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Old 08-28-2013, 07:33 AM   #1
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POCO Primary voltage


Hi;
Until I read "The Grid" by Phillip F. Schewe, I thought that the primary lines in a residential area carried 13KV, but in the book, he says 7.2Kv is the standard for primary distribution lines (in residential areas).
So I'm wondering where I got the 13Kv from. Is there a different primary voltage used for industrial, or did I just get wrong info from the getgo?

BTW; I highly recommend this book for anyone who wants a comprehensive education on how the power grid works, its history, and its future, including discussion on reducing CO2 emissions. This has to be the most complete read on the subject I have found, and it is available at your local library.

FW

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Old 08-28-2013, 07:36 AM   #2
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I imagine it varies by region and load on the lines. I thought they were 4kv.

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Old 08-28-2013, 08:10 AM   #3
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The city here is in the process of switching from 7200 to 13000.
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Old 08-28-2013, 11:47 AM   #4
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7200V is probably the most common primary distribution voltage in non-industrial areas. That is measured phase-to-ground. The corresponding phase-to-phase voltage is 12,480. 13.2kV, 13.8kV, and 14.4kV (double the 7200V standard) are also common. Anything over 15kV to ground requires additional safety precautions and more expensive equipment, so is not commonly used for distribution, only transmission. Old systems were 2400V phase-to-phase (delta configuration) or 4160V phase-to-phase (2400 to ground, but used in a delta configuration). 4160V is no longer commonly used for utility distribution, but is very common for internal use in industrial facilities.
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Old 08-28-2013, 05:08 PM   #5
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Originally Posted by mpoulton View Post
7200V is probably the most common primary distribution voltage in non-industrial areas. That is measured phase-to-ground. The corresponding phase-to-phase voltage is 12,480. 13.2kV, 13.8kV, and 14.4kV (double the 7200V standard) are also common. Anything over 15kV to ground requires additional safety precautions and more expensive equipment, so is not commonly used for distribution, only transmission. Old systems were 2400V phase-to-phase (delta configuration) or 4160V phase-to-phase (2400 to ground, but used in a delta configuration). 4160V is no longer commonly used for utility distribution, but is very common for internal use in industrial facilities.
That makes sense. I guess I got the 13KV figure from the phase to phase voltage, not phase to neutral. I used to know how to calculate voltages in multi-phase systems, but have long since forgotten the formula. I do know it involves algebra though

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Old 08-28-2013, 07:46 PM   #6
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here in s.c inthe country we have 14.4 kv for distribution,this is REA,i belive SCE&G is 13.2 kv. then some are 4800 volts. this is for the really older areas. some city power is 7.2 kv.some REA'S are still at 7.2 kv.
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Old 08-28-2013, 07:47 PM   #7
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here in s.c inthe country we have 14.4 kv for distribution,this is REA,i belive SCE&G is 13.2 kv. then some are 4800 volts. this is for the really older areas. some city power is 7.2 kv.some REA'S are still at 7.2 kv.this voltage is from phase to neutral
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Old 08-29-2013, 12:42 PM   #8
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I used to know how to calculate voltages in multi-phase systems, but have long since forgotten the formula. I do know it involves algebra though
Phase-to-phase is phase-to-ground multiplied by the square root of 3 (approximately 1.73).
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Old 08-29-2013, 02:30 PM   #9
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This is an interesting thread. I've always heard 7200 and 13kv tossed around. Is there a way to tell simply by seeing the overhead lines/transformers?
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Old 08-29-2013, 04:23 PM   #10
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This is an interesting thread. I've always heard 7200 and 13kv tossed around. Is there a way to tell simply by seeing the overhead lines/transformers?
Sort of, in general. 7.2kV insulators are often noticeably smaller than higher voltage ones. 4160V insulators were sometimes even smaller still. But you often see 7.2kV lines on larger insulators in some places just due to availability. If the insulators are a single mushroom-shaped thing, then it's probably not more than 7.2kV - although it might be up to 15kV since those insulators are often class-rated for 15kV. If the insulator is taller and has multiple ridges, then it's probably more than 7.2kV - but not always!

The prediction is much more accurate on transmission lines. Suspension disc insulators are good for approximately 10-20kV per disc. This is nonlinear - more discs, more voltage per disc. So a 49kV line may have 4 discs, and a 500kV line 20 discs.
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Old 08-30-2013, 12:40 AM   #11
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This is nonlinear - more discs, more voltage per disc. So a 49kV line may have 4 discs, and a 500kV line 20 discs.
That takes care of the voltage, but a few other factors come into play.

Disc failures happen thru natural or man-made causes. Throw in a few extra to allow for it. On a 500kv, we went 4 extra.

Pollution (dust, salt, ash, stack emissions, etc) you might add any thing between 0 and 20 depending on the evaluation.
So, in the heaviest pollution areas, you might have 44 discs total.

That is at sea level. Toss in another 3 percent for every 1000 feet of elevation. Always round up. So, at 3000 feet you could end up with as many as 48 discs in every string. (30-32 was more typical for that elevation).
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Old 09-04-2013, 08:12 AM   #12
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What type of insulators are used for those long-distance lines that run as high as 1.2MV (million volts)?

FW
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Old 09-04-2013, 11:54 AM   #13
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What type of insulators are used for those long-distance lines that run as high as 1.2MV (million volts)?

FW
The same type of suspension discs as for a 345kV line, but many more of them.
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Old 09-04-2013, 01:16 PM   #14
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Ahhhhhh nothing like the smell of ozone in the foggy morning near the ocean and every salt covered insulator stack has arcing. Was never an electrician by trade, but often wondered what was the safe distance to keep between me and the pole on those mornings walking from the parking lot to the ship.

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