Question

A three phase fully controlled bridge converter is feeding a load drawing a constant and ripple free load current of 10 A at a firing angle of 30°. The approximate Total Harmonic Distortion (%THD) and the rms value of fundamental component of the input circuit will respectively be

Answer 1

Load current $=I_0=10A$
RMS value of total source current

$I_s=I_0\sqrt{\frac{2}{3}}=10\sqrt{\frac{2}{3}}=8.165A$
Supply current $i_s$ can be expressed by fourier series,

$i_s\left(t\right)=\sum _{n=1,3,5}^{\infty }\frac{4I_0}{n\pi }\sin \frac{n\pi }{3}\sin (n\omega t-n\alpha )$

RMS value of fundamental current,

$I_{s_1}=\frac{4I_0}{\sqrt{2}\pi }\sin \frac{\pi }{3}$

$\Rightarrow I_{s_1}=\frac{4\times 10}{\sqrt{2}\pi }\times \frac{\sqrt{3}}{2}=7.8A$

Total Harmonic distortion

$=\sqrt{{\left(\frac{I_s}{I_{s_1}}\right)}^2-1}\times 100$

$=\sqrt{{\left(\frac{8.165}{7.8}\right)}^2-1}\times 100=31\%$