[Beowulf] harmonic mitigators vs PFC?

Mark Hahn hahn at physics.mcmaster.ca
Sun May 23 10:40:30 PDT 2004

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.

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.

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...)

thanks, mark hahn.

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