Practical Classroom Demonstrations Of Power Quality Issues

Several simple classroom demonstrations safely and conveniently illustrate common power quality issues: voltage distortion, voltage flicker, and current harmonics. A voltage distortion demonstration shows the everyday effects of having many personal computers or switch mode power converters in a small area. A flicker demonstration uses a light bulb to illustrate dramatically line voltage variations caused by the fusing system of ordinary laser printers. A current harmonic demonstration conveniently shows the effect of a capacitive filter on the harmonic content of the ac line current. All demonstrations use only laboratory equipment and instrumentation that is readily available in an academic setting. Introduction In the past few years, the quality of power has become an important issue for the electric utility companies. Customers have come to notice deviations from the expected single frequency, constant amplitude, sinusoidal voltage that is the nominal product of their local utility. Sensitive power electronic loads have become an indicator of less than perfect voltage quality. Often, the effects of imperfect voltage quality may seem insignificant to utility engineers. Unfortunately, these effects may range from mere irritation of a blinking digital clock to lost or spoiled products. In many cases, ironically, these same power electronic loads cause the "dirty power" as well. Certain industries, for example, the semiconductor process industry and certain food processing industries, are especially sensitive to power quality issues. They often write power quality specifications with economic premiums and penalties into their contracts with the electric utilities. In fact, several of these customers with sensitive loads have begun to request the teaching of power quality as part of the undergraduate power engineering programs. Power quality problems occur on the utility distribution grid at voltage levels that are too high for safe investigation by most third-year and fourth-year students. Power quality problems often are intermittent in nature. However, this paper presents demonstrations that show power quality phenomena in a fashion that is both practical and safe. Three demonstrations to illustrate the following issues are given: voltage distortion on the residential and commercial power distribution grid, flicker and sags, and current harmonic distortion. Required equipment is ordinarily within the capability of any undergraduate program electrical engineering program: an oscilloscope, a low frequency spectrum analyzer, a simple function generator, a laser printer, and an ordinary three-wire extension cords. Elementary Demonstration of Voltage and Current Distortion There is noticeable voltage distortion of the utility voltage in most distribution locations. This is particularly true in buildings with the combination of older wiring and a large number of P ge 351.1 switch mode power supplies. Nearly all personal computers have switch mode power supplies. Many academic buildings contain several computers and such buildings are more than a few years old. This makes a typical academic building an excellent place to show the aggregate effects on the utility system of large numbers of switch mode power supplies. This demonstration can be set up in seconds in a typical academic setting: an undergraduate electronics or power laboratory or in a classroom. Equipment for this demonstration is fairly simple and common in an academic environment: a transformer that steps 120V single phase down to a safer level, such as 12 Volts, and an oscilloscope with standard voltage probes and clamp-on current probes. A special cable helps in capturing the current waveform. The cable is a short, three-wire, 16 gauge or larger, extension cord, approximately 20 cm in length with the outer jacket stripped off a portion of its length, but with the individual insulation intact on the three insulated inner wires of the cord. Enough of the outer jacket should be removed to permit the clamp-on current probe to fit around each insulated inner wire individually. Setup is per Figure 1. The special cable is placed in series with the oscilloscope's power cord or in series with the power cord of any other convenient equipment containing a switch mode power supply. Placing the special cable in series with the oscilloscope's power cord uses the oscilloscope as both experimental apparatus and measurement apparatus, requiring one fewer piece of equipment. Figure 1 Experimental Configuration for Measuring Voltage and Current Distortion However, students find the demonstration more pertinent if the special cable is in series Osci l loscope