How is a pole mounted transformer wired?

infomike

I'm having trouble understanding how a pole mounted transformer is wired. I am astounded at how little information is online about one of the most common devices on the grid.

The typical single phase transformer has a single wire connection from one high voltage power line. To have a complete circuit on the primary side of the transformer, there must be another wire to which it's connected. Am I correct in assuming that the other connection is to the multi grounded neutral? If so, does the multi-grounded neutral carry current for all 3 phases of the high voltage lines? Does this MGN wire originate from the substation transformer? It appears that the the secondary, neutral tap from the transformer is also connected to the MGN, which also seems confusing.

FOIWATER

You're right one end of the winding is presented to the high voltage line, and the other must be attached somehow to the source.

In the event of a three-phase transformer, the return path is on another phase. A returns on B, B on C, and C on A. This is the case for a delta connected three-phase. A wye connected three phase has all the returns from the coils connected to the neutral point of the source, through a neutral wire. A single-phase, as is the case in one pole mounted distribution transformer, as well uses the neutral from a three phase system. The substation feeding the system has a wye with grounded secondary transformer, and even in the event your xformer is single-phase, that phase is more than likely part of a three phase system.

As for the part about the secondary being grounded, it's true as well. It's not so confusing. They center tap the secondary of the coil, so as to provide a means of connected 120 or 240 volt loads. Then they ground the center tap for protection reasons. Your appliances and wiring housing (junctions boxes) are all bonded to ground in your house. So in the event of a fault, such as a live wire contacting something metal, that bond will allow current to return to the neutral point of the transformer through ground, rather than through you.

You know what I mean?

infomike

I got most of that, Foiwater, and I understand the 3 phase transformer, but it's the typical single phase transformer that sits out on my street that I am not clear on. You say that it's probably part of a 3 phase system? I know that it only taps one phase of that 3 phase system, but the primary windings seem to also be connected to a MGN that runs along all power poles in the distribution system. Is that the return? I would also like to know which role of the MGN plays in this entire system.

dlgoff

If your pole transformer looks like this,



it's wiring looks like this,



With one terminal of the primary and the secondary center tap connected to ground. So yes, all the pole transformers have a primary winding connection to ground as does the secondaries center tap; a "multi grounded neutral (MGN)" as you suggest.

dlgoff

I've always loved this Science Channels "How It's Made" episode.

FOIWATER

Yes that is the return, I will associate myself with dlgoff's post as well.

The role of the MGN is so that a single-phase primary has a return so current flows, back to the substation transformer.

infomike

I added to the diagram to show my understanding of the wiring? Is this correct?

So, you are saying the the multi-grounded neutral wire provides a return for all 3 phase lines? Since, this wire appears to be thinner than the other 3 wires, I am assuming that it does not carry a lot of current.

I have seen many sub-stations that are not attached to the MGN. Therefore, I am wondering exactly where this wire originates.

dlgoff

Substations without grounds? Are you sure?



http://en.wikipedia.org/wiki/Electrical_substation

infomike

Substations all have grounds, but what I am refering to is the MGN (multi-grounded neutral).

I want to make sure that we are all discussing the same thing. A neutral is not necessary the same thing as a ground. (Of course, most of you know that) The MGN is a wire (current carrying, I believe) that runs parallel to the 3 phase wires on power lines, usually below them. There are wires that run down the poles to the ground that are attached to the MGN of course which keeps the MGN at the reference voltage.

So, my original observation is that I don't normally see the MGN wire go to some substations.

jim hardy

from op yes to all 3.

from last post I lived in a part of the counrty with lots of lightning. The neutral was run above the main wires to act as a lightning rod.

Look carefully and see if your MGN doesn't go at least to the structure(usually galvanized steel) holding the first insulators of the distribution lines.
When you build a substation the first thing you do is dig way down and lay a crosshatch mat of big wire mesh. That establishes a ground plane for the substation. Then you fill to level of substation grade. All structural steel is tied to that mat by big conductors that are usually not insulated, they look innocuous and simply disappear down into the fill. So the MGN need only be tied to the galvanized steel tower that holds the insulators for the phase lines. That steel is tied to the mat underneath everything. Next time you're around a substation look at the feet of the towers for that bare "earthing" wire.

One purpose of the ground mat is to limit the voltage drop along the earth should a fault occur, such as a wire falling to earth. They want the volts per meter along ground to be low. A walking man's two feet span roughly a meter and they want the voltage between his feet under fault conditions to be insufficient to electrocute him.

Of course it also provides a substantial surface area for dissipating lightbing harmlessly well below substation equipment. The whole substation "rides" the potential gradient like a magic carrpet.

old jim

jim hardy

ps reason the secondary neutral is tied to primary MGN is this

if transformer shorts primary to secondary they want the primary fuse to blow immediately lest your house wiring be subjected to distribution level voltage.

infomike

Thanks for the explanations, Jim.

As to this:
In my area, there is a wire above the high voltage transmission lines that as far as I know, does not normally carry current, but acts as a lightning attractor. Often, these poles also carry a MG neutral line. My understanding was that lightning attractors were not designed to carry current as neutrals do.

So, does the MGN wire to the substation connect to the wye-connected junction of the 3 phase coils of the transformers? I've heard that a neutral wire is needed for unbalanced loads.

jim hardy

Yes, most likely through that ground mat.
yes. If you draw three equal length vectors 120 degrees apart they will add to zero.
Think of those vectors as representing phase current, so long as phases are balanced there's no current left over to flow in the neutral.
Of course perfect balance is unlikely in something random like residential use so the 'leftovers' do return via neutral. The majority of the load current does get cancelled as demonstrated by that vector trick. So neutral is often a smaller conductor.

Study the wires while you're out walking and see how they distrinuted the loads in your neighborhood. Where i grew up each block was fed by one phase, maybe four transformers for the twelve to sixteen houses on that block.

infomike

I checked a couple of substations and did find that the neutral wire went underground with the power lines. I assume that it connected to the ground mat that you mentioned.

It's definitely making more sense now. As far as my neighborhood is concerned, there are only two of the phases tapped, each for about 30 homes.

jim hardy

Someday you'll see a substation crew there. Ask them for an introduction to the layout and about where that neutral goes. A utility crew showed me which breaker fed my neighborhood.
Of what practical use that was i don't know but the joy of understanding was, like the Visa commercial says, "priceless".

ibid.
Actually, that's quite nice to hear. Thank you.

infomike

I actually saw a crew there once, but I was afraid that they would think that I was a terrorist if I asked questions like that.

Mike_In_Plano

What a sad, aweful, and unfortunately true thing to say about our times.
To ask a straightforward question about an oddity has always been enough to set the occassional mean spirit off.
Now, this person has an entire support system.