Pulse Width Modulation And Model Predictive Control Of Cascaded H-bridges Converters

Being widely used in industrial applications of Traction Drive System,High Voltage Direct Current Transmission(HVDC),Uninterruptible Power Supply(UPS),Power Electronics Transformer(PET),wind energy system,and photovoltaics,the topology,modulation scheme,and control algorithm of converters have emerged as attractive topics in power electronics.Among conventional multilevel converters(MCs),the topology of cascaded H-bridges(CHB)is widely used because of its high modularity,excellent scalability,low dv/dt,and minimal power electronics switching devices required with the same level number of output voltages.Taking CHB converters as the subject,the research on Space Vector Pulse Width Modulation(SVPWM)schemes and Model Predictive Control(MPC)algorithms of CHB converters is done.By analyzing the topology and operation principles of a single-phase CHB converter,and using a method which is similar to operation principles of Phase-Shift Carrier-Based PWM(PS-CBPWM),the single-phase multilevel Space Vector PWM(SVPWM)is simplified to single-phase two-level SVPWM through obtaining dwelling time of voltage vectors based on principles of single-phase two-level SVPWM.After a Single-phase seven-level SVPWM is realized,the single-phase nine-level and eleven-level SVPWM are obtained by easily modifying some parameters.The feasibility of the presented single-phase multilevel SVPWM scheme is validated by a Hardware-in-loop(HIL)experimental platform.Adopting the similar method to a three-phase CHB inverter,a simplified three-phase multilevel SVPWM scheme is presented.After analyzing operation principles of a three-phase CHB inverter,each H-bridge is regarded as one arm of a three-phase three-level inverter based on the output voltage's waveform corresponding to different switching states.Thus,each three-phase H-bridge unit can be treated as a three-phase three-level inverter,so that each unit can be seperately modulated through three-level SVPWM.When each three-phase H-bridge unit is being modulated by using three-level SVPWM,based on the symmetry of six sectors in the Space Vector Diagram(SVD),dwelling time of voltage vectors and switching time of PWM signals are obtained through the calculation in the 1^st sector.Then,the switching time used for the generation of PWM signals is obtained by mapping the switching time obtained in the 1^st sector to the real sector in which the reference voltage vector locates.With advantages of fast prcessing speed,good hardware stability,and strong scalability,an FPGA-based implementation scheme is carried out.Correctness and performance of the presented three-phase multilevel SVPWM scheme is verified by a three-phase seven-level CHB prototype.In order to eliminate steady-state error,improve dynamic performance,and enhance scalability of conventional PI-based control algorithms,a modular MPC algorithm adopted to a single-phase CHB rectifier is presented by treating each H-bridge as an single two-level rectifier.Deviding each sampling period into several intervals and adopting serial calculation mode,the mathematical optimization problem is solved within an interval module by module.For the calculation of the optimal modulation function of each H-bridge,the capacitor voltage balance term is embedded,and the balance ability is adjusted by changing the value of weight factor.In addition,to reduce the sampling frequency of source voltage and current,a compensation scheme is presented to predict future values of source voltage and current,which are used to calculate the optimal modulation function of following H-bridges.In order to verify the performance of the presented algorithm,the comparison between the presented algorithm and transient current control algorithm is carried out.Compared with the transient current control algorithm,the presented algorithm eliminates the steady-state error of source current and achieve deadbeat compensation.In addition,to reduce the system complexity,increase the system capacity using limited I/Os,and expand the application field of the presented modular MPC algorithm,a modular MPC algorithm combined with capacitor voltages estimation adopted to dc-side sensor-less single-phase CHB photovoltaic inverters is presented.The estimated value of the capacitor's voltage for a single H-bridge is obtained through calculating the increment of the CHB inverter's output voltage.The performance of the presented algorithm is validated by comparing with conventional PI-based current control algorithm through an HIL experimental platform.As could be known,the presented algorithm accurately estimates capacitor voltages,while achieveing source current tracking and capacitor voltages balance.