With the growing reports of energy demand exceeding supply, the attractiveness of energy conservation devices has begun to resurface. The claims of such to save energy, along with the impact on the quality of the power, should be treated with a cautious eye before rushing off to install the latest and greatest.
While many devices live up to their claims, some are worth a second look. You might get energy savings, but with increased susceptibilities to power quality phenomena. Or, the devices themselves may contribute to power quality problems. Or, they may not really save energy, decrease your power factor, and cause potentially damaging transients.
In the not-too-distant past, most electricity in the United States was consumed by electric motors, with fluorescent lights coming in second. It is not surprising that these two areas are where significant development of more energy-efficient devices took place. Adjustable speed drives (ASDs) became the front end to many electric motors. Electronic ballasts replaced magnetic ballasts in fluorescent lights. Even compact fluorescent lamps became popular alternatives to incandescent lights.
ASDs have a rectified front end that converts AC to DC through diodes and/or SCRs. On a three-phase system, this results in six paths or poles of conduction (3 x 2). This results in harmonic currents of the 5th, 7th, 11th, 13th, 17th, and 19th levels, etc. These follow the rule of h = n x p +/- 1, where “h” is the harmonic number, “n” are integers of 1, 2, 3, 4, 5..., and “p” is the number of poles. These harmonic currents can produce in harmonic voltage levels (if the harmonic source impedances are large enough) that exceed IEEE 519 recommendations, as well as compromise the performance of other equipment on the same circuits.
ASDs are also known to be susceptible to tripping off-line on power factor (PF) cap correction switching transients. While the ASD didn’t cause the transient, the motor probably wouldn’t have experienced any problems when powered off the line instead of the ASD.
Fluorescent lights have been around for decades. The harmonic currents take on a large number of different waveshapes. These harmonic currents have taken on new dimensions with the introduction of electronic ballasts. Distortion current levels of 15 to 20 percent can become 60 to 80 percent.
At a manufacturing/ research facility, a contractor was offering to replace all of the ballasts with new magnetic ballasts that would save the company more in energy costs that the old lighting had consumed. Besides defying the laws of physics, the ballast turned out to have 85 percent harmonic current. Placing hundreds of these high-harmonic emitters through the facility would have compromised the low-level signal tests being conducted in the research lab, and possibly overloaded the service transformer.
Then there is the compact fluorescent lamp. Tests on a number of different bulbs uncovered one with some interesting characteristics. The “equivalent light output of a 100W incandescent bulb” consumed 100W at a power factor of 0.55. If zero watts savings with reduced power factor weren’t enough, there were transients during the warm up phase of nearly 400V. (See Figure 3.) And, there are the infrared emissions during start-up of the bulbs that can make TV and other infrared remote control temporarily inoperable.
Clearly, not all energy-efficient devices have such results. And conservation is by far a quicker solution to the pending watt shortages, as compared to building new generating plants. However, conducting a few simple tests on sample devices before signing a contract may save you more money in the long run.
BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.