TELECOM Digest OnLine - Sorted: 3 Phase Power

3 Phase Power

John McHarry (jmcharry@comcast.net)
Sat, 11 Mar 2006 23:24:22 GMT

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.

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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