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[Frank Koenig]

A common sight in these parts, especially in older, light-industrial neighborhoods, is a utility pole with two distribution transformers (often referred to as "pots" or "pigs"), one large and one small. I got curious about these and it turns out that they supply the infamous high-leg delta service. The big pot supplies the (typically) 120/240 V single-phase part just as if it were single phase service. Its secondary's center tap is connected to the neutral and grounded. The small pot has one end of its 240 V secondary connected to one end of the big pot's secondary and the other end supplies the high leg or stinger. So the three-phase part is an open delta configuration. There is no third transformer and the entire load is carried by just two of the primary-side phases.

More trivia: The big number painted on the side of almost every distribution transformer in the US is its capacity in kVA (kilo Volt Amperes). It's interesting to me how small the kVA rating can be in relation to the notional size of the service. For example, our vacation place in the Sierra Foothills has 200 A service. At 80% load that works out to 38.4 kVA. The lone transformer on the pole has a big "15" on it. I guess PG&E figures we won't turn on everything at once, at least not for very long. Question: does anyone know what, roughly, the thermal time constant of typical small distribution transformers is?

In leafy suburbs, like Palo Alto, the utility connects the secondaries of multiple distribution transformers in parallel. In this case undersizing the transformers is a pretty safe bet since the the larger the number of customers sharing the transformers the less likely it is that they all fire up their plasma cutters at once. I suppose it also increases reliability since a transformer failing open won't cause an outage. On the other hand, if the primary side fails open between transformers you could end up with quite an interesting circuit. I guess the utilities have thought through this scenario

--Frank

[John Roberts {JR}]

Question: does anyone know what, roughly, the thermal time constant of typical small distribution transformers is?

No but they have decades of experience with consumer demand. I wonder if things change after everyone on the block plugs in the Tesla to recharge at the same time.

In leafy suburbs, like Palo Alto, the utility connects the secondaries of multiple distribution transformers in parallel. In this case undersizing the transformers is a pretty safe bet since the the larger the number of customers sharing the transformers the less likely it is that they all fire up their plasma cutters at once. I suppose it also increases reliability since a transformer failing open won't cause an outage. On the other hand, if the primary side fails open between transformers you could end up with quite an interesting circuit. I guess the utilities have thought through this scenario

--Frank

Open primary just means hot voltage gets driven backwards through the transformer from the connected secondaries.  An open/disconnected fuse on the hot side, would not kill the primary circuit voltage until all primaries are opened up. 

Interesting instant karma QC for transformer windings. If one winding's number of turns is off relative to the others I suspect the thermal capacity would get tested pretty quickly.

JR

[Kevin Graf]

On the subject of pole pigs and electric car charging:

"What they don’t tell you about owning an EV"
Feb. 1, 2012 Leland Teschler | EET
Sometimes life seems to be a series of "gotchas," and there doesn’t seem to be an exception for owners of electric vehicles. If EVs attain the kind of wide-spread popularity their promoters hope for, buyers of Chevy Volts, Nissan Leafs, and similar vehicles could find unexpected bills for pole-mounted distribution transformers showing up in their mailboxes.

more at:
http://powerelectronics.com/content/what-they-don-t-tell-you-about-owning-ev   

[Kevin Graf]

On the subject of 'High Leg Delta' Jim Brown sometimes writes (in E-mail groups) about the problems that they can cause.
One sample post:

An especially nasty power system connection, called High Leg Delta,
uses 240V Delta 3-phase to feed single-phase power to residences and 3-phase power to
businesses on the same street. That system uses a center-tapped transformer
(at the street) to feed single phase, so WE get two legs of that 3-phase
power plus a neutral. That neutral carries all the ground current, much of
it harmonics, from the 3-phase system!

Neil Muncy first taught me about the mess that High Leg Delta could make in
an audio studio. He was troubleshooting a buzz problem in an urban studio
that he diagnosed as ground currents from that High Leg Delta system going
to ground via a pipe that ran under the studio, almost directly under the
guitar booth. The current created a huge magnetic field that got into
everything.

Jim Brown

[Mike Sokol]

As I've written on this and other forums in the past, I first encountered a high-leg delta panel about 40 years ago in the Old Mill Inn. This was in the scary days of open service panels and local bands connecting their PAR lighting to the panel bus with jumper cables. I had a meter and measured 120-120-208 from neutral to each of the 3 phases, so I didn't tap into the "high" leg. But another band did tie into 208 with their 120-volt lights and blew up a lot of bulbs in a few seconds.

See: http://en.wikipedia.org/wiki/High-leg_delta for a pretty good primer on how this works.

The High, Red, or Wild leg is supposed to have orange colored PVC tape on the insulation to designate its "wild" status, but I wouldn't count on that. ALWAYS meter any 3-phase panel from neutral to L1, L2 and L3 since you never know when something strange is going to be there. In fact it's good practice to meter ANY panel you're in. And if you're not a licensed electrician, you really have no legal leg to stand on if something goes wrong. And even if an "electrician" drops out camlocks for you to tie into, I would meter those as well.

I know I'm paranoid, but am I paranoid enough...

[Jonathan Johnson]

A common sight in these parts, especially in older, light-industrial neighborhoods, is a utility pole with two distribution transformers (often referred to as "pots" or "pigs"), one large and one small.

There is a significant cost advantage to the power company using this configuration. Not only do they save on the cost of an additional transformer, they may only need to run two phase legs to the transformer bank rather than three as in typical three phase service. If the nearest three-phase distribution is several miles away, eliminating the materials and installation cost for that third wire can save a big chunk of change, especially for public utilities bound by REA rules to provide power to farms at low cost.

[Stephen Swaffer]

Mike,

One of the few customers I have with a high leg delta used to a be a state of the art church building in the 70's. Still has the old tube organ with leslie speakers, building was wired to broadcast and receive sat broadcasts, etc.  I often wonder how much trouble they had with noise?  More seriously today, I wonder about the safety of the system.  Main panels are 2 200 amp 3 phase panels, one of which has no high leg delta phase, rather phase B is fed to both bus B and bus C.  Panels are full, but actual load is hard to discern since so much stuff has been disconnected and hard to know when we are really loading the system.  Tracing wiring is a nightmare-they have something like 20 spares pulled into the one panel and used it to pull a bunch of switch wiring for lighting through-no inspectors around here in those days!  Is feeding 2 buses with one phase a common practice?  Adding to my concern over the wiring safety now, is the building is being used as a youth  center and recently added a shelter for teens in need of a place to stay-so there is potential for over night stays on the third floor of this wood frame building.

Incidentally on transformer sizing-I find it interesting how they handle industrials around here.  I installed  a 3000 Amp 480 V  (2.5 KVA) service for a customer to accommodate new equipment.  I had to install 8 parallel runs of 500 MCM copper from CT cabinet to panel.  Utility ran 4 parallel runs of AL 500 MCM from a 1.5 KVA transformer and called it good-said their calculations showed it would never be loaded that heavily.  The service was sized appropriately by the NEC-I don't understand the discrepancy-except that the utility gets to write their own rules and decide how much to spend.

[Frank Koenig]

I had a meter and measured 120-120-208 from neutral to each of the 3 phases

Hi Mike, I think that might be closer to 317 V from N to stinger. -F

I take it back. It is 208. Sorry to confuse things.

[Mike Sokol]

Hi Mike, I think that might be closer to 317 V from N to stinger. -F

No, I think a 240-volt high-leg delta configuration gives you 120-120-208 with 208 from neutral to the high leg.

V_{bn} = \sqrt{{V_{ab}}^2 - {V_{an}}^2} \approx 208 V

It's been a while since I've done the vector math or actually measured a panel, but that's what I remember. I'll recheck my long-term memory later.

[Mike Sokol]

Hi Mike, I think that might be closer to 317 V from N to stinger. -F

I take it back. It is 208. Sorry to confuse things.

Frank
Not to worry. We all need to double-check our math and memories at times. In fact, I really like it when my college students challenge the math. It's a great opportunity to double-check the numbers and logic. I'm not an expert on 3-phase power, even though I've been around it for 40 years and hooked up a lot of 3-phase machines and motors. We really don't need 3-phase power for sound and lighting, but often tie into it without knowing. So anything we can do to educate our sound and lighting crews about 3-phase power is a good thing.

[ProSoundWeb Community]