The Research On Protection And Stability Control Of VSC-MTDC Systems

In the future,the Power Grid in China will gradually develop in the direction of the integration of source-network-load-trade and the coordinated of transmission and distribution networks.With the rapid development of Voltage Sourced Converters(VSC),VSC-MTDC technology has attracted widespread attentions in abroad.At the same time,it has been used in some scales recent years.Due to its multi-source power supply and multi-ways to achieve a wide range of energy interconnection.VSC-MTDC is favored by many scholars.In the transmission level,it can promote large-scale renewable energy consumption.Meanwhile,it promote large-scale optimization and complementarity of energy to give reliable asynchronously interconnected power grids in different regions.In the distribution level,it can constitute a multi-terminal flexible DC distribution network with different voltage levels,which provides a good solution for the closed loop operation of the distribution network and further improves the power supply reliability.Therefore,VSC-MTDC technology has gradually been the key technology for building the next generation of smart grid.DC protection and control strategy are the key technologys to ensure the safe and reliable operation of VSC-MTDC systems.In this paper,the VSC-MTDC systems consisting of two-level voltage source converter are taken as the research object.The main works includes the operation control analysis,fault transient analysis,system simulation,protection,fault location and control strategy for stability improvement.The specific work can be summarized as follows:1)The background and significance of the VSC-MTDC systems are analyzed.The advantages and features of the VSC-MTDC systems are expounded.The related topological structure and operation control principle are analyzed,which provides research basis for the subsequent research on the protection and control technology;2)Deriving the different operation control methods and voltage control strategies of VSC-MTDC systems,which to construct precise electromagnetic transient simulation model and to study the operating characteristics under different control strategies.At the same time,the typical fault characteristics and the fault transient process are analyzed;3)Analyzing the protection strategy for VSC-MTDC systems.Meanwhile,we propose a new protection strategy which install dc reactors at both side of the dc lines to form the physical boundary.A fault location method based on fault transient information is proposed for VSC-MTDC systems.Finally,the parameters of the dc reactors are discussed from the perspective of the DC protection operation time and the related device time-limit and the small signal stability of the system.The simulation verifies the effectiveness of the proposed method.4)Considering the stability of the system caused by the dc reactors for the VSC-MTDC systems.The small-signal stability model of the VSC-MTDC systems under different control strategies is constructed hierarchically.The influence of the stability of the VSC-MTDC systems caused by dc reactors,droop coefficient and power delivery are analyzed.Finally,to improve the system stability of the VSC-MTDC systems,an improved droop control strategy is proposed to improve the stability of VSC-MTDC systems.