[Beowulf] harmonic mitigators vs PFC?

[Jim Lux]

----- Original Message -----
From: "Mark Hahn" <hahn at physics.mcmaster.ca>
To: <beowulf at beowulf.org>
Sent: Sunday, May 23, 2004 10:40 AM
Subject: [Beowulf] harmonic mitigators vs PFC?


> Hi all.
> we're doing a new machineroom, which will dissipate around 250 KW.
> I'm having a hard time addressing the question of whether such a load
> (all modern PFC computers) needs harmonic mitigation.  the electricians
> all just say "your loads are nonlinear so you need HM".  I can't help
> think that the cost of HM hardware translates to more than a couple more
nodes!
> and merely because the PSU is implemented with nonlinear components
doesn't
> mean that the load is noisy.

You mention 800 nodes, etc., So you're talking about around 0.2% of your
total hardware cost?  Seems like a no-brainer to go with the recommendation
of the people who are concerned, if only for political reasons.

>
> to my thinking, the active PFC found in current computers means precisely
> that HM is not necessary, since a PFC of .97 (so says my KillAWatt)
indicates
> that only 3% of the power is drawn outside the ideal sine envelope.
> further, we're talking about O(800) seperate power supplies, and their
> harmonics are probably not perfectly synchronized, and so sub-additive.

Actually, the mechanism is such that the harmonics ARE synchronized.  It's
not the harmonics of the switching element, it's the harmonics of the
non-sinusoidal line current being drawn by a diode feeding a capacitor input
filter, which looks very peaky, particularly at light loads.  Those
harmonics are multiples of the line frequency, and, since your 250 kVA load
is almost certainly being fed from three phase, the neutral or circulating
currents for the 3 x line frequency (called triplen) harmonics are of great
concern.


>
> can anyone offer advice or references on this?  we can apparently get a
full
> answer by hiring a power consultant to bring in some kind of fancy power
> digitizer which will give us a plot of our load waveforms and presumably
also
> a spectrum.  so far, we're going along with the HM plan, but mainly
because
> it'll clean up the power coming in, and perhaps permit us to ride out some
> flickers (the compute nodes won't have UPS-protected power...)

Got an oscilloscope?  Just hook it up to measure the current going into one
of your nodes.  If it's a digitizing scope (any will do, because you're
looking at lowish frequencies), grab a few cycles worth of the data, and run
a transform to look at the power spectrum.

Somewhere, about 3 or 4 months ago, someone posted a link to a very nice
explanation of managing harmonics in this sort of situation.