RE: Household NEUTRAL is not really a return path

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Original poster: Steve Conner <steve@xxxxxxxxxxxx>

> the neutral is not really a return path

> Uh . . .  no!  The neutral *IS* the only return
path.

Whadya know! You're both right! :) In an ideal world,
with a perfectly balanced 120/208V three phase system,
the currents going into the neutral from all the
various loads cancel out before they get back to the
distribution transformers. The neutral wire is
sometimes made undersized to save money.

In practice, it's never quite perfectly balanced and
some neutral current flows back to the transformers.
This ought to be allowed for by using three 120v
windings connected in star, so the neutral current
just goes back into the star point.

But a much more serious problem is harmonic
distortion. Odd harmonic currents don't cancel in the
neutral- they reinforce. Installations with a lot of
discharge lighting or switched mode power supplies
(well in the days before active PFC anyway) can run
enough harmonic current to melt an undersized neutral
wire.

The final question is where all that unbalance and
harmonic current goes when it gets to the distribution
transformers. After all, the high voltage side is just
three phases with no neutral. As far as I know, the
unbalance gets transferred to the high voltage side
where it makes the three line currents different.
However they must always vector sum to zero since
there's no neutral.

The harmonic currents are usually kept out of the HV
side by using delta connected high voltage windings. A
delta winding looks like a short circuit to an odd
harmonic.


Steve Conner
http://www.scopeboy.com/