Feed-Forward Control for Active Voltage Balancing in Electric Drives with Five-Level NPC Converters

This paper presents a feed-forward-based control system for actively balancing the dc link voltages of an electric drive with five-level neutral-point-clamped (NPC) converters. The dc link capacitor voltages are balanced through a bidirectional dc/dc converter. The space-vector-modulation technique of the NPC converters contributes also to the voltage balancing in some operating conditions. This paper presents a feed-forward-based control system for the dc/dc converter that offers robust performance with low steady state error between the dc link capacitor voltages. The control system consists of voltage and current controllers for regulating the dc link voltages and currents of back-to-back-connected three-phase five-level NPC converters. This paper also describes the methodology to estimate the currents at the dc side of five-level three-phase NPC converters, since measurements of the dc link currents are necessary for controlling the dc/dc converter. The structure of the proposed control technique is presented and a 50kW system is simulated on Matlab/Simulink and validated experimentally with a low scaled prototype system.