ECN Forum Forums General Topics of the Trade General Discussion Area "power miser" lightbulb power reducers

wa2ise is right.

Balancing the neutral depends on the current waveforms, positive or negative, adding up to zero through the whole cycle. With pure resistive loads, that's exactly what happens. In that case the worst situation is all the loads on one phase. The neutral will have no more current than the hot phase involved. When these loads are evenly distributed over three phases, the neutral ends up with zero current (and the same happens when each phase has the same power factor on lagging loads).

When the loads do not present a pure sine wave current, then you begin to see problems. This is harmonics. And half-wave diodes make a LOT of harmonics.

If each phase has lots of half-wave load, all in the same direction, then there is very little current overlap time, and the neutral ends up conducting at separate times for all three phases, in the peak 33% of each phase's cycle (and maybe more, depending on the diode characteristics). The current in phase A (at the times its diodes are conducting) cannot return through phase B or phase C when the diodes in those phases are not conducting, and so the neutral gets the current just as if phases B and C had zero loads (which is the case when their diodes are in blocking state).

If the diodes are installed half positive and half negative within each phase, then you may have some approximation of a full waveform, depending on when the diodes actually conduct. They won't conduct all the way to zero crossing, so there will still be some amount of harmonics and added load on the neutral.

Other hazards exist with half-wave loads, too.

On single phase, if all the diodes on pole A are positive and all the diodes on pole B are negative, the neutral will see the sum of current for both poles. If someone decided that because the diodes reduce the overall load, that they can now double the loads, then the neutral could be running as high as 200% of rating.

On any system, when the diodes do not have an equal amount of current in the positive and negative directions, you then have a DC component of current. That doesn't play well with transformers, especially when you have a lot of it.

I have not analyzed what this would do to current transformers, either in power meters or in ground fault detection devices.