Research On Railway Static Power Conditioner Based On Modular Multilevel Converter

Railway’s traction power supply system in China adopts the mode of power frequency 25 k V single phase AC power supply, which is a kind of asymmetrical power supply mode, with the development of high speed and heavy load of railway in our country, railway power supply’s power quality, such as negative sequence, reactive power, and harmonic became more and more prominent. Railway Static Power Conditioner(RPC) is one of the most outstanding solution in many power management. However, the traditional RPC was constructed with two / three level structure of VSC, which had the weaknesses of low voltage level, limited compensation capacity and device dynamic / static pressure. In order to overcome the disadvantage of traditional RPC, this paper have constructed a Railway Static Power Conditioner that was made up of two single-phase back-to-back modules multilevel converter(MMC)(MMC-RPC).Making the MMC-RPC directly access to traction network, which can achieve all the functions of traditional RPC and advantages of MMC topology. The strategy have eliminated the step-down transformer, improved the DC voltage, and reduced the DC current. This paper takes MMC-RPC as the research direction, the main work and innovative points of this dissertation are as follows:1) Establish the mathematical model of MMC-RPC. The structure of V/v traction transformer and Scott traction transformer have been introduced firstly. And two kinds of traction transformers’ current compensation and power compensation instructions have been calculated. Then the basic topology of RPC was introduced. And the basic working principle of RPC was analyzed by simplifying the equivalent circuit model of RPC. Further the work characteristic of the sub module topology have been discussed, and the related mathematical model of the single-phase MMC have been established. On the basis of this analysis, a new type of RPC with MMC topology was adopted and MMC-RPC’s mathematical model was establish.2) The double closed loop control strategy of MMC-RPC have been proposed. After constructing orthogonal virtual components by Second Order Generalized Integrator, double closed loop controller was designed in rotating coordinate system, which regulated the MMC-RPC to achieve negative sequence control and reactive power compensation of traction transformer.3) MMC-RPC’s direct power control strategy have been proposed. The mathematical model of the voltage and power of single phase system have been analyzed. A direct power control(DPC) without angular frequency and inductance parameters have been designed. And the DC voltage stability strategy have been designed under the direct power control strategy, which realized the accurate and fast decoupling control of power, comprehensive control of negative sequence, harmonics and reactive power of traction transformer.4) The PIR control strategy of two phase three wire MMC-RPC in time domain coordinate system have been proposed. The accurate control of AC compensation current was realized by using proportional integral resonant(PIR) controller. The control strategy of the MMC bridge arm loop with resonant control was also designed, which have improved the quality of current tracking.5) Frequency adaptive PIR control strategy for MMC-RPC have been designed in the Park coordinate system. The mathematical model of single phase MMC have been established after constructing virtual orthogonal components. The analysis of the harmonic current characteristics was in the synchronous rotating coordinate system, which have pointed out that the single-phase harmonic current in the dq coordinate system was a multiple of 4. According to this, the proportional integral resonant control strategy have been proposed, which required less number of resonant controllers. And considering the frequency offset of the power supply network, the frequency adaptive power splitter and the frequency adaptive resonance controller were designed, improved the ability of frequency offset of control strategy. The control strategy can realize the comprehensive compensation of traction transformer in changed grid frequency and complex situation of locomotive load mutation.6) The circulation of MMC-RPC have been analyzed quantitatively and the suppression strategy have been proposed. Due to the characteristics of the distributed capacitance structure in MMC, it had the circulation problem。At the same time, it was different from DC power transmission that MMC-RPC need to drive the odd harmonic current to the traction power system. The harmonic current through the bridge arm would make the component of the arm circulation to become more complex. Through the analysis of the compensation principle of traction power supply system, the mathematical model of single phase MMC have been established. Combined with the characteristics of MMC-RPC compensation current, the mechanism of the circulation have been analyzed, and the existence of the MMC-RPC in the presence of an even frequency multiplier was obtained. Besides, the current would flow into the DC side, which can cause the fluctuation of DC current and voltage. And on the basis of theoretical analysis, proportional integral complex integral controller(PI-CI) was used to design a kind of loop current controller which was suitable for the single phase MMC-RPC system.