Study Of HVDC-Flexible System Based On MMC

Voltage Sourced Converter based High Voltage Direct Current technology is called "HVDC-Flexible" technology in China. Based on turn-off devices and PWM technology, HVDC-Flexible technology can be very effective to solve the new energy into the grid, remote isolated power supply, asynchronous interconnection of power grids and other issues, compared with the two level structure, based on a modular multilevel inverter(MMC) HVDC-Flexible is design in modular, very easy to expand and switching loss is low, So it is more suitable for high volt age direct current transmission. In this paper, with the two ends MMC-HVDC system as subjects, studied on the basic working principle of the MMC-HVDC, design of the system parameter and control strategy etc.This paper introduces several kinds of topological structure of the HVDC-Flexible, analyzed the development status and prospect of HVDC-Flexible at home and abroad, summarized the research status on control strategy of MMC-HVDC, including modulation strategy, suppression strategy of circulation, control strategy of voltage balance and the control strategy of unbalanced grid voltage conditions.Analyzed the basic structure and working principle of the MMC. Analyzed the basic structure and working characteristics of the MMC-HVDC system, analyzes and establishes the low frequency dynamic mathematical model of the MMC-HVDC system. Analysis of the importance of system parameters design of the bridge arm reactor and sub module capacitor selection, introduced the method of choosing a bridge arm inductor parameter. Through the analysis of the relation between bridge arms instantaneous energy and bridge arm sub module voltage fluctuation, put forward a selection method of the bridge arm sub module capacitance.Briefly introduces the commonly used MMC-HVDC carrier phase shifting modulation strategy and the nearest level modulation strategy. According to the derived sub module voltage fluctuation component expression, combin ed with the nearest level modulation strategy, a detailed description of the causes of bridge arm circulation. And obtain distribution of the three-phase bridge arm circulation in normal condition and fault condition respectively. Put forward a two harmonic circulation suppression strategy of control strategy based on modify the module input formula, the two harmonic circulation seriously restricted. Finally, research on voltage balance control strategy of MMC-HVDC sub module of the bridge arm, put forward an optimized for simultaneous voltage balance control as far as possible to reduce the switching loss of power device based on the strategy of voltage balance in traditional control.Put forward a start-up control strategy of the two ends MMC-HVDC system.Studied the design of controller of outer loop and current loop controller of the system. Study on the power transmission characteristics of MMC-HVDC system under AC system fault conditions and the implementation method of the related control objectives. Analysis of the fault in AC system can also realize the two control objectives: to suppress the negative sequence current AC side and to suppress the DC side voltage and current fluctuation.Finally, set up a two ends of the MMC-HVDC system simulation model in the PSCAD/EMTDC simulation environment, the control strategy proposed in this paper has carried on the simulation analysis, the simulation results verify the theoretical analysis in this paper, and the correctness and validity of the proposed control strategy.