Study On Control Strategy Of DC Grid Based Voltage Source Converter

Flexible DC transmission technology is a technology based on Voltage Source Converter(VSC).Different from the traditional DC transmission technology based on thyristors,flexible DC transmission technology can achieve self-commutation and control active power and reactive power independently,with a greater freedom degree in control.It can be connected with weak AC system or even passive system.Besides,without the requirement of large-capacity reactive power compensation equipment,it has smaller size and more excellent performance,which has been widely used in many fields,such as asynchronous AC system interconnection,renewable energy consumption,urban distribution network remolding and passive system feeding,etc.In addition,flexible DC transmission technology is very conducive to constitute a multi-terminal DC transmission system,which can greatly improve system flexibility and reliability.Multi-terminal DC transmission system is the early form of the DC grid,and some projects have been completed and put into operation.The DC Grid based on VSC technology is a new technology.Various forms of energy and load are connected to the DC bus to exchange power through the converter.It can give full play to the advantages of various forms of energy in a wide area and wide range.The output fluctuation of renewable energy can be stabilized through the DC Grid and realize the reliable integration of large-scale renewable energy.The development of flexible DC Grid is of great significance for improving the utilization of renewable energy and the operational stability of the power system.The safe and stable operation of flexible DC Grid requires a reliable control strategy.This paper studies the control strategy of flexible DC grid based on VSC.The main contents of the dissertation are as follows:(1)The development history and technical advantages of DC transmission are described.The typical DC transmission projects in the world are introduced.Different principles and application range of traditional DC transmission and flexible DC transmission technologies are compared.The technical problems,application scope and development trend of flexible DC Grid are analyzed.(2)The operating characteristics of VSC are analyzed.The topology,working principle and control strategies of VSC are introduced.The flexible DC Grid technology based on VSC is analyzed,the parameter design method of flexible DC transmission system is introduced.(3)The system-level control strategies of two-terminal DC transmission system,multi-terminal DC transmission system and DC Grid are introduced,including master-slave control,voltage margin control and voltage droop control.Aiming at the shortcomings of master-slave control and voltage droop control,an improved droop control with good steady and transient performance is proposed.(4)The electrical model and control model of a flexible DC power grid based on VSC are built.Various control strategies are compared through simulation,and the effectiveness of the proposed control strategy is verified.