ECN Forum Forums Photo Discussion Forums Photos for General Discussion Primaries of pole mounted transformers

Thanks for the pics guys.



I would agree. 95% of the lines around me are 4 wire wye primary with grounded neutral. Do you know what primary voltage they use? In my area, I have noticed that some medium to small towns that are not served by the local electric co-op(s) are served by either their own municipal utility, or by the larger utility and have a small substation are more likely to be supplied with the 3 wire delta. (usually 2.4 kv) I think for several reasons: first, most of these towns have a steady population and the system is adequate. There aren't a lot of businesses that demand more than what a system like this handle. (many of these systems are quite old) There really is no need to replace it. Most of the distribution transformers that they use can be ran from many different voltages and systems. For example: take a two bushing transformer that can run on 2400 Delta, 4160Y2400 wye, 7200 delta or 12470Y7200 volt wye, plus some of the transformers have another tap for a 7.9 KV, the phase to ground voltage for a 13.8 kv wye system like in my town. 3 voltage taps, 5 different systems, one transformer.

A couple pics I shot in towns that have a 3 wire delta:
one older:


One newer installation:

This is a 3p wye

... and a 3p open delta center tapped (red leg)

Do power companies follow a standard method or rule, as to which transformer in a 3p open delta center tapped (wild leg) is to be on the leading phase of the two (120 degrees ahead) and which is always on the lagging phase (120 degrees behind)? Something like "The wild leg transformer shall be on the lagging phase of the 2 high voltage primary supply lines, as compared to the line the main transformer (the one with the grounded center-tap) is fed by". Or is it essentially random?

It is usually "B" but I am not sure it is really a rule.

It seams different utility co vary in there distribution norms . In the Centerpoint (utility co)of course 120/240 single phase is the most common, next is 120/240 open delta, with 120/240 delta and 480/277 Y next. Rare is 120/208 y. Some motel drops but most 120/208 is from customer owned transformers with 480 drops. Of the 100+ commercial customers in Centerpoint territory none have 120/208. Now in Entergy (utility co.)territory (north of Houston) again 120/240 single phase is most common, followed by 120/208 y then 120/240 open delta, then 120/240 delta & 480/277Y. My shop is in Entergy territory and is served by 120/208 y although I have only 120/208 single phase serving my suite.

It's amazing how much variation there is among utilities in their construction standards, as contrasted with work on the customer side of the meter.

For example, in my area (Dominion Virginia Power territory), there seems to be no effort to identify phases on distribution lines or to keep them in the same positions on the pole. It's common to see primary conductor positions interchanged where the line turns a corner. Linemen refer to wires only by their relative locations on the crossarm at a given point: "road phase", "center phase" and "field phase". Even on service drops from aerial three-phase transformer banks, I don't see any phsae identification. It appears that phase rotation is only established when connection is made to the cuttomer's service wires.

In contrast, PEPCO, the neighboring utility to the north (serving DC and its Maryland suburbs), seems to identify phase positions consistently on its primary lines. On certain poles I see wooden placards with nailed-on metal letters "A", "B" and "C", which I assume are phase labels. I haven't looked closely at PEPCO's construction so I don't know how the identification is carried through the primary and secondary lines.

Around here the PoCo used red, white or blue " labels" on some crossarms or at the bottom of the pole to identify what phase is what, especially if there is more than one circuit...
Even the single phase pad-mount transformers have either a red, white or blue sticker on them to identify what phase they are supplied from.

A.D

Where phases cross at angle poles was apparantly a necessary 'roll' of the wire to maintain separation. And you are right, very rarely are phases marked on overhead. Anytime a corner/junction pole is changed out, a major part of the work is making sure everything is put back exactly the same, so that customer rotation is not changed, and so that everything phases out for backfeeding purposes.

As far as marking phases on service drops, it really doesn't matter to us except that on newer meters the high leg, if there is one, needs to feed through the meter on the far right (the customer can move it to the middle on the next wire landing). Although, if you see quad (or triplex) service wire (underground or overhead), the wires are marked Phase A, B, and C from the factory.

Obviously, if work is performed on a three phase service, rotation will be maintained.