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Author Topic: Pulling power for ONE PA, from two different sources?  (Read 18059 times)

Frank DeWitt

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Re: Pulling power for ONE PA, from two different sources?
« Reply #60 on: July 31, 2015, 09:21:02 am »

(Late response to a old thread)
 So a transformer here in New Zealand, typical of the rest of the world, has two high voltage terminals on it.  A USA transformer has one.

So in the USA, neutral is distributed via a wire.  And... the neutral to your home is also connected to that same neutral wire, so your home neutral is connected to the high voltage distribution network.  And that neutral distribution network is deliberately connected as far and as wide as possible.  It is also connected to the ground through plates and rods, at a substation, at the pole where the transformer is, and at your home.  The idea is that it is one big equipotential zone.

(Note to electrically savvy people - I have chosen to misuse the term "high voltage")

One of the wonderful things about the USA is that nothing is everywhere.  Or as someone said, Standards are a wonderful thing.  Everyone can have there own.

The transformer in front of my house has two wires connected to the input.  (Two large isolated posts on top)

Each post is connected to one of the two wires going from pole to pole

I don't know how common MGN is but it isn't used in my neighborhood.
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David Buckley

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Re: Pulling power for ONE PA, from two different sources?
« Reply #61 on: July 31, 2015, 06:41:13 pm »

One of the wonderful things about the USA is that nothing is everywhere.  Or as someone said, Standards are a wonderful thing.  Everyone can have there own.
USA's a big place, so yeah, I'm sure there's a bit of everything there.

The transformer in front of my house has two wires connected to the input.  (Two large isolated posts on top)
Th wires between the poles, are the insulators for the wires the same, or is one smaller than the other(s)?  Is there another wire going between poles lower down?
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Stephen Swaffer

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Re: Pulling power for ONE PA, from two different sources?
« Reply #62 on: July 31, 2015, 11:30:44 pm »

I'm curious.  With no common return neutral-the main purpose of which is to trip protective devices-what happens when one of the phase wires shorts to ground-or to a metal object like a power pole or fence?
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Steve Swaffer

Frank DeWitt

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Re: Pulling power for ONE PA, from two different sources?
« Reply #63 on: August 01, 2015, 09:24:31 am »

USA's a big place, so yeah, I'm sure there's a bit of everything there.
Th wires between the poles, are the insulators for the wires the same, or is one smaller than the other(s)?  Is there another wire going between poles lower down?

Both wires use the same size and number isolators.  One runs on top, the second one is about 2 ft down,  about 8 ft below that is a telephone cable.  No others wires for about a mile where the wires on my street tie into others.  All installed in the 80s
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David Buckley

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Re: Pulling power for ONE PA, from two different sources?
« Reply #64 on: August 01, 2015, 09:25:56 am »

I'm curious.  With no common return neutral-the main purpose of which is to trip protective devices-what happens when one of the phase wires shorts to ground-or to a metal object like a power pole or fence?

I take it this is in relation to high voltage wires?  What happens is breakers open.  There is a return path through a plate under the substation, and the breakers are operated by gizmos called "protection relays" which are much more sophisticated than a simple circuit breaker.  Where pure three wire (no neutral) distribution is used with a "wye/wye" transformer, the return path is often artificial, created by a small zigzag transformer to generate a neutral especially for fault detection, and there is often a resistance in the earth line to limit fault current.

The cute and really danagerous bit is that faults do occur on high voltage lines, and particularly with overhead lines, they can be for non-bad reasons, such as wind blowing the lines too close together, or a bird getting its wings between the wires or landing on a transformer, or indeed lightning hitting the lines.  These are transitory, annoying faults.  But they're a fault, and they cause the line to drop out of service.  So, there's a thing called a recloser.  A few seconds after the breakers open, the recloser commands them to shut again, and nine times out of ten, power is restored without incident.  The tenth time the fault is permanent, such as a real short or lines on the ground, perhaps from a vehicle knocking over a power pole.

Once you know about this its quite scary.  A few years back on the TV news there was a story about a hot air balloon that flew into overhead transmission lines.  Big flash, balloon and basket sits there trapped in wires.  I knew exactly what was going to happen a few seconds later, but morbid curiosity prevented me from not watching, as the reclosers did what reclosers do, the arcing destroyed whatever was holding the basket up, and the basket and occupants fell to their death.
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Re: Pulling power for ONE PA, from two different sources?
« Reply #64 on: August 01, 2015, 09:25:56 am »


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