John McHarry <jmcharry@comcast.net> 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.
In the USA, the fourth wire is called the "MGN" ("multi-grounded
neutral"). http://www.annsgarden.com/poles/poles.htm#def
The MGN isn't necessarily grounded at each pole. In my cable TV days,
the rule-of-thumb used by power companies was "every tenth pole; every
transformer pole; every corner pole; every end pole." At any pole
with a full ground, all other facilities on the pole (such as telco
and CATV cable shields) had to be grounded to the same ground.
And yes, for safety reasons, distribution primary is (almost always)
wye connected. Consider:
- If one phase (say, phase X) of a wye-connected distribution circuit
fails, all phase-X customers have no power, but phase-Y and phase-Z
customers are not affected.
- In one phase (say, phase X) of a delta-connected distribution circuit
fails, the phase-X conductor downstream from the point of failure
becomes a floating bus. Customers across Y-Z are not affected, but
X-Y customers are suddenly in series with X-Z customers. Big voltage
drop. Unhappy customers.
I wrote "almost always" above because I've heard of at least one case
of delta-connected distribution. Some fellow in New Jersey posted a
story about it here on T-D several years ago -- apparently, he was one
of the customers with half-voltage. Maybe he'll post his story again.
> 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.
Single-phase primary distribution is quite common on residential side
streets in the USA.
> 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.
Not necessarily. A separate line is sometimes used in situations
where several mercury-vapor (or sodium-vapor) lamps are installed in a
relatively small area (say, a city block or an interstate
interchange).
The lamps are wired in series, and each lampholder has a cutout
circuit that shorts the circuit across a failed lamp. Presumably,
there are current-limiting devices somewhere in the circuit that keep
the total current constant.
Far more commonly, however, streetlights of any species (incandescent,
mercury, or sodium) are simply wired across any conveniently-available
115-volt secondary distribution circuits -- the same circuits that
feed nearby residences and small commercial customers. Each
streetlight has its own photocell (at dusk, the lights come on at
different times). A burned-out lamp simply goes out, but it doesn't
affect any other streetlight. A photocell may short out (or get
covered with bird poop), in which case the streetlight burns all day.
Of course, there will be a dedicated 115-volt secondary distribution
conductor pair ("duplex") for any streetlight that happens to be on a
pole that wouldn't otherwise require secondary distribution.
Other pole-mounted 115-volt loads are wired in the same manner:
traffic signals, pedestrian lighting, CATV power supplies, seasonal
decorations.
> 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.
The primary windings may be delta-connected, but the secondary
windings are (in my experience) always wye connected. But I suppose a
power company could provide a delta connection to a customer that
specifically requested (and paid for) it.
> 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.
Why would ambient air temp affect load balance? I would think that
load balance would be determined by the customer's instantaneous
demand.
> 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.
Are wye-delta and delta-wye transformations ever used in secondary
distribution circuits? The ones I've seen were always in primary
distribution or sub-transmission.
Neal McLain
Neal McLain (nmclain@annsgarden.com)