On Thu, 30 Oct 2008 19:50:45 -0500, "Henry Kolesnik"
wrote:

Hey Ken & Grumpy
I started a new thread from the 7.5 VAC xfmr, because the subject has
drifted. You guys mentioned a DC component in pole pigs. Sometime ago
I heard that if an AC circuit had too many switching power supplies on
it, that would cause the power company all kinds of problems. What were
or are the problems and what was the evidence and what did the power
companies do to solve the problem?

There could be transformer core saturation problems if large half-wave
rectifier systems are used. In Europe old televisions had only a half
wave rectifier to generate the anode voltage for the tubes.
Fortunately, since the users would plug in their televisions both
ways, usually both half cycles were loaded within an area.



In a balanced three phase system the neutral current cancel out and
the neutral wire is loaded only then the phases are not equally
loaded. For this reason, the neutral wire is often smaller in large
underground cables.

Any full wave rectifier system generate strong odd harmonic currents
each time the capacitor is charged.

However, when each phase is loaded with switching mode power supplies,
which generate string 3rd harmonics, these harmonics do not cancel,
but instead the 3rd harmonic neutral currents from all three phases
add up, possibly overloading the neutral wire.

For this reason, at least in Europe, large power supplies must have a
power factor correction circuit to extend the conduction angle during
each half-cycle.

Paul OH3LWR