Question for a lineman...

[wyliesdiesels]

I realize there's probably not many lineman on this site but here goes:

Many but not all of the 3-phase 208Y/120 transformer banks in my town are hooked up differently than the usual way of wiring wye transformer banks! Instead of the service neutral being hooked up to each of the 3 transformer taps opposite the tap each phase leg is connected to, some banks have the neutral coming off the center tap of each of the 3 transformers. In other words, the center taps of all 3 transformers are hooked together to make the system neutral.

My question is why are they wired like this?

My guess would be that only half of the secondary coil is being used because of rated voltage?

And these banks are definitely not delta connected secondaries! If they were, the system neutral would only be coming from the center tap of 1 transformer.

I will try to snap a pic next time I come across one.

 

[Rtw5150]

Are they connected in a delta wye configuration?

 

[wyliesdiesels]

Yes, delta primary, wye secondary. I'm not sure how this matters as the wye banks in town that are wired the usual way on the secondary are also delta connected on the primary side!

Come to think of it, I've only once seen a 3 phase bank that was wired wye primary! And its in town here.

 

[nehog]

...

And these banks are definitely not delta connected secondaries! If they were, the system neutral would only be coming from the center tap of 1 transformer.

I will try to snap a pic next time I come across one.

Do take a clear picture (or a bunch of them) and I'm sure one of us will answer the question. I'm reluctant to give an answer right now because I'm not 100% sure of what you are asking--a picture will clear things up.

 

[superdel]

The xfmrs on the poles are in a delta configuration to drop 240 3 phase into business and small shops and make the power easier to transmit. (480 is usually in industrial parks and such). Since there are residential services usually mixed in, they take a line off each side of the xfmr and give it to you as 240 single phase, one line for each side of the breaker panel. They then take the center tap and use it for your neutral. So you have a reference for 120 in the panel. Usuallly one transformer on the pole is bigger than the others. This is the one they use for the residential. If there are no businesses or shops in the neighborhood, they are doing this same thing on all 3 xfmrs and they will all be the same size.
I would think that the power company doesn't supply 240 in a wye configuration.
To echo nehog, I am not sure what you are asking or really looking at. More clarification and a picture would help. Specifically with what you are trying to do as the end game.
But, I hope my information does help.

 

[rlitman]

(480 is usually in industrial parks and such)

Wow, really. Around here, I've never seen anything near that low at the pole.

11,200 is the lowest I've seen on the pole. At work, that goes Delta Wye down to 480/277 for our service, and that is stepped down to 208/120 within the building.

I too really need to see a schematic of what you're describing.

 

[Stuart in MN]

Wow, really. Around here, I've never seen anything near that low at the pole.

I assume he's talking about the secondary voltage, from the transformer to the building.

 

[superdel]

Our power provider comes out of the sub station at I am pretty sure 69kV. Between subs it is 115kV. The 69 is then spread out through the town and dropped to 240 on the poles and fed to each house. The power company bugs it into the weatherhead for you and it is then dropped into a meter mounted on the side of your house. Newer housing develpments have the lines run underground.
For industrial parks, they either drop the 69 into your own switchyard so you can do what you want with it, or if you don't have your own switchyard, I think (but am not sure) that they give you 13kV and you have to have your own transformer to distribute 480 in house.

 

[rlitman]

I think (but am not sure) that they give you 13kV and you have to have your own transformer to distribute 480 in house.

That depends entirely on the power company. They all have different standards for neighborhood circuits.

 

[foolishpride]

I believe Superdel has is correct. They aren't 208/120 rated transformers. They are more likely 240 Volt secondary rated transformers, with 3 or 4 secondary bushings. So they are only using half the secondary windings for their 120 Volts.

I've included a nameplate on a transformer with 4 secondary bushings.

 

[wyliesdiesels]

The xfmrs on the poles are in a delta configuration to drop 240 3 phase into business and small shops and make the power easier to transmit. (480 is usually in industrial parks and such). Since there are residential services usually mixed in, they take a line off each side of the xfmr and give it to you as 240 single phase, one line for each side of the breaker panel. They then take the center tap and use it for your neutral. So you have a reference for 120 in the panel. Usuallly one transformer on the pole is bigger than the others. This is the one they use for the residential. If there are no businesses or shops in the neighborhood, they are doing this same thing on all 3 xfmrs and they will all be the same size.
I would think that the power company doesn't supply 240 in a wye configuration.
To echo nehog, I am not sure what you are asking or really looking at. More clarification and a picture would help. Specifically with what you are trying to do as the end game.
But, I hope my information does help.

Your response tells me that u didn't read my post at all. I clearly stated the banks I'm talking about are 208Y/120 not 240 delta! These banks DO NOT feed houses!

To be more specific, the neutral in a 208Y/120 transformer bank usually is connected to the X2 or X1 bushing of all 3 transformers, with each phase leg coming off one transformer, and its tied to the bushing opposite the neutral, X1 or X2.

The banks I'm referring to have the neutral connected to the X0 bushing of all 3 transformers NOT X1 or X2! And, no, these are not 4 bushing transformers!

So I'm curious why this is!

 

[Terry454]

Guys, I can't tell you about the transformers -- it has been too long since I went to that school, and my field work was never that kind of stuff., so i don't remember.

Power companies distribution voltage depends on the company. In the Chicago area it is 13KV more or less. In more rural areas it is 34Kv -- longer distances to go.

69kv and up is generally considered transmission voltage, and in my travels around the country I have rarely (never say never) seen 69kv used as a distribution (to your home transformer) voltage. 24kv and 34kv are common distribution voltages.

FWIW: Transmission (as was said: between the substations) voltages can run from 69,000 to 760,000. At 760Kv the disconnects in the yard are 29 feet long. you do not want to make a mistake with that stuff. Common transmission voltages are 138kv and 365kv. There are 230kv and I am sure some others in specific places for certain reasons.
Terry

 

[wyliesdiesels]

I believe Superdel has is correct. They aren't 208/120 rated transformers. They are more likely 240 Volt secondary rated transformers, with 3 or 4 secondary bushings. So they are only using half the secondary windings for their 120 Volts.

I've included a nameplate on a transformer with 4 secondary bushings.

That's what I thought, that they are 4 bushing transformers but they only have 3 bushings each and they are feeding a 208Y/120 service! They are all the same size- 37KVA! And like I said each phase leg comes off one transformer bushing and is NOT connected to either of the bushings on the other 2 transformers so I know its wired in a wye configuration not delta!

 

[billsr4]

The winding in a standard 12/240 volt tranformer are as follows:
2 sets of windings

A------ B C--------D

120/240 VOLT A= 120V B&C TIE(CENTER BUSHING) NEUTRAL D =120 VOLT

120/208 VOLT A&C TIE 120 VOLT B&D TIE NEUTRAL

YOU WILL END UP WITH 1 BUSHING NOT USED AND THIS WILL ONLY BE IN A 3 PHASE BANK CENTER TAP (NEUTRAL TIE ON ALL 3 TRANSFORMERS , LEFT BUSHING FROM EACH POT WILL BE 120 VOLTS PH TO NEU AND 208 VOLT PH TO PH

 

[Jarcese]

^Beat me to it.

To answer the question of the capacity of the transformer; Full windings of the secondary are being used so you would not diminish the rating of the transformer.

Page 32 of the GE Transformer book

http://www.alexanderpublications.com/images/samples/Dist.Trans. Handbook.pdf

Edit: I don't think that's the GE book but looks pretty close.

 

[Zeke]

At 760Kv the disconnects in the yard are 29 feet long. you do not want to make a mistake with that stuff.
Terry

Whoa, I guess not. What does that equipment look like?

 

[wyliesdiesels]

^Beat me to it.

To answer the question of the capacity of the transformer; Full windings of the secondary are being used so you would not diminish the rating of the transformer.

Page 32 of the GE Transformer book

http://www.alexanderpublications.com/images/samples/Dist.Trans. Handbook.pdf

Edit: I don't think that's the GE book but looks pretty close.

I don't recall anyone asking about transformer capacity! I know how to calculate that- 37kva*3*2.8=~311 amps

Man, some people really don't pay attention on this board!

 

[Terry454]

Whoa, I guess not. What does that equipment look like?

I wish I had pictures. It is the kind of stuff you need a truck beside to get some perspective. The bus bars and wire sound like frying bacon, even on a dry day. Transformers are bigger than the house I live in, and that is for each phase.
Terry

 

[mrb]

Whoa, I guess not. What does that equipment look like?

probably something like this

 

[Jarcese]

I don't recall anyone asking about transformer capacity! I know how to calculate that- 37kva*3*2.8=~311 amps

Man, some people really don't pay attention on this board!

Calm down. I was just trying to help answer a question. Somebody asked about looking like only half the coil was being used. I assumed he thought the capacity would be cut in half, I guess I was wrong.

Congratulations on knowing how to do math, you should be very proud of yourself.

 

[coolreed]

I realize there's probably not many lineman on this site but here goes:

Many but not all of the 3-phase 208Y/120 transformer banks in my town are hooked up differently than the usual way of wiring wye transformer banks! Instead of the service neutral being hooked up to each of the 3 transformer taps opposite the tap each phase leg is connected to, some banks have the neutral coming off the center tap of each of the 3 transformers. In other words, the center taps of all 3 transformers are hooked together to make the system neutral.

My question is why are they wired like this?

My guess would be that only half of the secondary coil is being used because of rated voltage?

And these banks are definitely not delta connected secondaries! If they were, the system neutral would only be coming from the center tap of 1 transformer.

I will try to snap a pic next time I come across one.

If a business is using a three phase input from the utility (delta-wye) they cannot have three nuetrals, so the nuetrals are tied together. Keeping in mind the nuetral is the center tap for each tranformer output from the utility.

 

[wyliesdiesels]

If a business is using a three phase input from the utility (delta-wye) they cannot have three neutrals, so the neutrals are tied together. Keeping in mind the neutral is the center tap for each transformer output from the utility.

No I'm talking about something different. What I'm talking about is a different way of wiring a wye transformer bank.
----------------------------------------


Ok so I took some pics of 3 different wye transformer banks in town that are wired differently than the usual way of wiring a wye bank!

Notice how the service neutral is connected to the X2 bushing of all 3 transformers. The normal way of hooking up a wye transformer bank has the service/customer neutral connected to the X3 bushing of all 3 transformers and the 3 phase legs are each connected to 1 X1 bushing. Its almost as if only half the secondary coil of each can is being used since X2 is the center tap bushing!

 

[Jarcese]

That is the normal way of hooking up a 120/208 bank. The windings are changed inside the transformer. Look at the transformer book that I posted and the post above my first reply. The secondary coil is not cut in half. If you hook the neutral up to x1 and the high side to x3 and tape the middle bushing without changing the windings then you have a grounded delta straight 240v bank. If you don't change the windings and hook a bank up like the pictures then you only have a 120 bank which is useless.

 

[wyliesdiesels]

While on the subject of pole mounted transformer wiring, do any lineman have a clue as to what the contraption in the following pictures is for? It does not feed any houses or businesses and there is no industrial equipment in the area like large well pumps! There are 2 conduits running down the pole with wiring from it- one goes into a live meter main mounted eye level on the pole with no other conduit coming out of it and the other goes into the ground. I'd say there's at least 5 of these contraptions throughout town. The large transformer, lower on the pole, has 4 bushings, 2 on each side!

 

[Jarcese]

I have never seen anything like that where I'm from. By your description my best guess would be a primary metering cluster. Looks like those black square things are CTs and the transformers are being used as PTs on the pole for measuring power usage which feed down to the meter so a meter reader can come see how much power is being used on the primary side.

Our primary metering is an all in one PTs and CTs that feed down to the meter and then anything beyond that including primary wiring belongs to the customer. I'll see if I can find a picture...

 

[Jarcese]

New style metering cluster:
http://abbmvit.blogspot.com/2010/12/primary-metering-units-where-instrument.html

 

[wyliesdiesels]

That is the normal way of hooking up a 120/208 bank. The windings are changed inside the transformer. Look at the transformer book that I posted and the post above my first reply. The secondary coil is not cut in half. If you hook the neutral up to x1 and the high side to x3 and tape the middle bushing without changing the windings then you have a grounded delta straight 240v bank. If you don't change the windings and hook a bank up like the pictures then you only have a 120 bank which is useless.

Ok, thx. So I reread some of the comments on the first page about 4 bushing transformers. Should all 4 bushings be visible on the outside of the cans? Cause in the pics I posted, there is only 3 secondary bushings on the outside of each can. The few 4-secondary-bushing transformers I've seen had all 4 secondary bushings on the outside!

I have never seen anything like that where I'm from. By your description my best guess would be a primary metering cluster. Looks like those black square things are CTs and the transformers are being used as PTs on the pole for measuring power usage which feed down to the meter so a meter reader can come see how much power is being used on the primary side.

Our primary metering is an all in one PTs and CTs that feed down to the meter and then anything beyond that including primary wiring belongs to the customer. I'll see if I can find a picture...

Ok, so I can see how this could be for primary metering. The only thing is, this contraption is @ the end of the distribution lines because cut off fuses are open that connect the downstream distribution lines. Doesn't metering equipment need to be in line to measure the load? Also, I forgot to mention that the primary lines are 3 phase(3 wires) but only 2 are connected to this contraption! And, if this is for metering a customer, it would have to be a really small load cause the wires entering the riser conduit look like #10 or #8!

 

[Jarcese]

I have never seen a 4 wire transformer in a bank. Usually a 4 wire transformer is the whole bank inside one transformer. The reason there aren't more 4 wire transformers is because if it breaks you have to replace the whole thing. If a single transformer in a bank goes bad then you just have to replace the transformer and in some bank setups you can leave that transformer out of the bank for the weekend if there are no crews around to replace it with 57% capacity of the full bank.

Hard to tell what's going on with that metering but the conduit going down the pole is most likely just the control cord and 120/240 going to meter cabinet not the service.

Could be out of service and they're just being averaged or they may have disconnected that service because a building was torn down and they never removed the equipment assuming it would be hooked up again one day.

Edit: I took a closer look at the pictures and it looks to be in and running. Primary goes through the fuse, then through a CT and then to the primary load. PT is tapped off the load side.

There are only two phases because it's delta primary, no 3 phase available which is wierd because usually only industrial customers have primary metering. It could have been to some government housing or something only needs single phase service.

 

[Jarcese]

Sorry to double post again but I see the other phase now. They're probably only measuring two phases and calculating the load on the third phase.

 

[foolishpride]

That is the normal way of hooking up a 120/208 bank. The windings are changed inside the transformer. Look at the transformer book that I posted and the post above my first reply. The secondary coil is not cut in half. If you hook the neutral up to x1 and the high side to x3 and tape the middle bushing without changing the windings then you have a grounded delta straight 240v bank. If you don't change the windings and hook a bank up like the pictures then you only have a 120 bank which is useless.

Correct. But I think you mean the Taps are changed inside the transformer. These will change your secondary voltage in 2.5% steps. Sometimes they must be changed internally on the small cans, and other of the bigger transformers will have a selectable tap changer on the outside. This will give you up to +-5% of the nominal voltage.

 

[foolishpride]

Ok, thx. So I reread some of the comments on the first page about 4 bushing transformers. Should all 4 bushings be visible on the outside of the cans? Cause in the pics I posted, there is only 3 secondary bushings on the outside of each can. The few 4-secondary-bushing transformers I've seen had all 4 secondary bushings on the outside!!

Yes. See post 10.

 

[Jarcese]

Correct. But I think you mean the Taps are changed inside the transformer. These will change your secondary voltage in 2.5% steps. Sometimes they must be changed internally on the small cans, and other of the bigger transformers will have a selectable tap changer on the outside. This will give you up to +-5% of the nominal voltage.

Yeah I did mean to say taps. I re-read my post for misspellings but I skipped checking my info.

 

[wyliesdiesels]

...in some bank setups you can leave that transformer out of the bank for the weekend if there are no crews around to replace it with 57% capacity of the full bank.

Yes, that would be delta secondary banks which around here are no longer offered! If a transformer in a wye bank fails, the whole bank is dead!

Hard to tell what's going on with that metering but the conduit going down the pole is most likely just the control cord and 120/240 going to meter cabinet not the service.

Could be out of service and they're just being averaged or they may have disconnected that service because a building was torn down and they never removed the equipment assuming it would be hooked up again one day.

Edit: I took a closer look at the pictures and it looks to be in and running. Primary goes through the fuse, then through a CT and then to the primary load. PT is tapped off the load side.

There are only two phases because it's delta primary, no 3 phase available which is weird because usually only industrial customers have primary metering. It could have been to some government housing or something only needs single phase service.

There's 2 conduits- 1 goes to the meter cabinet, the other goes down the pole and into the ground. The meter is getting 2 wires and the conduit going underground has 3 wires so the load is 3 phase, if I remember right. Why would 3 phase not be available(there's 3 primary lines overhead)?

Yes, it is live. And like I said earlier, there is no industrial customers in the area; its all houses; no government buildings either. And there's @ least 5 of these metering setups throughout town. Also, FWIW the digital meter kept going from total KWH to 'NREG' instead of saying what the amperes were.

Ok, back to the wye stuff. So if the wye bank was wired as in the picture, then the taps have to be changed. So what about the other way of wiring a wye bank where the service neutral comes off of the X3 bushings of all 3 transformers and the service phase legs each come off one x1 bushing- do the taps have to be changed then?

 

[Terry454]

While on the subject of pole mounted transformer wiring, do any lineman have a clue as to what the contraption in the following pictures is for? It does not feed any houses or businesses and there is no industrial equipment in the area like large well pumps! There are 2 conduits running down the pole with wiring from it- one goes into a live meter main mounted eye level on the pole with no other conduit coming out of it and the other goes into the ground. I'd say there's at least 5 of these contraptions throughout town. The large transformer, lower on the pole, has 4 bushings, 2 on each side!

My guess (SWAG) is those are voltage regulators. The box lower on the pole is the control for automatic or manual operation.
Terry

 

[foolishpride]

Ok, back to the wye stuff. So if the wye bank was wired as in the picture, then the taps have to be changed. So what about the other way of wiring a wye bank where the service neutral comes off of the X3 bushings of all 3 transformers and the service phase legs each come off one x1 bushing- do the taps have to be changed then?

Taps only need to be changed if you are experiencing either a high or low voltage condition from your service. Lets say for example you have a 480 volt service, and you have some frequency drives that are voltage sensitive, and you are reading 492 volts on the input. You could raise the taps on the transformer bank to get the voltage down close to 480.

The other way of wiring a Wye bank that you are referring to would be 3 single phase transformers that are each 12,470 Volt primary, and 480 secondary rated. This would be a 480/277 Bank. The transformers could have 3 secondary bushings, with the center bushing being the center tapped winding of the 480.

 

[foolishpride]

My guess (SWAG) is those are voltage regulators. The box lower on the pole is the control for automatic or manual operation.
Terry

The can on top, and the bigger one on the bottom look like transformers. The one on the right, I'm not so sure. Could be an oil switch, which would have 2 bushings on top. A voltage regulator would have 3 bushings on top.

 

[wyliesdiesels]

My guess (SWAG) is those are voltage regulators. The box lower on the pole is the control for automatic or manual operation.
Terry

This is definitely not a VR. There's VRs in the area, they're very large and have 3 bushings on top. The boxes you're referring to, left of the large black transformer, are disconnects. You can't see it in the pic but when I was looking up @ the pole, I could see the handles on the sides of the boxes!

Its definitely metering equipment as there are CTs!

Next time I see an MID lineman I'm gonna ask 'em what its feeding!

 

[wyliesdiesels]

...The other way of wiring a Wye bank that you are referring to would be 3 single phase transformers that are each 12,470 Volt primary, and 480 secondary rated. This would be a 480/277 Bank. The transformers could have 3 secondary bushings, with the center bushing being the center tapped winding of the 480.

The only thing is, the banks I've seen wired in this configuration, with the neutral coming off the X3 bushings not the center tapped X2 bushings, are feeding a 208Y/120 service going to commercial businesses. I will take a pic if needed!

 

[Jarcese]

You can change either tap inside the transformer so the high side can be on x1 or x3 depending on tap position. Neutral is going to be on the middle for 120/208. Around here the high side if always on x3. Basically it's just whatever the common practice is in the area. You could get screwed up if you don't change the tap position yourself and there was no standard throughout the company on tap position.

I wasn't looking at the picture close enough and didn't see the other phase. Forget that part that I said about no 3 phase on the pole, I was wrong.

That metering could feed primaries that go for miles until there is any riser or transformer feeding a service. They could also be just randomly measuring load on the system. Looks like possibly 4kv so it's not likely that they have any way of tracking load out in the middle of a circuit. Most companies won't waist money on SCADA(remote controlled switching and load reading) equipment on old 4kv garbage. Too many possibilities to keep guessing what it feeds, I guess.

When you ask what it feeds, just don't ask too many questions. Most lineman I know and/or work with are not very nice to people. I on the other hand would probably talk to you all day. You're better off finding a troubleshooter who most likely knows more about the system and talks to customers all day. I feel like the better people understand how things work, the less likely they are to be annoyed when their power is out. Most line chews assume everyone who isn't a lineman is stupid no matter who much someone actually knows.

 

[wyliesdiesels]

...Neutral is going to be on the middle for 120/208...

What's odd about that is I have seen 2 ways of wiring the neutral around here and that's the whole reason I started this thread! The other way has the high side connected to the X1s and the neutral connected to X3s! I will take a picture because it sounds like this is an odd ball way of doing it since u said the "neutral is going to be on the middle for 120/208!"

That metering could feed primaries that go for miles until there is any riser or transformer feeding a service. They could also just be randomly measuring load on the system. Looks like possibly 4kv so it's not likely that they have any way of tracking load out in the middle of a circuit. Most companies won't waist money on SCADA(remote controlled switching and load reading) equipment on old 4kv garbage. Too many possibilities to keep guessing what it feeds, I guess.

This is an old electric company having started up some 100+ years ago! So it might be some old equipment setup not worth taking down.

When you ask what it feeds, just don't ask too many questions. Most lineman I know and/or work with are not very nice to people. I on the other hand would probably talk to you all day. You're better off finding a troubleshooter who most likely knows more about the system and talks to customers all day. I feel like the better people understand how things work, the less likely they are to be annoyed when their power is out. Most line chews assume everyone who isn't a lineman is stupid no matter who much someone actually knows.

All the MID guys I've talked to were really cool but I think they were troubleshooters!