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Server Power Supplies Take an Ugly Turn
Posted on: December 8th, 2011 by Bill

Well, it’s official.  I have lived long enough to see history repeat itself.

So this is what they mean when people mention the benefit from the wisdom gained from age.  It’s more like the movie “Groundhog Day”, where you are stuck in a tragic comedy.

On several occasions in the past six months, our team has run into significant power quality issues with 3rd order harmonics in a few of our data centers, both in the wholesale colo and enterprise markets.  Third order harmonics in data centers is like saying that it’s going to be hot in the desert – you know it’s true, but you hope it’s not.

We have come to discover that the commodity servers from Dell have been the culprit of some pretty ugly load-generated, third-order harmonics..

Power factor correction has been the boogeyman for servers and data center for the past three years.  Stemming from the need to run engine-generators at or below unity power factor, or a lagging power factor.  Power factor improvement was a byproduct of effciency improvements demanded by the marketplace.  This is accomplished by capacitors.  However, the amount of capacitance being injected into critical power system is starting to exhibit some darned strange things.  In solving one problem, the server manufacturers, as well as the OEM power supply vendors, are creating others.  It’s like the Will Smith movie, “I Am Legend”, where the vaccine for cancer ends up turning everyone into flesh-eating zombies.  Great.

Here’s what we are seeing.

  • Rectifier walk-in setting up a harmonic feedback on a 10s interval in transformer-less UPS modules.  And this is happening on 100% transistorized IGBT modules, not old 6-pulse units.
  • Harmonics in large multiples of the funamental, just like the 1990’s and early Dot Com years.  In fact, we dug 15 year old waveforms out of the files, and the new power supplies have a waveform exactly just like the crap that used to be supplied back then.
  • Power factor correction in the server power supply is now “swtichable” by the customer.  If the power factor correction and power supply were engineered correctly (as so aptly pointed out by one of our Engineering Directors Clint Summers), why is it switchable in a passive critical power electrical system.

When you peel back all the hysterics back, the bottom line is that the THD and the amplification of the harmonic disturbance into large multiples of what’s expected and from the fundamental frequency are well outside of industry-stated standards.

Most data center electrical infrastructure systems can deal with relatively strong harmonics.  We have seen some issues with zig-zag neutral reference operations.  But we have also seen this pop up in PDU operations as well.  Our position here is that the Z-Z would not have an issue if the power supplies worked according to the industry tolerances in the first place.  It’s like arresting a home owner for murder when the mob dumped a dead body on the guy’s front lawn.  You are blaming the wrong party.

The best of all, your power professionals have sufficient power quality metering that allows complete diagnosis of these issues.  So, don’t let that server sales guy pull an ole’ on you.  The matador goes home and the bull ends up being steak dinners for everyone!

Make sure that you demand the power spectrum data for the server power supply, both in a waveform format as well as the summary of frequency values, similar to what we’re showing below.

And for you reading enjoyment, please see the redacted power survey from the site.  I thought I was back in 1999 when I read it.  Note the PU multiples!  Holy crap!

Sorry for the inverse order of the pages.  Enjoy!

And Merry Christmas everyone!

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