An Advanced Modulation Technique Featuring Common Mode Voltage Suppression for Three-Phase Neutral Point Clamped Back to Back Converters

Abstract

Three-phase back-to-back converters are widely applied in various industrial, commercial, and domestic applications, such as AC motor drives. Due to the non-sinusoidal voltages they generate, a common mode voltage (CMV) appears, leading to problems in electrical drive systems and high-frequency applications. The CMV and rapid voltage changes can cause serious problems, including leakage currents flowing through the parasitic capacitors inside the motor, electromagnetic interference, shaft voltage, and bearing currents that reduce the motor’s lifespan. In general, research to reduce these effects is divided into two methods: modifying the drive system’s physical structure or improving the inverter’s control algorithm. Pulse Width Modulation (PWM) methods are commonly used in control algorithms of converters to reduce the CMV. However, adding pulse amplitude modulation to the PWM helps reduce the CMV. The technique of simultaneous pulse width and amplitude modulation of space vectors is proposed in this paper to reduce the CMV and its destructive effects in drive systems. The proposed technique is based on the elimination of zero vectors and the inherent reduction of DC link voltage by amplitude modulation leading to a further reduction of the CMV; The obtained results of applying the proposed strategy to a three-phase back-to-back NPC converter with 738-watt steady-state operating point power showed the system’s sufficient behavior with the efficiency of 98.62 percent. Finally, the transient performance of the converter from no-load to full-load condition ensures its sufficient behavior for industrial applications.