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Low-Computation-Cost and Low-Complexity Pulse-Width Modulation for Multilevel Inverters
Compared with two-level inverters, multilevel inverters have some distinct advantages including stepped output waveforms with lower harmonic distortions, reduced switch stresses, lower instantaneous rate of voltage change, lower switching frequency, etc. Therefore, multilevel inverters have been used in various industrial applications, such as photovoltaic systems, wind power systems, electric vehicle chargers, industrial motor drives, etc. The performance of multilevel inverters, such as voltage balance of DC-link capacitors, efficiency, common mode voltage reduction, reliability, harmonics, etc., is highly dependent on the pulse-width modulation (PWM) methods. Therefore, the proposed research focuses on developing advanced PWM methods for multilevel inverters to optimize their performance in various applications. This work presents a new simple zero-sequence voltage injection method for the carrier-based PWM (CBPWM) of the three-level neutral-point-clamped (NPC) inverter. The injected zero-sequence voltage signal is simply determined by comparing the three reference voltage signals and the DC-link capacitor voltages, respectively. Theoretical analyses show that the CBPWM with the proposed method can balance the DC-link capacitor voltages of the NPC inverter and eliminate/reduce their ripples. Compared to existing CBPWM schemes, the proposed CBPWM has lower computation cost and does not need current information. This work also presents a new universal space-vector PWM (SVPWM) scheme with invariant computation cost for multilevel inverters. In the proposed SVPWM, the modulation triangle in Sector 1 is quickly identified in a 120o coordinate system transformed from the α-β coordinate system. Then, the switching states and duty cycles of the three vertices of the modulation triangle are determined by simple computations. The switching states in the other five sectors are obtained according to their relationships with those in Sector 1. The proposed SVPWM is independent of the voltage level of the inverter and does not require any prestored lookup table or iterative operation. Therefore, neither the execution time nor the memory usage of the proposed SVPWM increases as the inverter level increases. Moreover, the proposed SVPWM is universal and can be applied to any multilevel inverters with the PWM controls based on space vector diagrams. Both the proposed CBPWM and SVPWM are validated experimentally in comparison with existing PWM methods.
Chen, Fa, "Low-Computation-Cost and Low-Complexity Pulse-Width Modulation for Multilevel Inverters" (2020). ETD collection for University of Nebraska - Lincoln. AAI27956689.