Question for a lineman...

[Jarcese]

Try to find the GE Transformer book somewhere, it has every bank possible in it. The one I posted is doesn't have as many bank combinations. You can probably find the actual book for a couple bucks. I usually carry a few of them and give them out once in a while when people ask me detailed questions when I'm at work 'cause you can get caught up talking shop pretty easily.

I know with a grounded delta straight 240(or 480) bank you can put the neutral and high side on either bushing. Pretty sure it's not possible to get 120/208 3 phase without having the neutral in the middle. If you look at the book I posted, you can get 240 3 phase, 120 single, and 208 single phase out of a bank the way you described. With that bank you can have your single phase 120 for lighting, dryers and electric oven on 208(has to be rated for it obviously) and your air conditioning and elevators on 240 3 phase. This would be a good set up for an apartment building, but could be used anywhere as long as you have the correct rating. You would have to have one of the transformers with a neutral coming of x2 though.

I could be wrong on some things as I'm digging a little deeper into banks that I havn't seen in person or worked on. I've only seen them in books.

 

[Jarcese]

Here is a picture of the taps inside the transformer. You just unbolt them and move the connection to the winding to where you need it depending on the bank you need. This is the only picture I could find but you get the idea.

 

[foolishpride]

Here is a picture of the taps inside the transformer. You just unbolt them and move the connection to the winding to where you need it depending on the bank you need. This is the only picture I could find but you get the idea.

Your arrow is pointing to the shunt straps coming from the transformer windings that connect to the bushings. These have nothing to do with the tap changer. The tap changer is the black dial selector with the numbers 1-5 beside the date on your picture.

 

[Jarcese]

I didn't draw the arrow, I just found the picture online. When I was talking about taps where I misspoke earlier I meant the taps to the windings that connect to the outer bushings as showed. A tap changer raises and lowers the voltage but it doesn't change to 120/208. You have to move the straps on the bushing.

Where I work only step downs, pad mounts, and regs have tap changers. We do have single phase tap changers on 2.4kv to change to 7.9kv since they're the only two distribution voltage. Either way, tap changers are usually to change the size of the primary coil, it doesn't have anything to do with creating a bank.

Just to add on to why you see the high side coming off different side of the transformer is for polarity. Single phase transformers over 8600kv or anything over 200KVA regardless of voltage has subractive polarity. If you look closer at a higher voltage transformer, where you would normally see x1 you will see x3 or x2 on a step down. It doesn't matter where you change taps to windings as far as a customer is concerned. The standard is there because if you bank say a 900kva bank with a 333 bank on 4kv you will create circulating current and blow the **** out of everything you just built. Ask me how I know that one

 

[foolishpride]

I didn't draw the arrow, I just found the picture online. When I was talking about taps where I misspoke earlier I meant the taps to the windings that connect to the outer bushings as showed. A tap changer raises and lowers the voltage but it doesn't change to 120/208. You have to move the straps on the bushing.

Do you mean internally move the straps. You never have to move these internally. Only change the way you connect the bushings on the outside of the transformer.

Where I work only step downs, pad mounts, and regs have tap changers. We do have single phase tap changers on 2.4kv to change to 7.9kv since they're the only two distribution voltage. Either way, tap changers are usually to change the size of the primary coil, it doesn't have anything to do with creating a bank.

Regulators are a tap changer. They are simply an autotransformer.

Just to add on to why you see the high side coming off different side of the transformer is for polarity. Single phase transformers over 8600kv or anything over 200KVA regardless of voltage has subractive polarity.

Do you mean Additive polarity is standard for all single-phase distribution transformers 200 kVA and below having high-voltage ratings of 9000 volts and below.
Subtractive polarity is standard for all single-phase distribution transformers above 200 kVA irrespective of voltage rating.
Subtractive polarity is standard for all single-phase transformers 200 kVA and below having high-voltage ratings above 9000 volts?

 

[Jarcese]

You have to move the straps or taps to the windings for 120/208. Look at a diagram for a 120/208.. How can you accomplish that on the outside of a 120/240 transformer? If the straps are already moved by somebody other than the lineman, then no you wouldn't have to do anything. The way our company works is they come in as 120/240 or 277/480 from the factory and the transformer shop changes the straps in house, tapes lugs, removes ground straps if necessary, and then stamps the voltage on the outside. Somebody has to move those straps one way or the other to get 120/208, 240, or 480. It's a union thing, that's why our lineman don't do it on the job...

If you can show me how its's done, I'll believe you but I've never seen it..

Yes I realize how a regulator works.

Yes I agree, you stated the standard for polarity better than I did. I do believe the standard is closer to 8600 volts though.. Im not looking to get into an argument on every aspect of how transformers work. I was just pointing out why you might have a high side or neutral in different place and why you wouldn't want to bank different polarities together.

I was supposed to be on vacation this week and I end up talking about electricity half the time..

 

[foolishpride]

You have to move the straps or taps to the windings for 120/208. Look at a diagram for a 120/208.. How can you accomplish that on the outside of a 120/240 transformer?

On a 120/240 transformer with 3 secondary bushings, connect the center bushing of each transformer together and ground it. Don't use the X3 bushing. Then your 3 phases are X1 of each transformer.

If the straps are already moved by somebody other than the lineman, then no you wouldn't have to do anything. The way our company works is they come in as 120/240 or 277/480 from the factory and the transformer shop changes the straps in house, tapes lugs, removes ground straps if necessary, and then stamps the voltage on the outside. Somebody has to move those straps one way or the other to get 120/208, 240, or 480. It's a union thing, that's why our lineman don't do it on the job...

I've seen a few transformers that had dual wound primaries, (13,200 Volts x 6600 Volts) that in order to change them to match the voltage, you took the top off and moved a copper plate jumper from different studs. But I've never seen them with dual wound secondaries.

I love union tradesmen. I are one.

 

[billsr4]

Most of your 167.5 kva and some 100 kva transformers will have 4 secondary bushings .
You can make up your taps on the outside with jumper wires and have a delta 120/240 volt or a 120/208 volt secondary.

A 120 / 208 volt secondary is going to give you the full kva rating from each tansformer at 120 volts.

 

[VDubJoe]

We get all our single ph trans 120/240 3 bushing. If we are building a 120/208 wye bank. We open the lid parallel the windings by redoing the straps. Do not use the center bushing. Buss all three X3 together for the nuet. and x1 for hot legs.
We do it all onsite.

Joe

 

[Jarcese]

This is why how electricity works is so hard to grasp for people. You ask what you think is a simple question and you come up with 20 answers and none of them are wrong. There are so many ways to build something and get the same result... That's why I love doing line work, no pesky electrical codes to get in your way Just be safe and don't build something that's going to hurt someone else..

I can see how you can make 120/208 on the outside of a 4 wire can, but I still don't see how you get 120/208 out of the 3 bushing from the outside. If you dont change the straps and hook it up like a 120/208 then you only have a 120 bank, no 208 available. Maybe your transformers come ready for 208 and nobody at your company has every had to change anything?

 

[Jarcese]

We get all our single ph trans 120/240 3 bushing. If we are building a 120/208 wye bank. We open the lid parallel the windings by redoing the straps. Do not use the center bushing. Buss all three X3 together for the nuet. and x1 for hot legs.
We do it all onsite.

Joe

There's the answer to the question about no neutral in the middle on 120/208. I didn't know you could hook it up that way but it makes sense, just never really thought about it until you said it. We move everything to the right side of the x-formers and use x2 for neutral, x3 for high side.

I guess I was wrong in a couple of my previous posts... That's why I tried to preface my posts by saying "Where I work."

I actually learned a few things about different transformers and bank set ups from this thread.

 

[foolishpride]

We get all our single ph trans 120/240 3 bushing. If we are building a 120/208 wye bank. We open the lid parallel the windings by redoing the straps. Do not use the center bushing. Buss all three X3 together for the nuet. and x1 for hot legs.
We do it all onsite.

Joe

I too was unaware that you could parallel a 3 bushing secondary transformer. I thought that the X2 bushings was connected to the center tapped winding. I guess you could remove the internal strap connected to the X3 bushing, and take the strap from the X2 bushing, and move it to X3.

But this would be the same as using only the X1 and X2 bushings. I've never seen or heard of it being done this way.

 

[Jarcese]

I too was unaware that you could parallel a 3 bushing secondary transformer. I thought that the X2 bushings was connected to the center tapped winding. I guess you could remove the internal strap connected to the X3 bushing, and take the strap from the X2 bushing, and move it to X3.

But this would be the same as using only the X1 and X2 bushings. I've never seen or heard of it being done this way.

I was wrong too about getting 120/208 without changing winding taps on 120/240 3 bushing. You're right, you do get 120/208 but you only get half the capacity. That's the reason we reach in and change them so you can get full capacity of the transformers.

Things can be confusing when you're only taught one way to do things and nobody tells you why, which is the case where I work. You have to really want to learn on your own if you want to know "why" rather than "how."

 

[wyliesdiesels]

There's the answer to the question about no neutral in the middle on 120/208. I didn't know you could hook it up that way but it makes sense, just never really thought about it until you said it. We move everything to the right side of the x-formers and use x2 for neutral, x3 for high side.

I guess I was wrong in a couple of my previous posts...That's why I tried to preface my posts by saying "Where I work."

I actually learned a few things about different transformers and bank set ups from this thread.

This is the WHOLE reason I started this thread- was because I've seen wye banks wired 2 ways on the secondary side! When some of u kept saying that neutral connected to the x3 bushings doesn't work, I was really confused because I've seen it that way and I was gonna take a picture.

So, back to my original question- why do some wye banks have the neutral connected to the X2 center tap bushing while others have the neutral connected to the x3 bushings?

From some of the responses, I've gathered that it has to do with the voltage rating of the transformers and the possible changing of the bushing connections inside the can! Am I right?

Thx for all the responses BTW! This has been an informative thread!

I was wrong too about getting 120/208 without changing winding taps on 120/240 3 bushing. You're right, you do get 120/208 but you only get half the capacity. That's the reason we reach in and change them so you can get full capacity of the transformers.

Things can be confusing when you're only taught one way to do things and nobody tells you why, which is the case where I work. You have to really want to learn on your own if you want to know "why" rather than "how."

I said that on the first page, that it looks like only half the secondary coil is being used because of the neutral being connected to the X2 bushings instead of the X3s and someone shot me down! I guess I WAS right in that only half the coil would be used unless the internal bushing connections were changed!

 

[Jarcese]

Yeah sorry, I either gave some bad info or didn't elaborate enough. I actually went into our school to make sure I have the right answer for you.

There are a handful of combinations to get 120/208 out of a 120/240 3 bushing. Inside the transformer on the winding taps are the letters ACBD, you can't see them in the picture posted. You can put A and C together and B and D together any way you want using any two bushings. So you can have 6 combinations(ground ******** outside must be moved if x2 is not neutral):

Additive:
x1 neut x2 high
x2 neut x3 high
x1 neut x3 high

Subtractive(x3 is on the left):
x3 high x2 neut
x2 high x1 neut
x3 high x1 neut

You can change polarity either way by swapping neut and high side. You just don't want to do that because you won't be able to bank with another bank if it has a different polarity. In some cases you have to change the polarity to get something to bank like 333 step downs with 167.5 step downs on 7.9kv single phase because they come standard with two different polarities.

If you leave the windings as is A by itself C+B nuetral and D by itself standard 120/240, you only have 4 combinations with half the capacity and third bushing is live:

x1 high x2 neut
x2 neut x3 high
x1 nuet x2 high
x2 high x3 nuet

Hope that helps. That's my revised answer...

Edit: Wylie, I know the original question was if they are using half the windings. The answer is that you can(which I didn't realize until later in this thread), but you wouldn't.
Main reason is cost. If you want to build a 150 bank and you didn't parallel the windings(use the full windings), then you would have to put up three 100s in series instead of three 50s in parallel since you are using half the coil. If a 100 costs $2500 and a 50 is $1500, then you are wasting $3000 per bank. Not a big deal if you're a small municipal with a couple banks. Imagine a huge company like National Grid doing this. They would waste hundreds of millions of dollars.

 

[wyliesdiesels]

Ok, thx! The 2 different wye banks I've seen here are x1 high and x2 neut, x1 high and x3 neutral! I will take a few pics next time I drive by the banks I'm thinking of and post them here!