Sources of harmonics
Harmonics are produced by nonlinear loads or devices that draw non-sinusoidal currents. An example of a nonlinear load is a diode, which permits only one-half of the otherwise sinusoidal current to flow. Another example is a saturated transformer, whose magnetizing current is no sinusoidal. But, by far the most common problem-causing nonlinear loads are large rectifiers and ASDs.
Nonlinear load current waveshapes always vary somewhat with the applied voltage waveshape. Typically, the current distortion of a nonlinear load decreases as the applied voltage distortion increases – thus somewhat of a compensating effect.
As a result, most nonlinear loads have the highest current distortion when the voltage is nearly sinusoidal and the connected power system is “stiff” (i.e., low impedance).
The fundamental current angle, which is almost always lagging, is adjusted to yield the desired displacement power factor. Harmonics phase angles are adjusted according to the time shift principle to preserve waveshape appearance.
Classical Nonlinear Loads
Some harmonic sources are not related to power electronics and have been in existence for many years. Good examples are:
For economic reasons, power transformers are designed to operate on or slightly past the knee of the core material saturation curve. The resulting magnetizing current is slightly peaked and rich in harmonics.
The third harmonic component dominates. Fortunately, magnetizing current is only a few percent of full-load current.
The magnetizing current for a 25 kVA, 12.5kV/240V transformer is shown in Figure 1. The fundamental current component lags the fundamental voltage component by 66°.
Even though the 1.54Arms magnetizing current is highly distorted (76.1%), it is relatively small compared to the rated full-load current of 140Arms.
As with transformers, machines operate with peak flux densities beyond the saturation knee. Unless blocked by a delta transformation, a three-phase synchronous generator will produce a 30% third harmonic current.
There is considerable variation among single-phase motors in the amount of current harmonics they inject.
Most of them have THDI in the 10% range, dominated by the 3rd harmonic. The current waveform for a refrigerator is shown in Figure 2.
|Title:||Understanding Power System Harmonics by Prof. Mack Grady (Dept. of Electrical & Computer Engineering University of Texas at Austin)|
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