In an electrical system, a component of the current or voltage which does not consist of the fundamental frequency is harmonic. Source of harmonic content in the systems are power electronic equipment, non-linear load, surge, saturated magnetic material, and inrush currents.
In general, any nonsinusoidal can be expressed as a Fourier series :
Fourier series in equation (1) can express as follow
where,
Using equation (2), RMS voltage is :
Here, $V_{1rms}$ component is with the fundamental frequency. So, only $V_{1rms}$ is desirable and other components are considered as harmonics.
In equation(8) $V_{oh}$ is the RMS value of harmonic content in the waveform. Dividing equation (8) with respect to the RMS value of the fundamental component will give total harmonic distortion (THD).
where g is the distortion factor
Displacement factor (DF) (Fundamental power factor)
Where, $\theta_1$ is angle between $V_{1rms}$ and $I_{1rms}$
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