What are these power pole transformers for?

[letterman]

In rural areas I often see on power poles these things that look like small transformers, which are not near any buildings. They are on the primary lines, which I understand are generally around 15KV. These look similar to the transformers used by homes and buisnesses, except they are smaller, particularly smaller in diameter. They only have two insulators on the top. They hang on a pole. The wire coming to that pole enters one of the insulators, the wire leaving that pole (going to the next pole) is on the other insulator. In other words they are in series with the wires and on the HOT side. The ground wire (lower on the pole) is not affected at these poles, it just continues on.

What are these things for? What do they do?

I should mention that I have seen THREE of them on poles that furnish

3phase power (4 wires with the ground). Each of the 3 wires goes to one of these things.

Thanks

[TD]

[e-mail] wrote in news: [e-mail]:

They're put there by the government to spy on you.

[Jim Redelfs]

They are line conditioners. They correct voltage drop.

[Steve B.]

Here's an answer from our family electrical inspector...

Steve B.

"Oil circuit breakers and/or re-closures?

95% of things in series (on power lines) are some type of switch. Often you will see they also have a fuse in parallel to them (but open)..the fuse is in series with the power line also.. the fuse is there, to be manually closed, in the event the C/B goes bad or needs servicing.

Because of the voltage, if the breaker needed to open, especially under "fault" (short) the amperage it needs to break might be thousands of amps, therefore , the operation is all contained in a canister filled with non-conductive oil. The contacts opening in oil, without air, prevents flash & damage to the C/B.

They could be re-closures or a combo beaker/re-closure. Re-closures are critters that, as their name says, reclose the line. They are set for a certain amount of times (usually 3) so if something happens like a squirrel gets on the transformer, the squirrel shorts, the breakers opens, light go out.. the "blast" at the transformer ends with the squirrel flying, clearing the "fault". The re-closure, in a preset time, closes the line back minimizing the outage.

[Nate Nagel]

That makes more sense than a transformer to compensate for voltage drop; one would think that that would require a ground/neutral connection.

nate

[bud--]

A little longer answer:

Imagine rural distribution starts from a point with distribution wires going out like a tree. If you have a fault at some point you want minimal disruption of the distribution. Everything downstream from the fault would be disconnected and everything else would be on.

You could put fuses at 1 mile, 2 miles, ... from the feed point to isolate the fault. The problems is that a fault at 5 miles is likely to blow the fuses at 4 miles, 3 miles, 2 miles, 1 mile. The outage is not minimized.

A method commonly used is to put "sectionalizers" at 1 mile, 2 miles, 3 miles. At the source is a "recloser". On a fault, the recloser opens.

If there was fault current at the 3 mile sectionalizer, the fault is beyond that sectionalizer and when the recloser opens that sectionalizer will open.

The recloser then closes. If there is still a fault the recloser opens again. If there was fault current at the 2 mile sectionalizer it will open. The recloser connects again....

Sectionalizers 'count' the reclosures and open depending on how they have been set, which depends on how many sectionalizers there are to the recloser. After the closest sectionalizer would have opened, if there is still fault current the recloser stays open. The result is only the minimum required part of the system is killed.

Sectionalizers always open when the circuit is dead so they don't have to open on fault currents. That means they can be pretty small. That is probably what you saw.

Reclosers do open on fault current - multiple times. They have to be quite large, more like a refrigerator or larger.

You need a sectionalizer (and recloser) in each hot wire. With 3 phases on the pole you need 3.

[Pete C.]

You can tell the regulators pretty easily, they are typically fairly large and they nearly always have a decent size dial gauge mounted at the top angled down which indicated the transformer tap is currently selected. They normally also have a conduit down the pole to a small control box.

[mkirsch1]

As was mentioned earlier, the fuse is there to bypass these devices in case the device needs to be serviced.

[Craig]

you have the correct answer, the others are ... well "offline" LOL A picture is worth a thousand words, but capacitors usually are square and are used for power factor correction, line boost transformers are just as common and sometimes you see both on the same pole

[David Nebenzahl]

On 5/20/2008 8:00 PM Craig spake thus:

As I pointed out elsewhere in this thread, there's no way they can be transformers, "booster" or otherwise.

Unless you want to explain to us how those a transformer can work with only 2 connections ...

[Joe]

They are power factor correction capacitors. Every long service line must have these at predetermined intervals to keep the current and voltage in phase. In industrial plants some distance from a substation, machines like welding rigs will even have built in capacitor power factor correction to reduce current draw. Visit any power company supply yard and you will see the capacitors stored with the switches and distribution transformers. You may find a friendly engineer in the office to explain the deal for you. HTH

Joe.

[M Q]

If they are in series with the lines, they are not power factor correction capacitors. To be power factor correction capacitors they would have to be connected line to line or line to neutral.

To be

[David Combs]

That final paragraph -- could someone explain them in a bit more detail?

Thanks,

David

[TKM]

Power factor-correcting capacitors are usually rectangular metal boxes fastened on a cross-arm near the top of the pole.

Is there a small loop on the devices in question? If so, the device is a switch/fuse that is designed to open the line at that point. The utility crew has a long insulated pole that hooks on to the loop so they can operate the switch from the ground or bucket truck.

If there is no loop, I'll guess that the cylinder is a lightning arrestor that will short the high voltage from a lightning strike to ground.

TKM