Line current characteristics of three-phase uncontrolled rectifiers under line voltage unbalance condition

Three-phase uncontrolled rectifiers with capacitive filters are highly sensitive to line voltage unbalance, drawing significantly unbalanced line current even under slightly unbalanced voltage condition. This paper presents an analysis of this "unbalance amplification effect" for an ideal rectifier circuit without ac- and dc-side inductors. A novel way of modeling the voltage unbalance is established by introducing a deviation voltage superimposed on balanced three-phase line voltages, which allows simplified analysis of the rectifier operation. With proper approximations, closed-form expressions for symmetrical components of the fundamental line current and correspondingly the current unbalance factor are derived in terms of the voltage unbalance factor, filter reactance, and load current. The equivalent rms value of the third harmonic current is also derived. The analysis clearly shows high sensitivity of the current unbalance to the voltage unbalance, and provides a basic guideline for rectifier circuit design. The validity of the analysis is confirmed by simulation.