| Compared with traditional transmission technology,flexible DC transmission technology has more flexible operation modes and better system controllability.In view of the obvious advantages of the Modular Multilevel Converter(MMC)topology,the High Voltage Direct Current(HVDC)system began to adopt the MMC topology,thus forming the MMC-HVDC system.The MMC-HVDC system combines the advantages of the modular multilevel topology and DC power transmission system.It is suitable for high-voltage and high-power applications.It also has broad application prospects in the fields of new energy grid connection,grid interconnection,island power supply,and multi-terminal DC transmission.The analysis and study of MMC-HVDC system is of great significance for the construction of future power grids.And the fault analysis of this system is also an important research content.The research on MMC-HVDC system internal faults and AC side faults has been relatively rich and mature.However,the DC side faults,which are the common faults in the system operation,need to be further deepened.This article will carry out the following research for the DC side faults of MMC-HVDC system.(1)The topological structure of the MMC is analyzed and related mathematical models are established.The basic working principle of the MMC is explained,and the basic control strategies of the MMC are introduced from three aspects: the modulation strategy,voltage equalization control and circulating current suppression.Based on this,the working principle of the MMC-HVDC system is introduced,and the control strategies of the MMC-HVDC system are explained from three levels: the system level,converter station level,and converter valve level.(2)Analyze the fault mechanisms of three kinds of faults on the DC side of the MMCHVDC system.Establish the corresponding mathematical model,analyze the characteristics of the fault current,and propose an analytical formula for the fault current.Among them,taking bipolar short-circuit fault as the research focus,the current characteristics before and after the fault blocking are analyzed.And the analytical formula of the bridge arm current after the converter station blocking is given.The accuracy of the theoretical research is verified using the PSCAD / EMTDC simulation platform.(3)Taking Zhangbei four-terminal flexible DC transmission project as an example,the DC-side fault of the multi-terminal MMC-HVDC system is analyzed.And an analytical calculation method of the DC-side fault currents of the multi-terminal flexible DC system is proposed.First,an equivalent circuit network of a multi-terminal DC system is established.Second,the pole to pole fault and unipolar grounding fault on the DC side are analyzed.The circuit network after the faults are established,and the corresponding pre-fault matrices and fault matrices are obtained to solve the branch currents.Then,another idea of solving the fault currents in the frequency domain is proposed.Finally,the theoretical derivation is verified by a simulation platform.(4)The transient characteristics of the DC-side fault current are affected by a variety of factors.The effects of the fault location,electrical parameters,and AC system feed on the fault current are analyzed and discussed.When analyzing the impact of the AC side feed current on the fault current,the equivalent discharge circuit proposed in Chapter 3 is improved and a correction factor is introduced.The improved fault current analytical expression is obtained.The simulation platform is used to verify the correctness of the theoretical analysis. |
PDF Full Text Request