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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Frank Koenig on December 16, 2013, 11:58:42 AM
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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
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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
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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
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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
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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...
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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.
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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.
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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.
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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.
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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.
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Mike,
I could be wrong-but it seems like from measurements that part of the reason for the name "wild leg" is that the voltage to neutral is variable-depending on load balance and type (inductive-reactive-resistive). My understanding is that the only way the wild leg can be used is for a phase to phase 208 load-either 3 phases, or between the wild leg and either of the other 2 phases? Trying to run a 208 load from the wild leg to neutral is not a good idea.
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Mike,
I could be wrong-but it seems like from measurements that part of the reason for the name "wild leg" is that the voltage to neutral is variable-depending on load balance and type (inductive-reactive-resistive). My understanding is that the only way the wild leg can be used is for a phase to phase 208 load-either 3 phases, or between the wild leg and either of the other 2 phases? Trying to run a 208 load from the wild leg to neutral is not a good idea.
That's a really interesting question. Of course in a delta high-leg system you only connect 3-phase motors that don't require a neutral at all, or 120-volt loads between the center tap and either leg of the 240-volt transformer. There's no good reason to hook anything up to 208 volts between the neutral and the "high" or "wild" leg. But while there's not as much current available on this leg, I think that's due to the reduced size or the second "high" leg transformer.
But I've got an electrical buddy who was a substation engineer, and knows all sorts of weird stuff about 25 Hz 2-phase power at Niagra Falls, and other early electrical history. I'll ask him if he knows the history of the word "wild" as in wild-leg delta power. Maybe there's something to it.
If you want a real brain teaser, take a look at the Scott-T transformer, which was invented in the 1890's to convert earlier 2-phase 90-degree power to 3-phase 120-degree power without any moving parts. http://en.wikipedia.org/wiki/Scott-T_transformer
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Mike,
I could be wrong-but it seems like from measurements that part of the reason for the name "wild leg" is that the voltage to neutral is variable-depending on load balance and type (inductive-reactive-resistive). My understanding is that the only way the wild leg can be used is for a phase to phase 208 load-either 3 phases, or between the wild leg and either of the other 2 phases? Trying to run a 208 load from the wild leg to neutral is not a good idea.
One of the nightclubs I used to work in (a century ago) had a 208v wild leg that powered the AC compressors. The split phase 120/240v service powered everything else.
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Here's a thought. You guys check me on this. I don't do three-phase every day. And I have issues with polarity ;)
If you flip the polarity of one of the the transformers then the neutral-to-wild-leg is 317 V instead of 208 V.
(120 @ 0 deg) + (240 @ 120 deg) = (208 @ 90 deg)
(120 @ 0 deg) - (240 @ 120 deg) = (317 @ -41 deg)
Maybe this is where the "wild" reputation comes from.
Properly connected single-phase loads won't know the difference. Properly connected three-phase loads will just see the phase rotate in the opposite direction, which will cause plant technicians to flip two of the motor motor leads to get the motor to spin the way they want.
If this is correct then I think there's a joke in here about the polarity of the third leg.
--Frank
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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. ... Is feeding 2 buses with one phase a common practice?
My concern would be that someone could install two 120V circuits on adjacent breakers with a single, shared neutral (a fairly common practice). If the breakers are one on bus B and one on bus C (rather than one on bus A) they will both be on the same phase. This means that the shared neutral has the potential to be overloaded.
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a friend had a auto body shop in a 80 something year old commercial building surrounded by houses. i rewired the building for him. he had 2 seperate services coming in. a 120/240 3 wire single phase for standard 120v recepticals and lights and a 240v 3 wire 3 phase for equipment and machinery and motors. his old compressors had 3 phase 240v motors. theres also the 120/240 4 wire 3 phase high leg system. the high leg is on C phase at the service and here in cali the dept of building and safety require the high leg to be on B phase at the circut breaker panels. C gets orange phase tape. black , orange , red , white , green in the panels.
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It's interesting to me how small the kVA rating can be in relation to the notional size of the service.
In the eighties I was production manager for a singer who had attended school in a small town in Alabama. There was a homecoming concert planned for her at the gymnasium at that school and a section of highway was to be named for her with the Governor of Alabama to present the award. I live a short distance from that school and so was dispatched to do a site survey to be sure we could do a proper show in the gym. Checking for power availability, I found two 400a 3 phase disconnects across the hall in the industrial arts shop...GROOVY.
Showtime, Governor on stage ready to speak, lights are run up and POW! power to both sound and lights goes down! As luck would have it, there was someone from the local power company attending. A quick trip to his truck and he returned with a long pole to reset the breaker on the pot hanging from the pole outside the gym. It seems that the xformer on that pole could not supply even 200 amps!
We did the governor's presentation with only follow spots and audio while the power company hung a larger transformer and tied it in LIVE!
I suppose it is not uncommon to have higher breaker ampacity at the service than the transformer can supply.
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Properly connected three-phase loads will just see the phase rotate in the opposite direction
Well, I guess you were all waiting for the other shoe to drop as old Uncle Frank figures it out.
The trap I fell into was believing that flipping one of the transformer windings (source) is equivalent to flipping two of the motor leads (load). It is not -- I drew the picture.
The transformer polarity must be correct or three-phase loads will blow up. As a result the neutral-to-wild-leg must be 208 V.
I'll be quiet now an leave this to the electricians. Thank you all.
--Frank
Edit: spelling
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The transformer polarity must be correct or three-phase loads will blow up. As a result the neutral-to-wild-leg must be 208 V.
Isn't 3-phase power a lot of fun? :o
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wasnt their someone named "pole pot" ? dont drop your pole in the pot. i think i'm out of phase right now. Mr. Douglas was on top of a pole many times