The Inefficiency of PQ

Recently, a colleague from the NFPA 70B Electrical Equipment Maintenance Committee asked me for statistics on energy efficiency and power quality (PQ). Plenty of statistics are available on the financial impacts of power quality phenomena on productivity. For example, a typical textile mill will experience a $10,000 loss when a sag causes a difference in torque on the drums, resulting in the fiber breaking. Or consider financial institutions, such as the stock markets, where losses are measured in millions of dollars or more per minute of down time. But efficiency and PQ was a new question for me. The two are related in a number of cases. One of the most significant is in the mitigation devices used to minimize the effects of transients, sags, swells, interruptions, and harmonics. Obviously, unless we can disprove the conservation of energy laws, any device put in the circuit between two points will result in losses, hence lower efficiency than just a plain wire. Transient voltage surge surpressors (TVSS) devices are used today in distribution panels as well as within most electronic equipment. These devices "eat" up transients above a specified voltage level. However, in systems with significant voltage distortion that results in high crest factors or peaks of the voltage waves, these devices can be forced to clamp the voltage on each cycle. This results in efficiency losses, as that energy doesn't make it to the load. There are devices that can use the electricity more efficiently than is presently being used, though these also can have impacts when lightly loaded, such as the decrease in power factor of rectified input-switching power supplies in adjustable speed drives when lightly loaded. Many of these devices may actually introduce potential power-quality -related problems, such as increased harmonic currents. Harmonic currents result in higher losses in transformers, especially with higher order harmonics. A typical six-pulse converter (full wave, three-phase) will produce significant harmonics of the 5, 7, 11, 13, 17 and 19th, and so on. A more energy-efficient and lower harmonic distortion 12-pulse converter will produce harmonics of the 23, 25, 35, 37, etc. The eddy current and other frequency-dependent losses are a function of the square of the harmonic number (or frequency). Hence, the losses at the third harmonic would be a factor of 9, compared to the 13th, which would have a factor of 169, or 19 times greater losses than the third harmonic if the current levels were the same. The transformer must be derated, which may mean adding an additional or larger-size transformer to maintain the load. Transformers can also experience higher losses when there are half-wave rectifiers drawing unsymmetrical current, which is an effective DC offset on the transformer's magnetic curves. Harmonic currents can also result in fluting of the bearings of grounded motor shafts. This can result in increased drag, hence higher losses in the motor and lower efficiency. Voltage unbalance, often considered in the PQ realm of things, is an enemy of electric motors also. The NEMA MG-1 book can provide information as the significant impact of just a 3 percent unbalance. Some people consider sustained low-voltage levels or interruptions to be a power quality phenomenon, but these are more likely to be considered "reliability" issues from an electric utility stand point. Depending on the type of load, they can result in efficiency changes. Some people even consider power factor to be a power quality problem, and sell devices under the power quality banner that correct for it. Most of us PQ traditionalists don't see it that way; however, some energy-efficient devices do affect power quality, as well as power factor. For example, some of the compact fluorescent lamps used to replace incandescents can consume less energy, but at the expense of decreased power factors and even introducing transients during start-up. So, yes, Virginia, there is a bit of correlation between efficiency and power quality phenomena, though it would be a challenge to produce meaningful statistics on it. BINGHAM, manager of products and technology for Dranetz-BMI in Edison, N.J., can be reached at (732) 287-3680.

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