[Beowulf] Re: UPS & power supply instability

David Kewley kewley at gps.caltech.edu
Thu Sep 29 20:43:38 EDT 2005


On Thursday 29 September 2005 15:45, David Mathog wrote:
> > > I liked Jim's idea of trying a
> > > balanced, large resistive load on the lines.  A heater, an
> > > electric range.
> >
> > I want to say (but am not sure at the moment) that the
> > instabilities show up
> > at around 140kW.  I don't have 140kW of resistors available easily,
> > let alone a full-power load of 350kW.
>
> Sure, but if things are really, really, badly screwed up one
> might see a largish neutral current, percentage wise, no
> matter what size the resistive load.  Try three matched
> space heaters (one per phase) and see if the neutral current
> is zero.  If it isn't then you (or your electrician) can
> diagnose at least part of this problem without having to plug
> in a full load.

Now that *is* an interesting idea, and a doable one.  I can certainly 
try that, although I don't know exactly when. ;)

> For the experts out there: could a huge iron pipe, or a lot
> of iron rebar, produce this much of an inductive load if these were
> located just below the floor, say 6 inches from the power
> lines running to the racks?  The lab in question is on
> the bottom floor of the building so there could conceivably
> be something like that underneath it.  I'm thinking not, but
> then I've never worked with the kinds of currents that are present
> here.

If this magnitude of effect is important, then we probably should also 
think further about ground currents in the room, which we have measured 
at various points.  We have eliminated the one place where a delta-wye 
transformer secondary's neutral was tied to ground at two places.  Now 
the PDUs do not indicate any ground current, whereas with the two-tie 
topology they did.  All the rest of the equipment (e.g. HVAC) is 
3-phase, so should not be a problem.

But as much as 2-3A *is* flowing when we look with clamp-on meters at 
various ground cables.  This ground current is likely related to the 
fact that there are three ground paths to the room, all referred back 
to the same earth electrode point.  These paths are likely tied 
together in the room as well as at the remote earth electrode, creating 
a loop.

One ground path travels with the computer power lines.  A second is 
grounding the raised floor metal components.  And a third ground 
travels with the power to the HVAC systems.  To complicate that third 
path, the HVAC ground path splits between the HVAC main breaker box and 
the HVACs, one path going through a devoted UPS, and one directly 
connected to the HVACs (the two branches power different subcomponents 
of the HVAC).  Very likely the HVAC and PDU chasses are tied to the 
floor grid, thereby connecting all 2-3 paths back to the earth 
electrode.

If there's no leakage between neutral and ground anywhere, then the only 
other mechanism I can think of that would cause these currents is 
induction.  Can induction typically produce currents this large on a 
~1000 ft^3 ground loop?  Am I missing something?

David
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