Leading Power Factor Problem

[Gavin Parsons]

I have 2 x 120 Kva UPS ( 3 phase 400V ) supplying a computer room to provide
me with n + 1 in event of a power failure. However the load is now 100 Kva
but we have a problem with leading the power factor. I cannot put a big
motor on the UPS to correct it. Does anyone know of any other ways that can
correct this ?

Regards

Gavin

 

[Mark Empson]

Hello Gavin.
Can you be sure that you have a leading power factor? how was this
measured.?
I would expect that you have a distorted current waveform and so have a poor
harmonic powerfactor rather than a poor displacement power factor. Some
instrumentation can not differentiate between the two.

Best regards,
Mark Empson.
http://www.lmphotonics.com

 

[John Woodgate]

I read in sci.engr.electrical.compliance that Mark Empson

Can you be sure that you have a leading power factor?

I don't think he means that. I think he wants to add leading current to
improve the power factor.

how was this
measured.? I would expect that you have a distorted current waveform and
so have a poor harmonic powerfactor rather than a poor displacement
power factor. Some instrumentation can not differentiate between the
two.

You may be right, in which case he needs an active power-factor
corrector. A visit to the Claude Lyons web site might be in order.

 

[Chris Oates]

I read in sci.engr.electrical.compliance that Mark Empson


I don't think he means that. I think he wants to add leading current to
improve the power factor.

Don't think so - he's trying to introduce introduce lag
with a motor

 

[Gavin Parsons]

It is a main server room with lots of racks.
The leading power factor was measured by the UPS engineer with what I dont
know.
Unfortunately i cant put a motor on it as it is a comms room and yes we do
have distorted wave forms due to harmonics to the24th level but we are using
harmonic filters to try to corect this.
I hope this adds more fuel to the fire

Gavin

 

[Rusty]

I have 2 x 120 Kva UPS ( 3 phase 400V ) supplying a computer room to provide
me with n + 1 in event of a power failure. However the load is now 100 Kva
but we have a problem with leading the power factor. I cannot put a big
motor on the UPS to correct it. Does anyone know of any other ways that can
correct this ?

Regards

Gavin

Do you really have over 500 computers on that UPS? That is some
computer room!

Computers normally result in a lagging power factor. As somebody
else said, are you sure it is not a very distorted waveform? By
virtue of the way a UPS generates AC from DC, the waveform is often
very far from a sine wave. This is probably confusing any simple
instrument used to measure PF. You may really need capacitance, not
inductance, applied across your supply, if the PF is really causing
a problem with the supply company.

 

[Andrew Gabriel]

Do you really have over 500 computers on that UPS? That is some
computer room!

Computers normally result in a lagging power factor.

Hum, not for very many years. Switched mode PSU's result in a tiny
leading phase shift. However most of the low power factor is not
due to the phase shift, but due to the harmonic distortion of the
Current waveform -- the tendancy to draw current only near the
peaks of the voltage waveform (which is almost in phase -- the load
is weighted in the short period just up to the peak which gives you
the tiny leading phase shift).

As somebody
else said, are you sure it is not a very distorted waveform? By
virtue of the way a UPS generates AC from DC, the waveform is often
very far from a sine wave.

The pseudo sine wave output is likely to make the harmonic distortion
of the Current draw somewhat worse than it would be with a real sine
wave, and increase the small leading phase shift slightly.

This is probably confusing any simple
instrument used to measure PF.

If the device attempts to measure PF just by examining phase shift
(and I've seen devices that do this), it will be reading completely
wrongly with a Switched mode PSU, where the low PF is mainly not due
to phase shift.

Incidently, SMPSU's are available which have almost unity PF, and
you'll find those in larger more expensive computer systems, but
probably not in any PC system.

I always find it a big irony that UPS's go to such lengths to recreate
sine waves (with varying degrees of success), and SMPSU's really
couldn't care less. If anything, SMPSUs would work better on square
waves, and UPS's delivering square waves would be much easier to design ;-)
OK yes, there are problems with RFI emissions, switches working, etc.
May as well go the whole way and chuck out the middle man, and have the
computer's SMPSU work directly from the UPS's DC battery...

 

[Bill]

I have 2 x 120 Kva UPS ( 3 phase 400V ) supplying a computer room to
provide me with n + 1 in event of a power failure. However the load is
now 100 Kva but we have a problem with leading the power factor. I
cannot put a big motor on the UPS to correct it. Does anyone know of
any other ways that can correct this ?

Regards

Gavin

If you are sure that you have a leading power factor, one option would be
to extend the lines that are running to the computers. You will drop
voltage as well but the extra line will counter your leading pf. This will
only gain you a small amount of correction though for the cost of the wire.
If the capacitance is a larger problem then I would suggest going to an
active bank at the supply that will correct your problem.

Bill

 

[Mike Undercofler]

I have 2 x 120 Kva UPS ( 3 phase 400V ) supplying a computer room to
provide me with n + 1 in event of a power failugeneratorer the load is now
100 Kva but we have a problem with leading the power factor. I cannot put
a big motor on the UPS to correct it. Does anyone know of any other ways
that can correct this ?

Regards

Gavin

Gavin,

I'm guessing from your original post, and what I have gathered from the
followups I get here, I don't seem to be getting them all, that your
problem is a 3 phase 400Vac(?480 maybe? are you in the US or another
country?)feed into a ups system that feeds out to, by the sounds of things,
one heck of a computer room. You have a leading power factor due to the
load placed.

There is always the missing info thing to consider, but I'm guessing your
UPS is an active switching station, and inactive power unit (like a
generator that sits idle till it is needed and a small battery and
converter set up that holds power while the generator gets up and running).
If that is the case, an active power unit would be the best way to go. (the
computers run off a battery pack and converter full time, and the generator
kicks on when needed to charge the batteries) All depends on how much your
company wants to lay out for a battery room, and converter system. When it
comes right down to it, we all have to watch what we spend, even if it is
well worth it.

If you are simply looking for a cost effective way to beat a leading power
factor, your local power utility company (PUC) can probably supply the
engineering specs, free or at a modest charge, for a capacitor bank that
will do the job. Despite how much every tech in the world gripes about
them, all PUC's are helpful when it saves them money, and getting rid of a
leading power factor draw will save them money. Just call them and ask to
speak with an engineer in the generation or maybe distribution (depends on
the PUC I'd start with generation, explain what I needed, and go from
there) division. And be prepared to wait a few minutes till you get through
to someone that has some idea what you are talking about. (So don't make
the phone call on your lunch break )

Hope this helps, and if not, please drop a reply, or email so we can get
some more specs, and we *will* find something that does.

Mike Undercofler