In a recent article, we briefly discussed the concepts of efficiency and effectiveness with regards to power consumption. It was timely with all the press coverage lately about being “green,” reducing one’s carbon footprint, lessening global warming effects and the like. Being green-conscious isn’t just about the environment. It also is related to the green of the cash flow to the bottom line—profits. Green and green go hand in hand.
Effectiveness can be defined as “what power must be supplied by the electric utility to perform the work required by the consumer of such, whether a computer or an entire facility.” The source of this power doesn’t necessarily have to be from the electric utility alone.
Alternative energy sources continue to make progress on the cost-competitive basis against utility-generated power, especially with regard to escalating costs for fuel oil and natural-gas-fired plants. Because few people have a local water source adequate for hydro-powered generators, wind-powered generators and photovoltaic solar arrays are increasing as local power sources, depending on your geographic location. Whereas some of the earlier solar projects used separate areas on a building site to locate the panels, a number of large commercial properties, such as large retail stores, are putting them on the roofs of their facilities to avoid using up pricey real estate. Wind-powered generators generally require a separate location on the property, and they should have at least a 10 mph average wind speed and be paying at least 10 cents per kilowatt-hour for electricity.
According to the U.S. Department of Energy’s Energy Efficiency and Renewable Energy section, another advantage to producing one’s own green power is that “net metering currently is available in more than 35 states.” As explained on the Web site, “Net metering enables customers to use their own generation to offset their consumption over a billing period by allowing their electric meters to turn backwards when they generate electricity in excess of their demand. This offset means that customers receive retail prices for the excess electricity they generate.” More green and green benefits.
Of course, there still are the more traditional and perhaps easier ways to get double green. Most electric utilities have “demand-side management” programs that assist customers to reduce usage, as it is more cost-effective for them than building new generation and transmission capacity.
Avoiding an increase in the demand charge and offsetting costly power factor penalties are two things on the money side of green energy that are relatively simple to implement. The demand charge can be larger than the energy consumption portion of your facility’s electric bill. The utility must have the worst-case capacity available to keep the system stable under the peak load. Therefore, even though you may have only a peak value of “x” megawatts that occurs once a year, they have to size for the summation (using some weighted averaging) of all the demands in the area, so the generation, transmission and distribution infrastructure can handle such. That charge will stay on your bill for a year or more, even if you never come close to that peak demand again except for that one 15-minute interval.
As for the power factor, it is a good indication of how effectively you are using the power in watts versus what the utility has to provide, which is volt-amperes. A power factor of 0.75 is basically wasting 25 percent of the power that the utility must generate and send over the wires. Adding properly sized capacitor banks can bring that number back close to one, reducing costs and energy, another green and green win. Replacing old motors with energy-efficient ones powered by adjustable speed drives (ASD) is another method with easily calculated payback periods.
So how does all this green relate to power quality? One also must be cautious and diligent that one green doesn’t make the other flow the wrong way. Adding PF cap banks without understanding the harmonic currents in the facility may result in a resonance condition, which can amplify the voltage to a potentially destructive level. Similar situations can arise with the ASDs, which produce harmonic currents that can react with existing PF caps into a resonant condition. ASDs also can be more vulnerable to transients caused by the switching in/out of the cap banks, resulting in perceived overcurrent conditions that can trip the ASDs offline.
As for alternative power sources, the source impedance relative to the load impedance is an important set of parameters to know before employing such. The electric grid is referred to as “stiff,” meaning there is enough capacity at low enough of an impedance that sudden and significant changes in load current generally don’t bring the system to a grinding halt. If you increase your source impedance with an alternative power source, say from 0.5 ohms to 2 ohms, then the same motor starting up could result in a sag that is four times deeper than it was before. A sag to 94 percent that didn’t cause any equipment to malfunction can become a 76 percent sag, which may result in a costly interruption of the processes in the facility.
So, while going green, make sure the other green isn’t turning red.
BINGHAM, a contributing editor for power quality, can be reached at 732.287.3680.