Abstract:
The goal of this thesis is to obtain a simple and easily implementable scheme
for input power factor correction and harmonic reduction in a three phase
rectifier. A basic three phase diode rectifier having specified voltage(300V P-P)
is analyzed. The input current contains harmonic current . The input power
factor has also been found poor. The cause of having low PF and high THD for
a diode bridge rectifiers is related to non-linearity of the input current. Method
of re-shaping the input current waveform to be similar pattern as the sinusoidal
input voltage is done by the non sinusoidal input current injection technique. A
delta modulated inverter is used to neutralize the harmonic current by injecting
the difference of a reference sine wave and non sinusoidal input current in
opposite direction to the rectifier input current.
Conventional three phase diode bridge rectifier is connected with 3 phase
supply. The difference current (error signal) between the sine wave and the
non sinusoidal input current is injected to a common node which is compared
with a reference sinusoidal(desired) current. The error current is injected from a
current source inverter rectifier circuit, where, the free wheeling diodes and
capacitor is used as a dc source. Switching pulses for the inverter are
generated from the vertices of the error signal using delta modulation. A High
frequency filter is designed to retrieve the error signal from inverter output.
Thus the error current applied to the rectifier input in opposite direction to
neutralize the distortion of input current and make the input current having less
THD. Separate dc source is not used for the current injection. The concept of
static VAR compensation is used in this work to obtain dc source necessary for
providing the harmonic current for injection. The aim of the thesis of a three
phase rectifier having low THD and good power factor is achieved successfully.
