ECN Forum Forums General Topics of the Trade General Discussion Area Passive Harmonic filters

Thanks, but I suppose another downside to my idea is that the IEC ends of all the PC power cords for 120V or the 208V cords are the same, and if in the future someone replaces a computer fed by 208V, unplugs the IEC female from the back of the PC, and doesn't realize that the PC power supply on the new one needs to be set to "220V", and he reuses the old power cord still on 208V, he'll blow up that power supply.... When I've done this 208V or 230V thing I labeled the power cord near the female IEC connector "208V" or "240V", and I know to expect some of the cords to have the higher voltage. But someone else might not expect this....

[This message has been edited by wa2ise (edited 10-13-2005).]

[This message has been edited by wa2ise (edited 10-13-2005).]

You could always put a small DY transformer next to the PDU supplying some of the circuits and achieve the same result - with the advantage that all the external wiring stays as is without any dangers of PSU failure.

M.

Marc,
Other than isolating the harmonic problem to the secondary side of the transformer, how does the DY xfmr reduce overloading of the neutral in the PDU?
BTW, I still think that after additional loading, the problem will not be severe enough to warrant the neutral to be in excess of the phase conductors. Now that I think of it, I assumed that the PDU feeder had a full sized neutral or better, right?

Let's as pure example assume each workstation takes 2A and there are to be 24 workstations.

It is always advisable to assume the PCs are non-PF corrected and have very sharp conduction angles. This means the Neutral can carry up to 3 x phase current.

So, in no transformer scenario:

24 PCs over 3 phases = 8 PCs per phase = 16A per phase and possible 48A on Neutral.

If we put in DY xfos and split the load between straight feed and DY feed, then:

9 PCs over 3 phases = 3 PCs per phase = 6A per phase and possible 18A on Neutral.

Next 15 PCs are on DY. Secondary currents are 10A per phase and possible 30A on local Neutral. However, primary side (Delta) has 10A per phase but no Neutral current therefore total current on each supplied phase is 16A per phase (back to our original).

The trick is the current curve presented to each phase is a combination of 2-pulse and 6-pulse resulting in a much reduced harmonic content, and the whole system is capable of running on a 3P+1N cable.

As for the Ground/Earth currents, I suspect circulating currents and would need to do tests to establish where they were coming from (because there is just no way on 7.5A per phase he can combine to be in excess of 22.5A - and as is 13A is already on the Neutral, that Ground current must be coming from another system).

M.


[This message has been edited by marcspages (edited 10-13-2005).]

I'm not understanding how the transformer is connected into the distribution to result in the 9PC's over 3 phases then 15PC over 3 phases. Are you suggesting a isolation transformer serving a subpanel for the 15 PC's?

I haven't done the FFT on the input to a PC's switchmode power supply, but I believe that the entire draw is not at 180 hz relative to the nominal 60hz. Even if it was only drawing 180hz (and other triplens), I thought when you add the triplens, you can theoretically get a max of 1.73 times the phase current on the neutral. I will try to draw three phases of 180hz, 120 degrees out of phase from each other and see what I get.

[This message has been edited by Ron (edited 10-13-2005).]

Ron,

The DY is effectively an isolation transformer; The primary is connected to the incoming three phases, the secondary feeding a portion of the PDU that has been split from the original supply - except the Neutral is connected to the original Neutral so as to maintain the original wiring fed from the PDU.

Believe me, there is a huge amount of badly written material stating the maximum on the Neutral can be 1.73 (the primary mistake being they are relating this to the way voltages are calculated in a 3-phase supply). We're dealing with current, a completely different kettle of fish - I've even shown the Neutral taking 3 times Phase in practice and made many an engineer shudder!

What I omitted to add in the previous post is a DY xfos is much cheaper than a filter! (but you do require the load to be relatively constant - but most office situations are).

M.

PC power supplies can be thought of as being a bridge rectifier (at 230V setting) or a voltage doubler rectifier (at the 120V setting), feeding a big capacitor that has a more or less resistive load across it. The rectifier diodes only conduct at the very peak of the input AC voltage to "top off" the capacitor to replace what the resistance drained off between voltage peaks. This makes for current spikes happening at the input AC voltage peaks, and no current draw at all between peaks. These spikes last for about 5% of the cycle time of the 60Hz waveform cycle. Thus there is *NO* overlap from phase to phase. PCs on phase A draw their current spikes at the peak of A from A and the neutral. Then likewise for phase B, and then C. The neutral thus sees 3 times as many spikes as any one phase wire does, and over time will get as hot as the same guage of wire carrying 3 time the current of any one of the phases. You just need to think in terms of the time domain to see this.

Heard it said that the European equivalent of UL and/or FCC will soon require that switching power supplies draw current off the lines like a purely resistive load would. This technology exists; it just costs more....

Exactly! And you can even go to a maximum of 60 degrees conduction angle per half-cycle and still get a whole 3 times phase current on the Neutral (assuming all three phases are equally loaded with the same hi-tech load).

It's the thinking in real time that is a problem for a lot of folk, hence the introduction of Fourier analysis - but this has led to so much incorrect teaching. But lest you think I'm complaining, I'm not. It is this that keeps me in business sorting out issues others can't (tee-hee!). An example is "the 3rd circulating in the delta winding" - absolute rot! Sadly, I'm not prepared to publish my findings on this because there are institutes in the PQ fraternity with a little more clout than me busy plagiarizing my work left, right, and centre and calling it their own!

G5/4 is the primary driving force here in the UK to get power supplies to incorporate both PF and harmonic correction. The extra costs pays for itself in a very short space of time through electrical systems not failing through high harmonic content.

M.