TELECOM Digest OnLine - Sorted: Re: 3 Phase Power

Re: 3 Phase Power

Sun, 12 Mar 2006 05:32:08 -0500

John McHarry wrote:

> Apropos the earlier discussion in the 25Hz thread, I think both the US
> and Europe distribute three phase wye connected power. If you look at a
> distribution pole, at least before the phases are split out, you will
> see three well insulated wires, and a fourth wire that is earthed at
> each pole.

> I believe both systems provide single phase house current by attaching
> a transformer from one phase to neutral/earth. If you go down a street
> with three phase distribution you will see the transformers (pole pigs)
> attached to each phase in sequence. When you get near the end of a
> run, it may drop down to two, or even a single phase. If you are going
> down a street with streetlights, there will likely be a line below the
> main distribution lines carrying lower voltage for them.

I studied power a bit 30 years ago in but now my knowledge is a bit
more limited so excuse any technical mistakes. Here in Raleigh, NC and
I'm assume other CP&L/Progress Energy locations, they have been on a
campaign for a while to loop all the 3 phase feeds from the
substations so there are few if any "ends of runs". I don't know if
this started due to Hugo & Fran, just happened at the same time, or
was accelerated due to them. But it sure makes it faster to bring
folks back "on the grid". Plus as a rule they don't tap transformers
for houses directly off the 3 phase wires. They have runs of a one to
a few blocks attached to a feeder drop below the 3 phase lines which
helps keep the main lines running even when local problems
arise. These feeders are connected with fuses to one of the 3 phase

I learned most of this talking to a repair supervisor as it turns out
my pole is one of the first visited after a major storm. It has the
first disconnect switch for the 3 phase loop out of the substation.
And I'm on the substation side. Plus my transformer is still connected
directly to a 3 phase leg which means that I'm one of about 5 houses
in my area to get power before anyone else within about 1/2 mile or
so. I was told unless they had to change out the transformer they'd
leave it connected that way.

> When you come to a customer requiring three phase power, there will be three
> pole pigs, one off each leg. I think in Europe these are also wye
> connected, but some in the US are delta connected. Some places the power
> company, at least in the past, got cheap and used two pole pigs to deliver
> an "open delta" where the third phase is imputed. On a warm spring day
> with the sun shining and the birds singing, this works fine. When the
> loads get out of balance, all sorts of evil ensues.

> There is, however, a reason for both wye and delta connections. Non
> linear loads, and ever more are with the prevalence of switching power
> supplies, generating harmonics. Multiples of the third harmonic,
> called triplen (from triple n) currents add in phase on the neutral. A
> wye-delta transformation traps them and keeps them out of the upstream
> system, where large currents on the neutral can wreak havoc.

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