Modeling of Dual-PWM Adjustable Speed Drives for Characterizing Input Interharmonics Due to Torque Oscillations

The dual-PWM adjustable speed drives (ASDs) inject interharmonics into the distribution grid, causing low-frequency oscillation (LFO) in the grid. This article investigates the common-modulation process of motor load torque oscillation and how it resulting in interharmonic components in the front end of the back-to-back PWM-based ASDs. Particular emphasis is given to the theoretical analysis of interharmonic-causing frequency aliasing and fundamental root-mean-square (rms) LFO. Besides, based on the Bessel PWM switching function and common-modulation principle, a model of interharmonics generated by the dual-PWM ASDs is established. The proposed model, which takes into account the motor operating conditions and load torque disturbance, predicts the input interharmonic frequency locations and voltage rms LFO pattern with respect to bridge crane dual-PWM ASD systems. Simulation and experimental results on dual-PWM ASDs and distribution grid in the Shanghai Yangshan Deepwater Port area verify the effectiveness and feasibility of the model and the theoretical analysis.