Harmonics (electrical power)

Harmonics are electric voltages and currents that appear on the electric power system as a result of certain kinds of electric loads. Harmonic frequencies in the power grid are a frequent cause of power quality problems.

Causes

In a normal alternating current power system, the voltage varies sinusoidally at a specific frequency, usually 50 or 60 hertz. When a linear electrical load is connected to the system, it draws a sinusoidal current at the same frequency as the voltage (though usually not in phase with the voltage).

When a non-linear load, such as a rectifier, is connected to the system, it draws a current that is not necessarily sinusoidal. The current waveform can become quite complex, depending on the type of load and its interaction with other components of the system. Regardless of how complex the current waveform becomes, as described through Fourier series analysis, it is possible to decompose it into a series of simple sinusoids, which start at the fundamental power system frequency and occur at integer multiples of the fundamental frequency (as described in the main harmonic article).

Effects

One of the major effects of power system harmonics is to increase the current in the system. This is particularly the case for the third harmonic, which causes a sharp increase in the zero sequence current, and therefore increases the current in the neutral conductor. This effect can require special consideration in the design of an electric system to serve non-linear loads. [For example, see the National Electrical Code: "A 3-phase, 4-wire, wye-connected power system used to supply power to nonlinear loads may necessitate that the power system design allow for the possibility of high harmonic neutral currents. (Article 220.61(C), FPN No. 2)"]

In addition to the increased line current, different pieces of electrical equipment can suffer effects from harmonics on the power system.

Motors

Electric motors experience hysteresis loss caused by eddy currents set up in the iron core of the motor. These are proportional to the frequency of the current. Since the harmonics are at higher frequencies, they produce more core loss in a motor than the power frequency would. This results in increased heating of the motor core, which (if excessive) can shorten the life of the motor.

Telephones

In the United States, common telephone lines are designed to transmit frequencies between 180 and 3200 Hz. Since electric power in the United States is distributed at 60 Hz, it normally does not interfere with telephone communications because its frequency is too low. However, since the third harmonic has a frequency of 180 Hz, it and higher-order harmonics are high enough to interfere with telephone service if they becomes induced in the line.

olutions

There are a variety of solutions available to reduce the effects of power system harmonics. Passive filters are widely used to control harmonics, and are a simple solution, but over time may lose effectiveness as their components age, and may also become overwhelmed by harmonic sources throughout the network [cite web |url=http://users.ece.utexas.edu/~grady/Harmonics_Notes_Grady_June_2006_print.pdf |title=Understanding Power System Harmonics |accessdate=2008-09-05|last=Grady |first=Mack |year=2006 |month=June |format=PDF |pages=7.6] .

Further reading

References