Topband: Chassis Bonding
billw at waveform.net
Wed Jan 29 10:40:17 EST 2014
That's not entirely correct... Triplen harmonics are created by non-linear loads, but the voltage developed on the neutral as a result of them is a result of voltage drop between the source of the "problem" and the return which will be one or more utility transformers. The farther you are from the transformer the worse the issue will be (higher voltage developed). Triplen harmonics primarily result in additional heating in the neutral and transformer due to higher than normal currents (since they don't cancel). Triplen harmonics are also becoming less of an issue as many electronic loads now incorporate power factor correction. This used to be a very big deal in datacenters due to older computer power supplies (which are rectifier loads and very non-linear), but most new equipment isn't a problem.
Note that "high leg delta", when used to deliver 120/240 single phase power will *always* have the center tap grounded and that includes in industrial, 3-phase customer sites. If the center tap isn't grounded then you would have to have a corner-grounded system, but there is always, by code, a ground bond at the site. The only impedance-grounded system I'm aware of is the old 4800 volt primary system but in that case all of the step-down transformers on the poles also provide transformer isolation between the primary and secondary (since it's not possible to use autotransformers in that application).
Most of the newer (since the 70s) power distribution used by utilities is a wye on the primary, and the neutral of that wye is grounded at each pole and the substation. All customer (both single a three phase) services have their ground/neutral bonded to that common ground/neutral used by the primary. And note that from the utility's perspective, on the poles, the neutral and ground are essentially the same thing (at least in terms of the way the system is wired).
And our home and ham station grounds, when properly bonded to the service entrance, WILL share current with the utility system. I see about 7 amps at home that "comes from" the utility (but I have a ground ring, lots of rods, probably 500+ feet of copper in the ground and a well casing). It's not a problem, but it's there.
> > The voltage driving that current isn't high, it is caused by the
> > neutral voltage drop back to the substation, but it is a real voltage
> > and current that exists.
> No, it is caused by triplen harmonics combining on the neutral (and
> ground) of 3-phase systems to add rather than cancel. That's the source of
> power line "buzz" -- 180, 360, 540, 720 Hz. While we don't have 3-phase
> power in our homes, the power on the street is nearly always 3-phase, and
> often a very nasty variation of 3-phase called "high leg delta." It's 240V delta,
> with all three phases fed to light industrial customers, and one of the legs
> center-tapped to feed single-phase users.
> Those 3-phase users get no neutral and no ground, so their harmonic current
> finds its path to ground through OUR neutral. THAT'S the source of our
> "buzz." And yes, it's not unusual to see an amp or more of that stuff on our
> neutral. High leg delta is all over mixed residential/light business/industrial
> neighborhoods of nearly all cities, and it's what I've got in the Santa Cruz
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