Pole xformer connections

[quogueelectric]

We all see pole mounted xformers with the primary pigtailed to the insulator every day . We know that this is one side of the primary winding of the pole mounted transformer. What or where is the other side of the primary winding connected to???

 

[mdshunk]

The primary and secondary neutral is common on a one bushing pot. There's basically a bolt on the side of the pot, near the bottom, for the neutral connection.

There are two bushing pots, but they're generally used to make up a bank and not really for residential distribution. I have a few pole top transformers, so I'll try to take a picture tomorrow if I remember.

 

[mdshunk]

Here's a picture of one I just found on the net. It's a weird one, but maybe you'll get the idea. It's got a primary neutral bushing on the side near the top. It's got an EGC type bolt on the side, near the bottom. It's got a jumper on the outside, between the can and the secondary neutral bushing. Normally, that jumper is inside the can, in my experience.

 

[LarryFine]

Marc, most of the transformers I see have that neutral-can jumper on the outside, just as your picture shows it, between the 240v bushings. The connection on the upper right is the one I don't recall, but I will look around tomorrow.

 

[mdshunk]

Marc, most of the transformers I see have that neutral-can jumper on the outside, just as your picture shows it, between the 240v bushings.

I suppose it just depends on where your local POCO buys their stuff, or how they specify it when they're purchasing. Same idea, in any event.

 

[quogueelectric]

primary current

primary current

So judging by the pictures it would be safe to assume that the power company is using the ground as a return for its primary side current. By the way that was an awesome picture thanks Marc.

 

[Smart $]

So judging by the pictures it would be safe to assume that the power company is using the ground as a return for its primary side current.

I'll say it's safe :grin: [as long as you don't mean earth ground, but rather the grounded condcutor]

Here's a pic I took of a pot across the street from my house:

 

[iwire]

So judging by the pictures it would be safe to assume that the power company is using the ground as a return for its primary side current.

No, not really, they are using the 'MGN' (MultiGrounded Neutral) conductor as the return.

 

[LarryFine]

You'll notice in Smart's pic that the transformer has two primary bushings, which means that it couyld be used in either a Y or Delta primary configuration, but here it's used as a single-ended primary in a Y system.

Also, just below the secondary neutral bushing, you can see the primary grounded conductor is crimped to the secondary neutral conductor. Upward it picks up the primary and the primary lightning arrestor.

 

[mdshunk]

Every now and again I see two bushing pots used for residential drops. Seems like a waste. Might be a rural utility, that only wants to stock one type of each KVa size. Just a guess.

 

[dbuckley]

I must be missing something here.

What those picxs look like is that the MV supply is three phase wye, and 'ground' is used as the return leg. Surely this means that if the ground comes adrift on the pole the neutral wire to the home is then the return path through the GEC...? That sounds outrageously scary to me.

I know that ground return is used in ruralville for an overhead single phase MV supply, to half the conductor cost, but surely not in urban USA.....?

 

[mdshunk]

I know that ground return is used in ruralville for an overhead single phase MV supply, to half the conductor cost, but surely not in urban USA.....?

You're mincing terms. The "return" is not the ground (earth). The "return" is the neutral conductor, which also happens to be grounded at every pole. Called the "system multigrounded neutral. It's a seperate conductor. Earth return is not presently used in the US. In both my picture and Smart$'s pic, the hot wire is ABOVE the pot, and the neutral is BELOW the pot, running from pole to pole. Check the pics again.

 

[iwire]

Earth return is not presently used in the US.

Not intentionally anyway.

 

[quogueelectric]

Then how

Then how

Is the multi grounded neutral conducting back to the source which would be the high voltage generators at the power company? If it is not conducting through ground then where is the primary load going to on the grounded side of the primary winding? I dont see a neutral conductor going back to the power company. I suspect some of the current could come down the grounded wire under that nice wooden wiremold that they put on each pole that a xformer sits on I just emailed a friend of mine who is a lineman and got him on the job for info.

 

[mdshunk]

Is the multi grounded neutral conducting back to the source which would be the high voltage generators at the power company? If it is not conducting through ground then where is the primary load going to on the grounded side of the primary winding?

It's going through that neutral conductor a few inches below the transformer. It's a big bare stranded thing, about 4/0. Not sure how you can miss it.

Sure, a tiny little probably is going through the earth. Kirchoff's law still applies. One reason we do equipotential planes in ag buildings and pools.

 

[LarryFine]

Is the multi grounded neutral conducting back to the source which would be the high voltage generators at the power company?

Actually, the source of this primary power is the local sub-station, not the generating plant, similarly to the secondary neutral current only seeking the transformer secondary neutral.

If it is not conducting through ground then where is the primary load going to on the grounded side of the primary winding? I dont see a neutral conductor going back to the power company.

You may not realize it, but you do see it. In the first pic, it's the bare neuttral of the triplex in the foreground, with several crimps on it, and in the second pic, it's the top of the three secondary conductors.

In other words, yes, the primary and secondary circuits share the same conductor in some places, and each current uses only the portion of that conductor that is between the source and the load for that voltage.

I suspect some of the current could come down the grounded wire under that nice wooden wiremold that they put on each pole that a xformer sits on I just emailed a friend of mine who is a lineman and got him on the job for info.

As Marc said, a small amount may flow through the earth, but the system grounded conductor (neutral w/more than one phase) carries most of the current.

 

[quogueelectric]

I do of course see the wire

I do of course see the wire

It's going through that neutral conductor a few inches below the transformer. It's a big bare stranded thing, about 4/0. Not sure how you can miss it.

Sure, a tiny little probably is going through the earth. Kirchoff's law still applies. One reason we do equipotential planes in ag buildings and pools.

But usually it is only local to the loads it is serving. It looks like it is one big happy ground rod convention where all grounds from everywhere the poles the house services and the frames of the xformers all converge here. However the primary winding has got to have a continuous neutral all the way back to the substation minus the balanced loads of the 3 phases which cycle between themselves in this Y configuration for the primary windings. Or it will be traveling back through the earth I think

 

[dbuckley]

Called the "system multigrounded neutral. It's a seperate conductor. ... In both my picture and Smart$'s pic, the hot wire is ABOVE the pot, and the neutral is BELOW the pot, running from pole to pole. Check the pics again.

After a brief adjournment to check the pix, I now see what you mean. That is "interesting". Presumably this does mean that the MV is a wye supply.

Thats very different to The Rest of the World where the MV supply is pure delta, and single phase pole trannies go between any two MV lines, and three phase obviously across all three of the MV lines.

Yes, I was definitely missing something

(Updated to say) A bit of googling later, found a very interesting article: The Hazardous Multigrounded Neutral Distribution System (PDF), linked from this very site!

 

[quogueelectric]

Thank you sincerely

Thank you sincerely

After a brief adjournment to check the pix, I now see what you mean. That is "interesting". Presumably this does mean that the MV is a wye supply.

Thats very different to The Rest of the World where the MV supply is pure delta, and single phase pole trannies go between any two MV lines, and three phase obviously across all three of the MV lines.

Yes, I was definitely missing something

(Updated to say) A bit of googling later, found a very interesting article: The Hazardous Multigrounded Neutral Distribution System (PDF), linked from this very site![/QUOTE You found the missing link I was looking for and it was right under my nose all the time. I will enjoy this reading.

 

[wptski]

How do you interpet the markings/size of the can? Along the same lines, how many residences are one can? Or does that depend??