Research On Direct-Connected Converter Topology And Its Control Stategy Of Co-Phase Power Supply Traction Substation

Single-phase AC electrified railway using industrial frequency has become the main trend in the field of electric traction today because of its advantages in applicability, superiority and sustainability. The characteristics of single phase, nonlinear and random fluctuation of electric locomotive make the traction power supply system have many problems, such as negative sequence, harmonics, reactive power, "phase splits", communication interference, equipment capacity utilization and so on. On the whole, the main problems of the current electrified railway are mainly manifested in two aspects. One is the negative sequence which is the main power quality problems, and the other is phase splits. Co-phase power supply system with active compensation as an ideal power supply scheme, real time and strong flexibility, solves the electric energy quality and phase splits problem effectively. However, limited by the power electronic devices of unit's voltage level and the capacity, the active compensation converter often need to with a large number of matching transformer and multiple-converter technique to meet the requirements of the voltage level and the capacity. Namely, there is no direct-connected converter. The matching transformer has the function of increasing the current and reducing the voltage, it has no contribution to lower the compensator capacity. Which not only increase the investment in transformers and system loss, and bring the equipment covers area with the wasted space and heat dissipation problems, bring a lot of inconvenience to the project implementation, especially the space limit of the transformation of existing line is more obvious. Therefore, it is of great significance to carry out the research of active compensation direct-connected converter for co-phase power supply systems.The structure and the performance characteristics of the different types Sub-module in modular multilevel converter (i.e., MMC) are analyzed in this paper. And the MMC topologies of direct-connected converter based on half bridge and full bridge sub-module and cascaded H-bridge are proposed, which are suitable for all the traction substation connection modes of 120 and 90 degree. The different topologies in variable voltage, variable frequency, the coupling form, bridge arm connection types are compared. It is pointed out that the respective applicable occasions. In addition, how to select the numbers of sub-module and the design principles of the parameters of capacitor and inductor are also presented.Although the voltage level and power capacity are solved in the above section, the cost investment brought by the active compensation capacity cannot be ignored. So, the compensation degree, including negative sequence, reactive power and harmonic, is introduced into direct-connected converter, and the relationship between compensation capacity of the active power and reactive power compensators and the compensation degree is studied. It is pointed out that the satisfaction degree compensation can be implemented according to the requirements of the national standard on the power quality. In fact, different topology in the form of direct-connected converters, the capacities and numbers of IGBT device are different even though their compensation capacities are same. The compensation capacity utilization of IGBT device is discussed, and the optimization scheme of compensation capacity utilization based on IGBT is proposed. Moreover, the associated compensation capacity is closely related to traction substation connection modes, as well as the traction ports and load properties. The matching power capacity for direct-connected converters is discussed in different transformer connection modes. Additionally, based on the single phase combined co-phase power supply systems, a scheme of multiple phases combined power supply for urban rail transit is discussed. And the optimization method of active compensation capacity of all kinds of loads is presented. In the last part, according to the wiring forms of the traction substation, converters are classified into two categories: two-port type and three-terminal type. And their compensation characteristics and matching relations of capacity optimization are discussed respectively.The control strategies of direct-connected converters of the co-phase power supply systems are explored in this paper. The relevant research is divided into three aspects. Firstly, NLM (Nearest Level Modulation) and capacitor voltage sorting are discussed, and then the grouping and sorting method is presented in order to reduce the algorithm computational complexity. A voltage balancing control strategy for hybrid connection MMC based on bipolar power sub-module (BISM) is investigated as well. Secondly, to reduce the numbers of PI (Proportional Integral) controller, to optimize the structure of control systems and to lower the frequency of data accessing and processing, a method combined estimation of stored energy with CPS-SPWM (Carrier Phase-Shifted Sinusoidal Pulse Width Modulation) is put forward, which is also appropriate for a broad range of traction loads. And thirdly, in view of the undesirable effect of circulating currents among the bridge arms, a suppression strategy of circulating current is suggested based on estimation of dc circulating current and PR (Proportion Resonant) controller.At last, the simulation system of the co-phase traction power supply substations based on direct-connected converters is set up in Matlab/Simulink. And analvsis results are given in detail according to three types of converters of full power, active power and reactive Dower. which validate the correctness and effectiveness of the proposed methods.