Research On Operating Overvoltage Of Multi-terminal Ultra HVDC System

Compared with traditional end-to-end UHVDC transmission,multi-terminal UHVDC transmission uses multiple distributed terminals for power collection and multiple distributed receivers to absorb power,which solves the problems of large fault impact and difficult receiver absorption under high power.Therefore,multi-terminal DC transmission is a flexible,economical and reliable transmission method.Besides,in the process of building the energy Internet,the use of multi terminal DC transmission can significantly reduce the construction and operation costs of transmission lines,which is an important basis for the development of the global energy Internet.At present,the research on over-voltage of UHVDC System in China mainly focuses on the traditional two-end DC systems.The research on multi-terminal DC systems mainly focuses on coordinated control and protection strategies.Therefore,the research on overvoltage of multi-terminal UHVDC system is not enough.However,overvoltage research is the basis of insulation coordination,and the interaction between converter stations may change the overvoltage mechanism in multi-terminal DC system.It is of great significance to analyze the over-voltage of multi-terminal DC system in detail.This paper deeply researches the topology and control method of multi-terminal UHVDC system.In the electromagnetic transient simulation software PSCAD / EMTDC,a ± 800 kV four-terminal UHVDC system was established as the main research object,while a two-terminal system and two three-terminal systems(two different structures)of the same voltage level were built as the contrast.In these four systems,the typical AC side,neutral line and DC field operating overvoltage are simulated.By analyzing the mechanism of the fault,the most serious working condition of the fault is found out.The differences and similarities of overvoltage results among the four systems are compared,which shows that there are great differences between multi-terminal UHVDC system and two-terminal UHVDC system in some faults.