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Old 09-12-2016, 10:12 PM   #1
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Default Open delta open wye transformer bank

We have a delta wye transformer bank with a failing transformer.
The primary is delta the secondary is wye low resistance grounded with a ground fault CT between the center point of the wye and the grounding resistor.

There is not much load on the bank and I am thinking why not disconnect the faulty transformer and run open delta open wye wile the transformer is being repaired
My question is when you run open wye can you have a neutral and how is it connected. I have racked my brain and I cannot visualize a neutral on a open wye.

The Voltages are primary 138 KV and secondary 13.2 KV

Thanks
LC
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Old 09-12-2016, 10:36 PM   #2
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i know when you open a closed bank the capacity decreases.i have a book with the open service for a open y but don't for life of me rember how it's done.
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Old 09-12-2016, 10:39 PM   #3
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Ummm, isn't that kinda risky?
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Old 09-12-2016, 11:28 PM   #4
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The load is just not there and I am also thinking it could be a little risky and the decision is not mine to make. That is good for me.
I just cannot visualize how you could do it or even if you can do it.

LC
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Old 09-12-2016, 11:29 PM   #5
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I'd pass that call up to the EE department.

My gut says: don't attempt anything tricky.

It's already got 'issues.'

Don't compound them.
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Old 09-12-2016, 11:37 PM   #6
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You are correct. That is way way above my pay grade I was doing some wondering.

LC
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Old 09-12-2016, 11:43 PM   #7
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By the way I would like to welcome everyone to the Industrial Electrical Forum.

For those of you that have been away this weekend Cricket put it up late yesterday afternoon.(11 Sept)

LC
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Old 09-13-2016, 12:10 AM   #8
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Quote:
Originally Posted by Lone Crapshooter View Post
We have a delta wye transformer bank with a failing transformer.
The primary is delta the secondary is wye low resistance grounded with a ground fault CT between the center point of the wye and the grounding resistor.

There is not much load on the bank and I am thinking why not disconnect the faulty transformer and run open delta open wye wile the transformer is being repaired
My question is when you run open wye can you have a neutral and how is it connected. I have racked my brain and I cannot visualize a neutral on a open wye.

The Voltages are primary 138 KV and secondary 13.2 KV

Thanks
LC

It been a while what I did long time ago .,,

However there is a gotcha you will reduce the capaitcy about 35%

but big question is the type of transformer set up is this a tri core or indivual core transfomer ?

If latter it is easy to do it but tricoil core forget it it can get wacky with it.

The pf will go right out of the window so just be aware on that.

It kinda same prinpical with lower voltage used on distubation transfomer plus industrail transfomers.
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Old 09-13-2016, 12:47 AM   #9
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One reason that you do not do it is that there will be no voltage at all on one leg of the wye.
With an open delta you at least have three fairly closely matched line to line voltages.
Not very good if you have any line to neutral loads, which is a prime reason for using wye in the first place. Not particularly good for the 2/3 of any single phase line to line loads that connect to that line.
If you have a three phase motor which expects a wye supply, you still don't make any connection to the wye point of the motor, and so you would in effect be single phasing the motor.

You can drive the delta primary of a delta/wye transformer from an open delta with only the normal capacity loss.
But removing one transformer whose secondary is one leg of the wye just should not work.
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Old 09-13-2016, 10:41 AM   #10
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I agree with Inetdog: Open wye secondary isn't a functional 3 phase connection. You're missing a phase connection point and would be single-phasing your loads

The only way I know you can ever run an open-wye is if it's the primary on a delta secondary. That way you still have 3 phase points for your load, but you run at 58% because of missing winding capacity.
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Old 09-13-2016, 11:27 PM   #11
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well as i said if i could find my book & i did, such a thing is not possible. a open delta/wye is not possible , but the other way is. with the wye you loose one leg it is no longer three phases available.this is in a transformer connection book by GE. this is hi to lo. don't see why it wouldn't apply in any application.if somebody knows how it's possible would love to see a diagram.

Last edited by circuitman1; 09-13-2016 at 11:33 PM.
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Old 09-14-2016, 11:04 AM   #12
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Think of an open Y as the standard 2 legs of a Y in an apartment building.

In essence, it's single phase, not 3 phase.

If there are closed ∆ systems downstream of the open Y, they'll feed into the dead leg to some degree but not even close to balanced 3 phase.

The only time you can remove a transformer and still have 3 phase is if the load is a ∆, not the line.
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Old 09-14-2016, 03:06 PM   #13
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Quote:
Originally Posted by micromind View Post
Think of an open Y as the standard 2 legs of a Y in an apartment building.

In essence, it's single phase, not 3 phase.

If there are closed ∆ systems downstream of the open Y, they'll feed into the dead leg to some degree but not even close to balanced 3 phase.

The only time you can remove a transformer and still have 3 phase is if the load is a ∆, not the line.
If the load is a delta and the supply is a wye, you still cannot remove a transformer and have three phases. Instead you just have a single phase feeding a delta load. You need three points of connection fed from a three phase source, with or without a neutral in addition, to get three phases.
An open delta has only two secondary windings, but three output connecting points.
An open wye has two hots and a neutral. You can derive the third phase from that if you put in the right combination of transformers, but just attaching a delta load to it will require the neutral to be unconnected, leaving you just a single line to line phase.
If you connect a delta load directly to two phase lines and the neutral, you will have a three phase corner grounded source but with the wrong voltage and phase relationship. (That is the closed vector triangle will have one 120 degree angle and two 30 degree angles instead of three sixty degree angles.)
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Old 09-14-2016, 10:31 PM   #14
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[quote=inetdog;3210954]If the load is a delta and the supply is a wye, you still cannot remove a transformer and have three phases. Instead you just have a single phase feeding a delta load. You need three points of connection fed from a three phase source, with or without a neutral in addition, to get three phases.
An open delta has only two secondary windings, but three output connecting points.
An open wye has two hots and a neutral. You can derive the third phase from that if you put in the right combination of transformers, but just attaching a delta load to it will require the neutral to be unconnected, leaving you just a single line to line phase.
If you connect a delta load directly to two phase lines and the neutral, you will have a three phase corner grounded source but with the wrong voltage and phase relationship. (That is the closed vector triangle will have one 120 degree angle and two 30 degree angles instead of three sixty degree angles.)if you have a wye in & delta out , it is called open wye/ delta. the bank is at only 65% capacity i belive. they do it all the time here where there are small three phase loads but large single phase loads. they do it all the time.
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Old 09-14-2016, 10:34 PM   #15
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[quote=circuitman1;3212386]
Quote:
Originally Posted by inetdog View Post
If the load is a delta and the supply is a wye, you still cannot remove a transformer and have three phases. Instead you just have a single phase feeding a delta load. You need three points of connection fed from a three phase source, with or without a neutral in addition, to get three phases.
An open delta has only two secondary windings, but three output connecting points.
An open wye has two hots and a neutral. You can derive the third phase from that if you put in the right combination of transformers, but just attaching a delta load to it will require the neutral to be unconnected, leaving you just a single line to line phase.
If you connect a delta load directly to two phase lines and the neutral, you will have a three phase corner grounded source but with the wrong voltage and phase relationship. (That is the closed vector triangle will have one 120 degree angle and two 30 degree angles instead of three sixty degree angles.)if you have a wye in & delta out , it is called open wye/ delta. the bank is at only 65% capacity i belive. they do it all the time here where there are small three phase loads but large single phase loads. they do it all the time.
this is an open wye/open delta it is done all the time.this diagram can be found in the GE transformer connection book.
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Last edited by circuitman1; 09-14-2016 at 10:38 PM.
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Old 09-15-2016, 01:42 PM   #16
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[quote=circuitman1;3212394]
Quote:
Originally Posted by circuitman1 View Post
this is an open wye/open delta it is done all the time.this diagram can be found in the GE transformer connection book.
Open wye primary and open delta secondary is just fine (with limitations), yes.
The OP was asking about an open delta to open wye transformer bank. That is, removing one transformer completely from a delta primary/wye secondary transformer bank.
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