Research On Inline MMC-RPC And Electrified Railway Power Quality Control Technology

The main power supply method for electrified railways is single-phase power frequency AC for power supply,which is mainly due to its practicality,superiority,and sustainability.However,due to its single-phase,non-linear,and random fluctuation characteristics As a result,there are many problems such as negative sequence,harmonics,and reactive power in the traction power supply system.As a new type of energy fusion device,the railway power regulator has been proposed by Japanese scholars to realize the governance of the power quality of electrified railways.It has the characteristics of dynamic and flexible,and can effectively solve the power quality problem in traction power supply systems.However,due to the insufficient overall voltage level and compensation capacity of current railway power conditioners,a large number of matching transformers and multiplexing technologies of power electronic devices are required to enable railway power conditioners to be directly connected to the traction network without direct-connected converters.The matching transformer is usually a step-down transformer,which reduces the primary voltage to reach the rated voltage level of the power electronic device,but increases the current on the secondary side and increases the dynamic and static losses of the power electronic device during operation.Auxiliary facilities such as step-down transformers have caused problems such as increased investment,large equipment footprint,and heat dissipation,making the implementation of the project more difficult,especially the need to replan the site when retrofitting existing lines.Therefore,it is of great significance to carry out research on in-line railway power conditioners.The main circuit structure and sub-module working principle of the in-line MMCRPC are established and analyzed.Due to the symmetry of the MMC,a single-phase MMC is used for equivalent controlled source analysis.The analysis shows that the single-phase MMC has four-quadrant operating characteristics.The single-phase MMC was analyzed with a mathematical model of the circulating current,which provided a theoretical basis for the subsequent circulating current suppression strategy.At the same time,the system capacitor and bridge arm inductance parameters were designed based on the suppression of sub-module capacitor voltage fluctuations and the size of the circulating current.Research on control strategy of in-line MMC-RPC converter is carried out.Combining multi-level carrier phase-shift modulation with the sub-module capacitor voltage sequencing algorithm to form a new carrier-phase modulation strategy,which enables power electronics to have a fixed switching frequency,reducing the difficulty of controlling the sub-module capacitor voltage balance.It has certain practical engineering significance.Combining the results of circulating current analysis,a PR control strategy based on DC component prediction is proposed,and simulations verify that the second and fourth frequency doubling components in the circulating current have a good suppression effect.From the perspective of current and power,the principle of compensation instruction detection is analyzed,and the compensation target is provided for in-line MMC-RPC.Finally,taking MMC-RPC as the research object,two kinds of non-linear control theories are used to verify that MMC-RPC is non-linear.Based on this,a non-linear controller is designed and the stability of the system is verified using Lyapunov’s second law Sex.Compared with the traditional dual-PI control,the system has the advantages of high stability,rapid compensation,and fewer adjustable parameters.Matlab / Simulink has verified through simulation that the proposed nonlinear controller is stable,strong,and stable under dynamic interference.Downsizing and other advantages.