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Harmonics are frequencies that are integer multiples of some fundamental frequency in the circuit. If your system has a signal that is not a pure sinewave (single freq), then harmonics will exist.
If your system has a squarewave signal operating at 100 kHz, then you'll have a fundamental signal at 100 kHz and harmonics at 300 kHz, 500 kHz, 700 Hz, etc (since squarewaves contain only odd harmonics). Other waveforms will also have even harmonic components. So, if it's not a pure squarewave, you'll also see harmonic signals at 200 kHz, 400 kHz, 600 kHz, etc.
Dear nouman_rafi
Hi
In power electronics (some times) we will have non linear wave forms or destroyed component (such as the out put of triac when the turn on angle is not perfect) . according to the fourier series law , each non sinusoidal waveform , contained from some sinusoidal wave (such as an square wave). ok ? when you created non sinusoidal wave , it is usual to have some harmonics .
Best Wishes
Goldsmith
Here's a good little applet (requires java) that visually demonstrates the sinusoidal content of various common wave forms... you can play with the amplitude and frequency content of each of them Fourier Series Applet
If you generate a pure sinusoidal signal in your circuit, and you get harmonic content, then something is behaving in a non-linear fashion (if it was linear, a single sinewave input would result in a single sinewave coming output). Since there is some distortion to the pure sinusoidal waveform, other frequency content must be present (@goldsmith: Fourier series decomposition). This distortion/non-linear conversion can be caused by any number of things... a diode, transistor (all amplifiers generate harmonics), op-amp, etc.
To see the frequency content of a signal in action, drive an op-amp with a small sinewave. If you begin with the output signal within the supply rail voltages of the op-amp, you will get a nice clean sinewave output; seen on a spectrum analyzer as a single tone. If you increase the input signal level such that the output signal begins to clip (hits the supply rail voltages and "flattens"), then you will see harmonics begin to appear on the spectrum analyzer (even and odd harmonics). If you continue to increase the input signal such that the output appears to be a squarewave (near-vertical edge transitions and flat tops/bottoms), you will see strong contributions at the odd-harmonics, and less at the even harmonics.
The harmonics are generated due to non-linear operation mechanism of Power Electronics components i.e. the power electronics components gets ON and OFF at certain firing angle and due to this some electronic sparking (harmonics) are generated.
You can also get harmonics in power distribution systems where there are substations with varying power factors (phase relationships between voltage and current).
For example all voltages are in phase throughout the system but in one subsysttem the the power factor (cos phy, phy being the angle between the current and voltage sinusoid) may be 0.8, in another 0.5 possibly due to a large number of electric motors running with little to no load. This means you have a number of subcircuits drawing currents at varying angles. If a transformer upstream has an output impedence of some magnitude then superimposing the various unaligned currents will cause distortion on the supplies sinusoidal output. Spectrum analysis of this will reveal various harmonics.
Why is this bad? The phase difference between cuurents causes circulating currents in your distribution system causing heating and waste of energy
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