That is great.. Thanks for the URL. Much appreciated. I went through NY Trade School in 1957 and have been trying my very best to remember all of the 'stuff' I learned back then. Everything from motor winding to pulling and hooking up raw power. Now, at a retarded (eh.. retired?) state of being, it is fun, but sometimes hard to remember. It's kind of like the ole' dog, you know, he chases cars but cannot figure out what to do with them once caught? lol..
PJ
Oh - 4 phase is nothing more than 2 phase with a center tap on the generator winding. (Plus a polarity change) Each phase is a 90º displacement on the rotor/stator. (3 phase is 120º)
There are four nodes in this setup-- The fourth point (G) is usually there in the US to give the 120/240 split phase (B to G is 120, C to G is 120, 180 out of phase) G is the grounded node-current carrying, but tied to ground at the service enterence, usually white in the US- for the load (user) side of the hookup, but does not have an equivelant on the power co side of the transformers. It is derived from a center tap on the user side solely to provide the two 120 phases-- this transformer is the same hookup as a normal split phase hookup at a residence. The third power node, A, is 120 degrees ahead of B and 120 degrees behind C.
This leads to a bunch of interesting behaviour, such as the voltage from A to G is 208V, 90 degrees out of phase with the voltages B-G, C-G and B-C. Node A is often called the 'wild leg', and is supposed to be colored orange under NEC, so as to differentiate it from the 120 legs. Another effect is that, though this hookup (in the standard three tranformer version, not one of the two transformer versions) is very stiff against load for 240V loads, the two 120's are not very stiff, and if not fairly well balanced, will drift. For example, at my shop, the wiring was done with one building WAY out of balance (5000W of lighting on one 120 leg, and nothing on the other) and this shows as several volts different between the phases at the transformer.
Another neat feature is the ability for the supplier to REALLY cheap out on the hookup (see my other post)
The delta is sometimes done a different way: one of the corners will be the grounded node. This will not provide 120V directly, but requiers a seperate transformer. Not real common other that for motor load only hookups. This setup won't provide 208.
A note on DC- As of teo years ago, New York transit was still running DC rotarary converters that are about 100 years old-AC in, DC out, but not quite a motor generator like a welder. I believe the last went out of service recently, but I may be wrong. The hookups use have been thyristor converters (all electronic) since the 50's.
I read recently that the last of the Edison DC service was finally discontinued.
"If you're a history buff, and appreciate the technology that surrounds us all, you'll love reading "Networks of Power: Electrification in Western Society, 1880-1930" by Tom Hughes. Hughes takes us back to the days of fierce rivalry between Edison and Westinghouse; the early era of electric power generation and consumption where the battle of DC vs. AC consumer power was born and decided.
Hughes doesn't stop there. Also included in this well-footnoted edition are in-depth narratives of the evolution of commercial power systems in England and Germany through 1930. A well written, readable snapshot in time.
Compelling historical reading for the non-technologist as well as the student of electrical power commercialization."
At the transformer, as per the diagram in that excellent link. When you do the actual service lateral, you pull three primary cables and each one has an aluminum (usually) center conductor and a copper concentric grounded conductor. The grounded conductors are joined at the transformer and also bonded to the transformer case and ground rods.
The concentric on the primary cable along with the transformer grounds are not always a metallic return to the substation. Primary neutral and common neutral (common to primary and secondary, run in the secondary position) are metallic returns to the substation. You find primary neutrals in the older 4KV where they've used 2400V transformers phase to neutral (star or wye). We also run a common neutral system in our 21KV that we use 12000V transformers on.
A side note to the underground cable, is they are not getting the life expectancy out of it that they thought when they put it in. One of the problems that we've run into is some soil conditions dissolve the copper neutral on the older installations where it was direct buried. We've since gone to a jacketted cable and require everything to be in conduit now. Another problem they find is when the cable is faulting it's through hairline cracks through the poly. We've replaced a considerable amount of this at great expense, and have recently been working with a company that injects silicone in the end of the strands that fills these hairline cracks. At 5 dollars a foot, it's about half the price of cable replacement. This year we have some 40000' coming up for cable replacement and another 20000' that we are going to do the silicone injection on.
Do they mention anything in the book, "Networks of Power" about the Folsom Powerhouse? I was privileged enough in my career to work with a man that operated it. He operated it from 1948 til they closed it in
1955. I worked with him in the '70's when he was Chief Dispatcher with the company I work for. I took a tour of the powerhouse with him and I took my video camera. I have him telling a lot of wonderful stories of things like the collectors flying off the generators, sparks everywhere and him diving out the window.
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Today when you talk about a network in the electrical utility industry it usually refers to a system in the downtown area of large cities. This is different from the transformers in the suburbs, in that, the secondaries are all in parallel. So if you lose a transformer you don't drop any customers, you just lose KVA. But you have to run your network with more reserve than the largest transformer.
You're way above my head, are you talking about upstream of the customer-owned cable? In the typical case, the concentric grounds are the only return, right?
That's probably the way to go- I once pulled an old lead-jacketed primary run through about 400' of conduit from a man-hole cover to an underground vault. It was a real PITA but pulling the new poly-jacketed stuff in was easy and saved a lot of trenching and restoration. Still, the majority of installations in this area are direct-bury. The only advantage is that sometimes you can Biddle the cable and make a quick repair without pulling the whole line out.
Another problem they find is when the cable is faulting
They inject silicone in the center strand which fills the gaps in the white poly insulation between the center strand and the semiconducting layer? That's a pretty neat trick. Is it only practical for utilities? Do they have to redo the potheads/elbows?
It's been a few years since I read it, but I don't remember that. A lot of the book dealed with rationalization (standardization and the rationalization movement) of the power industry, the balance between competition and the need for a unified grid. On the technical side, much of what we take for granted, such as the concept of a feed with two hots and a neutral, (vs. one hot and one neutral) was once a bright new idea. With your background, I think you would enjoy at least some of the book.
Damn! I am just a lowly HVAC tech, and even I understood everythiing he posted too! (Must not have my mind muddied up with all the ed-gication stuff!) Greg
I don't know about everywhere else, but I rented a shop that had 208-3ph. It was just as Doug described. Phase to phase = 208 volt, any phase to neutral was 120 volt. Greg
It was probably set up as a warehouse originally. This was often done because all that was needed was power for lights. The 208 wye service has the added advantage that your load is more balanced since you have three 120 circuits to use not just two. Plus you can run just about any 3-phase power machinery.
The only drawback to the 208 wye service is that any 240 2-phase stuff that you use won't get the power for which it was designed. But then, for just about anything that you find in a shop, that's not a problem anyhow. For instance, if you've got a 240 2-phase air compressor it will run happily on 2 legs of the 208 service. The power output might be a bit less, but you'll never notice it - plus the manufacturer overstated it in the first place anyhow.
Oh, and one more thing: Where Doug was saying "Wrong" and "NO.." up there. You now know that the correct answers are "Right" and "YES.." don't you?
If you're using a MS-Windows based reader, make _sure_ that you've selected a *fixed*pitch* font ("fixedsys" is one, so is anything with 'courier' in the name).
I suspect the real problem is that your reader was trying to display the text as HTML or in a proportional-width font. If you can somehow force your reader to display the article as plain text in a fixed width font, you should be fine.
And anyone in Southern California that wants to see a working 2400 VAC 60Hz 6-phase to 600 VDC Rotary Converter station only has to go to the Orange Empire Railway Museum on an operating weekend.
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There's a nice little GE station there, with a completely restored and re-wound rotary converter (ask about how that restoration came about, it's interesting). And it's all self-starting and self-shutdown, done with a big drum controller - just hit the button and watch it come up.
Looks like something Rube Goldberg designed, complete with a ball-and-worm armature shaft wiggler to keep the brushes wearing evenly, and big live-front contactors with open arc chutes...
They now have a solid-state converter for everyday use, but they can still fire up the rotary - for demonstrations, if they're working on the other power plant, or if they're going to be running a lot of rolling stock at once.
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