Advanced Pulse Width Modulation For Modern Inverters

As the key equipment of clean energy conversion,modern inverter needs to perform both rectifier and inverter functions.This technology is crucial for achieving the strategic goal of "Carbon Neutrality" and has been widely used in different industrial applications such as electric vehicle charging,renewable energy resources power generation,and motor drives,etc.The performance of modern inverters will directly affect the reliability and efficiency of the whole system.However,traditional modulation strategies for inverters become increasingly hard to meet the performance requirements nowadays of high precision,high efficiency and high reliability.Therefore,the research will have great theoretical significance and practical value in implementing advanced modulation strategies for improving the performance and cost effectiveness of modern inverter without adding any extra devices and hardware costs.In this thesis,four advanced modulation strategies are presented and implemented to address the issues of efficiency enhancement,harmonic optimization,dynamic characteristics improvement and reliability enhancement in modern inverters.This thesis first summarizes and analyzes the commonly-used modulation methods and their corresponding implementation principles,and obtains the essential connection and difference between different implementations;secondly,the evaluation models of different modulation strategies in terms of voltage linear modulation range,switching loss,current ripple and common mode voltage are built,which lay the theoretical foundation for the later advance modulation researches;finally,four advanced modulation strategies are proposed for three typical circuit topologies including three-phase two-level inverter topology circuit,single-phase two-level inverter topology circuit,and three-phase Boost-Buck type two-level inverter topology circuit,which are theoretically analyzed,verified and compared with the traditional methods.The main contributions and innovations of this thesis are listed as follows:(1)A generalized tri-state PWM strategy with reduced common-mode voltage for three-phase inverters is proposed to address the issue of deteriorated switching loss and current distortion caused by conventional reduced common-mode voltage PWM methods,which is realized by the bipolar carrier PWM method in practice.This strategy is able to adapt to the change in power factor and ensures that the common-mode voltage is suppressed while changing the modulation waveform to clamp to the phase with the maximum current.Compared with other reduced common-mode voltage PWM strategies,this strategy can reduce 50% of switching losses,thus having the feature of minimum switching losses.Meanwhile,this strategy has superior input and output current performance,which can reduce the current harmonic distortion by 60% at low modulation index.(2)A carrier-based PWM strategy based on model predictive control is proposed to deal with the problem of large harmonics in conventional finite set model predictive control method.This strategy can simplify the implementation by applying the error between the reference duty cycle and the duty cycle to be executed at the next cycle in the cost function,and generate the corresponding modulation waveform based on the selected switching states.The magnitude and frequency of reference current can be tracked well in both steady-state and transient-state,which allows to improve50% of current harmonic distortion without compromising the dynamic performance of the predictive control.(3)A generalized discontinuous PWM strategy is proposed to enhance the power conversion efficiency of the single-phase inverter with active power decoupling circuit.This strategy can self-adaptively adjust the clamping position and duration of the switching signal based on the AC current measurements and the reference voltage,so that the appropriate phase with maximum instantaneous current is always clamped to reduce the 50% of switching losses.In contrast to other DPWM strategies,this strategy can further reduce the switching losses and achieve the performance of minimum switching loss.(4)A 2/3 discontinuous PWM strategy is proposed to address the problem of large switching losses and low reliability of the two-stage Boost-Buck type inverters.The modulation reference of the front-end stage can be generated by zero-sequence voltage injection,which makes two-phase of the front-stage AC-DC converter stop switching at any instance to reduce87.5% of switching losses.Meanwhile,the back-end DC-DC converter is operating in soft switching mode by variable switching frequency control to further reduce the system losses.Via a synergetic control strategy,the frontand back-stage converters can greatly reduce the size of the DC-link capacitance,thus improving the reliability and power density of the system.All of the above advanced modulation strategies are validated in both simulations and experiments,and are compared with conventional modulation methods,which proves the effectiveness and superiority of the proposed strategies.