| Electrified railway system is the hub of transportation and economic development.Electric locomotive traction transformer is an important part of electrified railway system.However,traditional power frequency traction transformer has large weight and takes up a lot of space,which greatly limited restricts the development of electrified railway with more comfort,higher speed and lighter weight.Therefore,the call for the development of power-free traction transformers has become increasingly strong.Since MMC is a converter with strict modular structure and good output characteristics,in recent years,MMC as a multi-stage converter without power frequency traction transformer has become a research hotspot in the field of traction locomotives at home and abroad.Although the advantages of MMC traction converter are obvious,the trains need to be repeatedly started and braked due to various factors such as entering and leaving the station,climbing,temporary parking,technical parking,etc.,and this uncontrollable start-stop factor will not only cause the traction network voltage to fluctuate greatly.Therefore,the switching times of the MMC traction converter's sub-module are increased,and the switching frequency and loss of the device are increased,thereby shortening the service life of the MMC traction converter,and the failure rate is increased,thereby reducing the safety and reliability of the system operation.Therefore,the use of appropriate redundant fault-tolerant schemes to improve the fault tolerance capability of MMC traction converter has important research significance.In view of how to improve the fault tolerance capability of MMC traction converter,the main research contents of this paper are as follows:(1)The topology,working principle,technical characteristics and overall control objectives of the MMC-based traction drive system are studied.The topology and working principle of the full-bridge MMC converter are analyzed in detail,and its dynamic mathematical model is established.And the circulation generation mechanism is analyzed.Combined with the characteristics of single-phase AC/AC conversion MMC converter,the bridge arm inductance parameters are designed to suppress the bridge arm circulation.At the same time,the common modulation method of MMC is introduced,and CPS-PWM is used as the modulation strategy of MMC traction converter.A CPS-PWM modulation method with ?/N units of phase shift of each arm of MMC traction converter is designed.(2)An MMC fault redundancy fault-tolerant scheme suitable for electric locomotive field is proposed.The locomotive braking energy is used to charge the spare sub-module to realize the real-time replacement of the fault sub-module,and the charging converter for redundant sub-module charging is designed.The topology and working principle of the charging converter are introduced in detail,and the mathematical model is derived and the corresponding control strategy is designed.In addition,the fault detection,fault location and replacement methods of single-phase full-bridge MMC sub-modules are described in detail.Finally,the effectiveness of the charging strategy was verified by PSCAD simulation software.(3)In order to keep the capacitance of the MMC traction converter sub-module constant and equal,a capacitor voltage equalization control strategy is designed,including the average control strategy and the balance control strategy.The effectiveness of the above control strategy is verified by simulation.Finally,the simulation model is established in PSCAD/EMTDC simulation software,and the various redundancy schemes of MMC traction converter are simulated and compared.The simulation results show that compared with other redundant fault-tolerant schemes,the scheme proposed in this paper deals with faults quickly.The redundant sub-modules do not require additional charging time and the transient time is short,which does not affect the system performance,so that MMC traction converter can smoothly complete fault tolerance. |
PDF Full Text Request