[Vwdiesel] ----( The Power Factor )
r.c.brown at ieee.org
Tue Apr 19 00:03:41 EDT 2005
Sandy Cameron wrote:
> This bugs the hydro co. because your meter shows less watts used than you
> actually used, and they have to generate more power than they can charge you
> for to supply your load..
> If it's serious enough (industrial users) they make you pay for giant
> capacitors to correct the power factor of your big inductive load.
> More and more appliances are using "run" capacitor motors to save power and
> run cooler.(Energy Star) The cheap split phase capacitorless fractionals run
> hot and waste half the power in heat.
Power meters will only measure "real" power (watts) but non-unity power factor will
result in "reactive power" usually measured in VARs (Volt-Ampere-Reactive). Reactive
power is not really power at all, since it basically represents those times in the 60
cycle waveform when you have current and no voltage (I*0=0) or voltage and no current
(V*0=0) or more correctly when current and voltage are out of phase with each other.
The big deal with reactive current is in loading of the system, power lines and
transformers are rated for so many amps. With lots of reactive current, you can hit
the current limit of the transmission line sooner for a given "paying" load.
The other main issue is transmission system stability. Transmission lines and
transformers are inherently inductive (lagging power factor), and to get power to
flow from the generating station to a load, the generator has to "push" power with a
leading power factor. The more your load is lagging, the less headroom there is in
the system. And, there is a limit to how hard you can "push" since once the voltage
and current get 90 degrees out of phase, you lose synchronization and the lights go out.
When I used to work for NASA on a project monitoring power usage with wind tunnels,
arc jets and other multi-100megawatt loads, we were hooked into a system with
Stanford Linear Accelerator and Lawrence Livermore to coordinate peak power useage.
NASA had a quarter-million dollar computer monitoring system in place for the various
users to log in and plan/coordinate power useage among the other users to minimize
power costs. Interestingly, we got almost free power from the federal power system
(Shasta Dam, etc.) for under 1/2 cent/KWh, but had to pay PG&E "wheeling" costs,
basically to rent their power lines. We were billed for power used, power factor,
and peak power used in a one month period. There were enough cost savings in power
factor reduction (via huge capacitor banks) and trimming power peaks to easily pay
for the cost of the monitoring system. I recall the other charges exceeded the cost
of the power itself, bills in the $500,000-$1,000,000/mo. range and this was in the
early '80s. I seem to recall that they figured the cost of running the 80x120 foot
wind tunnel (6 - 30,000 HP motors) was about $200/minute just for electricity.
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