Pole top transformers

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mdshunk

mdshunk

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I understand how two bushing pole top transformers work... much like the dry transformers inside electricians are used to on a day to day basis. Question is, how are the more common one bushing pole top transformers connected? Where does the second high voltage lead connect to? There's darned little on the net about power line construction. I figure the more I know about distribution, the better troubleshooter I might be. Thanks...
 
Re: Pole top transformers

I figure the more I know about distribution, the better troubleshooter I might be.
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I'm not sure this will help but here goes.

A pole mounted transformer only needs a 1? line to feed it. Assume you have a 25 kVA transformer fed from a 1? line and the primary is 13.2 kV. We use a wye system so the voltage to the neutral is 7.62 kV. We will fuse that transformer with a 15 T fuse that will carry 150% indefinitely (just for fun, figure out how much kVA it will take to blow the fuse). Internal to the transformer, one end of the primary coil is connected to the primary bushing; the other end of the primary coil is connected to the tank which in turn is connected to the system multi-grounded neutral.

There are three secondary bushings on that type and size of transformer. The secondary coil is in two parts with the parts in series with each other. The ends are connected to the outside bushings and the other ends are connected to the center bushing for the neutral.

For a 3?, 208Y/120 volt bank, the two connections that are brought out to the center bushing are now taken to the end bushings so the coils are now in parallel and the output is 120 volts. We then connect a transformer to each phase and one end to make the center point of the wye. :D
 
Re: Pole top transformers

The two primary bushing (big ones on top) trans can be connected either line to line (high voltage) or line to neutral on a wye distribution line (not so high voltage) depending on the line voltage and voltage class of the transformer. If it is going to be connected line to neutral, it is cheaper to use a trans with one end of the primary connected to a lug on the side. One less bushing, Usually lower insulation class winding etc.

So it depends on what is required for the service, what the loads are on the number of phases which are on the poles. There may only be one phase there, so the only connection possible is line to neutral. Branch lines are like this out here in the country. Or there may be 3 phases and the residential services are all on one and the commercial is supplied open wye/open delta 3 phase from the other two. That is how it is on the main line from the substation down our way. I think this is partly because the 3rd phase was only recently installed and because it keeps any flicker from the commercial customers starting motors etc. from affecting the residential.

someone else may have other reasons to add. I'm like you, I want to know if I look at the transformer bank, if one has a wire burnt off it or what.

Jim

edited to add primary to the description of the bushings.

[ July 03, 2005, 10:29 PM: Message edited by: ccjersey ]
 
Re: Pole top transformers

I appreciate that, guys. Then, is it fair to say that since a one bushing distribution transformer has the one primary lead connected in with the secondary midpoint, it this really just an AUTOTRANSFORMER rather than a transformer??

I realize that this may not be the best forum for lineman questions, but I did try to ask on lineman sites. There's not much love on those sites.

[ July 03, 2005, 10:28 PM: Message edited by: mdshunk ]
 
Re: Pole top transformers

autotransformer? I don't think so. The primary is not connected internally to the center tap of the secondary. That tap doesn't have to be connected to anything if you don't need the 120. To get 120, the center tap of the 240 volt secondary is referenced to ground ( along with the neutral of the primary) On the 3 phase delta bank only one transformer center tap is connected to the neutral, the other(s) are connected to the 240 bushings only.
 
Re: Pole top transformers

Then where does the system multi grounded neutral connected to the pot? Does it just have a bolt on the outside of it somewhere?
 
Re: Pole top transformers

you got it! cheap and effective.

The utility could use that one type transformer to build single phase services 120/240 3 wire, 3 phase wye services 120/208 3 phase 4 wire, 3 phase delta services 240 3 phase 3 wire and centertapped 3 phase delta (120/240 3 phase 4 wire). All with a wye or line - neutral primary.
 
Re: Pole top transformers

Actually, we don't use a single bushing transformer for the delta configuration. We use a two bushing transformer, connect the primary in a wye configuration, and float the primary neutral. On the secondary side, we will normally connect the mid-point of one of the phases to ground and the MGN for the "neutral" and get a 120/240 high leg delta. For smaller services, we will use just two phases of primary, single bushing transformers, and an open wye/open delta configuration. :D
 
Re: Pole top transformers

Originally posted by charlie:

For a 3?, 208Y/120 volt bank, the two connections that are brought out to the center bushing are now taken to the end bushings so the coils are now in parallel and the output is 120 volts. We then connect a transformer to each phase and one end to make the center point of the wye. :D
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Charlie -- how do you change the connections at the center bushing to get the 2 coils in parallel? Is this done inside the transformer tank?

Do you reconnect the internal leads to the secondary bushings inside the tank so that the 2 coils are in parallel and are both connected to the 2 outside bushings, leaving nothing connected to the center bushing?

I'm trying to understand what you do (and how) mechanically on the transformer to get the 2 coils in parallel, producing just 120V.

Can you tell by looking at the outside of the transformer what has been done?

208Y/120 around here is almost exclusively done with pad mounts -- I've never seen it done with pole-top transformers. I'd like to be able to spot it when I run across it.
 
Re: Pole top transformers

Do you reconnect the internal leads to the secondary bushings inside the tank so that the 2 coils are in parallel and are both connected to the 2 outside bushings, leaving nothing connected to the center bushing?
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Exactly, however, we use the center and an outside bushing. We then tape up the other outside bushing so it is obvious that is has been done.

When the three transformers are in a cluster on the pole, we will run a single conductor from transformer to transformer to transformer and then down for the neutral. We call that conductor a "ring bus" because of the way it looks from the ground. At a glance you can tell if it is a wye but you can't tell if it is 208 or 480. :D
 
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