On 6/17/13 5:33 AM, Peter Gottlieb wrote:
The current distortion from simple transformer-rectifier-capacitor power
supplies contains a lot of third harmonic content. In a 3 phase system
(as are all distribution systems for commercial and industrial) the
third harmonic ADDS in the neutral, or creates circulating currents in a
delta configuration. These currents, as you mention, can get very large
and were the cause of many transformer explosions in cities as these
power supplies became common. The transformer designs had to be
improved, but the PFC supplies make a big difference.
How many of you have looked at the power line waveform, especially in an
industrial or commercial area? Doesn't look much like a sine wave, does
it? So it's pretty funny to see audiophile outlets
(http://www.dedicatedaudio.com/power_outlets).
Peter
The "PFC correction" stuff is, as you say, more about harmonic content
reduction than actual power factor. The rules on the current waveform
came in as part and parcel of the power factor rules, so maybe it was
just a simpler way to explain it?
It's all about looking more like a resistive load.
The US National Electrical Code was updated about 15-20 years ago
because of the neutral current problem. In light industrial, office,
running 208/120Y is very common, the old codes allowed the neutral to be
smaller than the phase conductors (assuming that the loads would be
resistive and all balance out)
but with all those capacitive input filters, the current in the
neutral got pretty high and there were fears of fires and overheating (I
don't know if there were actually any fires, but poor voltage stability
and heating of distribution hardware is probably more likely).
Certainly, the utilities weren't wild about the harmonic currents, so
they almost certainly agitated for the change as well. (Imagine you're
a utility servicing a multitenant building, but the tenants all have
single phase service, which the utility spreads around the three phases.
The utility has the problem of the distribution transformers and the
triplex currents.
And, in fact, this harmonic thing is hard to fix in distribution
equipment anyway (some set of tuned traps?) so it does make sense to
push it to the user.
The issue also arises with fluorescent and other gas discharge lighting,
particularly with "electronic" ballasts (e.g. switchers). The old
"magnetic" ballasts (basically just a big inductor) sort of inherently
act as a low pass filter, and solve the harmonic problem by getting
warm. And, they'd have a very lagging power factor, but a fairly fixed
on that you could compensate with capacitor banks.
As folks transitioned to the newer ballasts, the non-sinusoidal
current problem probably got worse.
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