Harmonics and power quality: an overview

A practical view of harmonics and power quality: Part 1

By Howard G. Murphy, P.E., Drives World

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With the invasion of nonlinear loads, such as computers, variable speed drives and other switch mode power supplies, our ability to deal with unexpected equipment behavior is confused by a mass of technical jargon offered both as explanation and solution. Revisiting the past when amps were amps and volts were volts might help in applying previous experience to understanding the sometimes-mysterious issues of harmonics and power quality.

Figure 1

The early years

Life was simple when distorted voltage and current meant only too much or too little. Voltage and current were measured with simple meters. There was no need to worry about true root mean square (RMS) values, instantaneous peaks or distortion percentages. You knew 480 volts was just 480 volts and equipment with a 480-volt nameplate would work fine...most of the time. If the measured voltage was too high or too low, you could expect some extra heating or fuse blowing. If the measured current to a 100-hp motor was 124 amps in each of three phases, the motor was near full-load. If phase currents differed, the voltage supply was not balanced or there was something wrong with the motor. If measurement revealed more than full-load current, the motor was overloaded or the voltage was too low and the motor might run hot.
The resolution of voltage and current problems was selected from a small list of well-known solutions. However, fixing the problem could wait and production continued, maybe with a larger fuse, until a more convenient time to make the necessary changes presented itself. We knew that unbalanced voltages applied to a three-phase motor would result in unbalanced currents. Also, the motor might run a little hotter if the problem was not fixed immediately. This was acceptable for a short time so that production was not affected.

Technology shifts

Most of the traditional ways of dealing with voltage and current problems are being abandoned because of the confusing terminology and explanations being offered as solutions to the "new" problems associated with nonlinear loads.

By offering regulated voltage sources and transmission and distribution systems capable of handling huge currents, the local electric utility supplies most of the power needs. The main concern for the user is to maximize the amount of work being performed with the least amount of current transferred from the utility to the user's process.

Minimizing reactive current has always been a concern since utilities often impose economic penalties when your reactive current is too large. With a large reactive current, the low power factor the utility sees places too much of a demand on transmission and distribution systems. This reduces the allowable energy--real work--transferred from the utility to the customer and reduces utility revenues.

Figure 2

The impact of nonlinear loads

Life has become more complex with the introduction of electrical equipment using the latest technologies for power conversion, control and communications. Most equipment using these technologies changes AC line power to DC power. This equipment uses power in a nonlinear fashion. Power is still the product of voltage and current, but with nonlinear loads, current may be zero while a voltage is applied to the load. This unique relationship between voltage and current defines a nonlinear load.

Since there is a direct relationship between the current waveform and power--real work--being transferred, the area under the current curve or waveform represents the power transferred. Most office equipment, for example, converts or rectifies AC power to DC power and creates little reactive current. The current, although in phase, does not follow the sinusoidal shape of the voltage waveform and therefore is called nonlinear.

Figure 3

For office equipment, the area under the curve of the current waveform defines the consumed power. Figure 1 shows a typical nonlinear load current pulse for a computer, fax or copy machine. The area under this curve must be equal to the area under the curve for an equivalent linear load since both consume the same power. The rectification process reduces the width of the pulse to less than the 8.3 milliseconds, or half cycle, of a linear load. Since the width is reduced, the height of the pulse must increase. The value of the RMS current for the nonlinear load increases slightly. The peak of the current pulse may be two to five times greater than the normal value of 1.414 times the RMS value of a pure sinusoidal waveform.

Although the RMS value of the current is approximately the same, the peak value creates problems if fuses and circuit breakers are not rated for the larger peak value. These current pulses cause non-sinusoidal voltage drops throughout the distribution system. The resulting voltage drops add or subtract from the sinusoidal voltage supplied by the utility. This results in a distorted voltage that supplies other utility customers and other equipment on the downstream side of the distribution system. Solving complex problems in electrical systems and components begins with trying to understand their behavior when subjected to current pulses and the attendant voltage distortion.

The current pulses are not simple sine waves that can be measured with simple meters. When problems occur, electrical equipment cannot simply be restarted and run until a more convenient time to investigate and fix the problem.

Although the same types of electrical events occur, like random fuse-blowing, overvoltage and undervoltage trip, our well-known list of solutions for fixing electrical problems seems not always to apply. Our search for answers from equipment suppliers results in more confusion and more technical jargon. Although the technical jargon may be interesting, we need to concentrate on the fundamentals. Do we need to understand every aspect of harmonics or can we boil the technical jargon down to the basics and apply some simple rules in dealing with the problems that are likely to occur?

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