards.
Slava

 

[bdn2004]

This is a power plant. Two of the sources come off of generators that produce the primary voltage at 15kV and its transformed down to 4160V. We have another, newer generating system that produces power at 20kV, bumps it to 138kV, then it's transformed down to 4160V. That is the third source that is out of synch with the other two. At present they use the two that are in phase to do make before break transfers. The trick is to bring in the other feeder....There are some good answers here..but I'm still a little confused as how to do this.

 

[dpc]

Normally for this application at a power plant, one of the transformers is made with the 30 degree phase shift in the opposite direction. This is a common setup. But without looking at a one-line, I'm not sure this is exactly your situation.

If you already have the transformers, you may be able to reconnect them, as David Beach mentioned.

You might have a look at this: faq238-1154

 

[davidbeach]

2*n*30[°] shifts are available from delta-delta or wye-wye transformers. (2*n+1)*30[°] shifts are available from delta-wye or wye-delta transformers. In both cases n=0,1,...,5.

Within those two groups, you can get 120[°] changes simply by rolling the connections on either the primary or the secondary (but not both). To get the 60[°] shift that sets up the rest of the possibilities you need to swap two phases on the primary and two phases on the secondary. How the swaps on the primary and secondary relate to each other determines the total phase shift.

Time to start drawing vector diagrams. Work with the standard delta-wye transformer and work with it until you understand why you get the 30[°] shift. Then start changing leads and see what happens to the vector diagrams.

 

[waross]

It could be a mix of additive and subtractive polarity transformers. That gives a 180 degree shift that looks very much like a 60 degree shift.
But if this is from an independent generator why can't you use the governor control to synchronize?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

Bill, all of the generators would be synchronized to the outside world. My guess is that the two older generators are connected to the 4160V bus from the terminals of the generator with wye-wye or delta-delta transformers while the output of the newer generator goes through two delta-wye (or a delta-wye and a wye-delta transformer and one of the phase shifts is backwards. A one-line would be a great help.

 

[bdn2004]

I think the light just went off in my head to what you are telling me...by simply changing the hookups on the primary and secondary we can create the 60 degree phase shift on the secondary. That would be an easy fix at the two existing transformers and it would not hurt anything because they are captive to the load. That would bring them in phase with the alternate feeder.

Frankly that is a great solution and potentially could save us lots of bucks. Thanks!!

The alternate source generator is 300 MW and like DB states is hooked up to the outside world. I'm talking about one of its feeds that is about 10 MW of load. And even it comes off of a large 4160V transformer.

I will sketch up a single line and try to post it tomorrow. I've not done that before.

 

[waross]

Thank you David. I understand now and agree with you.
Yours

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

bdn2004,

On the transformer nameplates, there should be a phasor diagram that will show the phase relationship between the two windings. You need to copy down all of this information, then sit down and draw out the phasor diagrams for the entire system you are dealing with. Then you can determine if reconnecting of the transformers can provide a solution. It's possible, but it will depend on the transformer connections.