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Q & A: distorted electrical power

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Most recent answer: 10/22/2007
why we dont consider even harmonics in supply voltage?
- Amol shivaji kochale (age 23)
shahada, maharashtra, INDIA

I guess you mean that the strengths of the odd harmonics of the voltage of the electricity delivered by the power company are bigger than the strengths of the even harmonics. In the United States, the fundamental frequency of the electrical power is 60 Hz, and the odd harmonics are at 180 Hz, 300 Hz, 420 Hz, etc, while the even harmonics are at 120 Hz, 240 Hz, 360 Hz, etc. Many electrical devices work best when the power is delivered only at the fundamental frequency and has little extra "noise" from higher harmonics, and so the strength of the harmonics is often closely monitored.

The power company loves to assign blame for the presence of unwanted harmonics on nonlinear loads that their customers plug into the power grid. A linear load is one that draws current in proportion to the voltage delivered (it’s only a little more complicated than this but we’ll explain). An electric heater, for instance, a toaster, is just a resistor and the current through it is given by Ohm’s law I=V/R, where I is the current and R is the resistance. A fine point here: toaster heater elements and light-bulb filaments have lower resistances when they are hot, and so are "nonlinear" in this sense. But they heat up and cool down so slowly on the time scale of the 60 Hz power oscillations, that they do not introduce noticeable harmonics in the power. Another kind of "linear" current draw may be out of phase with the incoming current, such as in an electric motor operating synchronously with the current. A high current load can "drag down" the voltage on the power supply because of resistance in the power lines. As long as everything oscillates at 60 Hz, no one complains.

Nonlinear loads are those with more complicated relationships between their current requirements and the supplied voltage. An example of a nonlinear load is an electric discharge lamp (such as a fluorescent tube or a sodium street lamp). Once the voltage gets high enough across two electrodes, an arc is created and a lot of current can flow more easily than before the arc is created. This means that the lamp draws nearly no current for small absolute values of the voltage,and much larger currents for larger voltages. They do not care which way the current flows. If a whole city-full of street lamps is operating, the peaks and troughs of the voltage supplied by the power company may sag a little bit (in absolute value), flattening the sine wave out. The harmonic structure of this situation consists entirely of odd harmonics.

If the sign of the distortion to the electrical power follows the sign of the applied voltage, then odd harmonics are generated. If there is a distortion that is, say, positive for either positive or negative applied voltage, then even harmonics may be generated.

Here are some examples of common nonlinear loads

  • Transformer cores -- hysteresis and saturation of the iron create nonlinearities in the response. Power companies use transformers for transmission of power over long distances at high voltage.
  • Electric arc lighting
  • Rectifiers and switching power supplies for supplying DC voltages from the AC line.
  • Variable-speed motors which draw current at their own frequency which may differ from the electric line frequency (and thus generate even or odd harmonics or usually, both)
  • Dimmer switches on lights at home.

Mike W. and Tom J.

(published on 10/22/2007)

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