Research And Implementation Of PWM Modulation Technology In Medium Voltage DC Electric Propulsion System

Compared with the previous two levels converter,the converter of medium voltage dc electric propulsion system uses the multilevel converter structure with broad application prospects because of the unique advantages.Due to the modular structure of the cascaded H bridge converter,the general power device with low voltage can realize the high output voltage.Therefore,this kind of converter is more suitable for high power applications.The PWM modulation technology of cascaded H bridge converter has a direct impact on the overall performance of the propulsion system.As one of the traditional modulation techniques of cascade H bridge converter,PS-PWM technique has the worse harmonic performance of output voltage.And for the CD-PWM technique,it has better harmonic performance of output voltage but files to distribute power uniformly between power units and make the utilization of the switching devices identical in a power unit.Although some scholars have presented a hybrid PWM technique to uniform distribution of the power and improve harmonic performance of output voltage,the overall effect is not very satisfactory when the number of levels of the converter is changed.Besides,the DC voltage utilization of mentioned modulation techniques also remains to be further improved.According to the above problem,this paper adopted a novel hybrid PWM modulation technique.The modulation technique provides theoretical guidance and reference for the application of cascaded H-bridge multi-level converter of medium voltage dc electric propulsion system to achieve better voltage quality and motor control system.And the research work includes the following aspects:1.The structural characteristics of the minimum unit H bridge composed of cascaded H bridge multilevel converter and the working principle of single H bridge unit are studied.Furthermore,the structural characteristics of the cascaded H bridge multilevel converter are derived,and the characteristics of the output voltage are studied2.In this paper,the principle and characteristics of the traditional multilevel SPWM modulation technique are studied,and the traditional modulation technique is improved to get a new hybrid PWM technique.The modulation characteristics and modulation principle of the novel hybrid PWM technique are studied.According to the characteristics of carrier phase shifting same as thePS-PWM,the harmonic distribution characteristics of the output voltage are studied theoretically.3.Through simulation experiments,we studied the modulation performance of the novel hybrid PWM from two aspects of modulation ratio and carrier ratio respectively.By analyzing the different modulation techniques in the five levels and seven levels converter,under different modulation ratio of line voltage and line current harmonic performance and amplitude parameters such as size,validation of the proposed modulation technique has good performance.4.The novel hybrid PWM modulation technique was applied to permanent magnet synchronous motor propulsion control system.The performance of the control characteristics is studied.Through the study of the mathematical modeling of permanent magnet synchronous motor and the paddle load,a double-loop control system of motor is designed.In cascade multilevel simulation experiment of permanent magnet synchronous motor propulsion control system using the novel hybrid PWM modulation,the simulation results validate the modulation technology in the application feasibility and the accuracy of motor control system.