Honest energy: The danger low-power-factor loads pose for the energy grid
As typical household products become more sophisticated, the power factor of the load they represent decays—a trend that is exacerbating a growing stress on our electric-power infrastructure.
By Joshua Israelsohn, Contributing Technical Editor -- EDN, April 2, 2008
I had the honor and great pleasure of serving as a panelist at an APEC (Applied Power Electronics Conference) rap session in February in Austin, TX. Although the session's central theme was the power sector's focus on efficiency, I was intrigued to note that all of the panelists agreed on an issue concerning energy use that lies outside a strict definition of efficiency: watts out divided by watts in.
As typical household products become more sophisticated, the power factor of the load they represent decays—a trend that is exacerbating a growing stress on our electric-power infrastructure. The problem with low power-factor loads is that they tie up grid capacity in excess of the energy they use.
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An analysis of a load that draws nonsinusoidal current represents the current waveform as a harmonic series with components that are either in phase or in quadrature with the voltage waveform. Because the power grid must supply all of the current—the in-phase component, which the utility meter measures, and the out-of-phase or reactive component, which the utility meter does not measure—the grid load exceeds the metered load.
The so-called PA (apparent power) is the product of the rms voltage and rms current—measures that ignore relative phase (Figure 1). PREAL (real power) is the integral of the in-phase voltage-current product over each line cycle. PREAC (reactive power) accounts for the quadrature-current component. The PF (power factor) is a measure of grid-use efficiency, which you can calculate as the ratio of PREAL to PA, or
Conversely, the excess grid capacity that a load uses is
The figure shows a load with a PF of about 0.9, which corresponds to an excess grid use of about 12%. To put numbers to a few real objects, I measured some devices in my home (Table 1). Think of these numbers in the context of the electronic devices your company makes and consider the important role high power factors can play in electric-energy availability.
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The article references volt-amperes, which is what the power utility generators must deliver, even though the preferred load is watts (pure resistive). Both the generators and power lines have losses which are essentially proportional to current load, therefore bad power factor poses a delivery problem for the utility.
Most utilities make allowance for consumers to present loads to the line with as low as 90% p.f. without penalty. Most residential and small commercial loads do not represent a problem, or are already compensated to meet these requirements (p.f. correction capacitors in standard lamp ballasts, for instance).
But, if you are an industrial customer, your plant probably has two meters, one for kilowatts (KW) and the other for kilo volt-amperes (KVA). The plant will be billed for the greater of 90% of KVA, or 100% of KW. This means it pays industrial customers to pay attention to bad power factor.
There are interesting side effects. Bad loads are almost always inductive. In order to compensate, utilities can run their generators over-excited in order to produce leading power factor at the generator, again within current limits. Lumped capacitors (switched) distributed around the system can also be used for correction.
But the power grid is a dynamic supply and load system. I have a friend who has spent his entire career working in system stability research with our provincial utility. There are some hair-raising stories when one tries to model on these real-life systems - sparks and smoke on your lab bench are no match for problems at the 100's of megawatts level.
Greg Carkner - 2008-24-6 12:55:00 PDT -
Here is an interesting look at power factor.
None of this is new, and power factor correction (PFC) devices are well known. Low PF wastes generating capacity, transmission capacity, and we have to pay for this somehow. No, you are not getting something for free, and it is definitely not green in any sense.
www1.eere.energy.gov/industry/bestpractices/pdfs/mc60405.pdf
David Pateman - 2008-14-5 06:08:00 PDT -
Hold on a sec. Am I reading the article correctly? With other than a resistive (or other 'proper') load, are we actually getting more useful power than we are paying for? Is there a way to do this with gas and water also? Hey, I want to be green... in my wallet!
Jim - 2008-10-4 17:23:00 PDT -
About:
Honest energy: The danger low-power-factor loads pose for the energy grid
Very basic level of the info plus one "innovation" that shows that the author is not an engineer
I am referring to the “Real power”.
Nicholas Adams - 2008-10-4 15:50:00 PDT -
Just a note. In the table you compare the PF's of tungsten-filament lamp and a fluorescent lamp noting that the compact fluorescent lamp has a PF of .56. What I remember seeing is that the compact fluorescent uses 28 watts to provide the same light output as a 100 watt tungsten-filament lamp which is 72% less power. So even with the PF hit of 0.56 the compact fluorescent is still consumes half the power (28w/.56)for the same amount light As a 100W tungsten-filament lamp. So it is still green (green lite?).
Chris Frantz - 2008-10-4 12:14:00 PDT
