Fault Characteristics Analysis And Fault Current Calculation Method Of DC Lines In MMC-MTDC System

Modular multilevel converter based multi-terminal high voltage direct current(MMC-MTDC)transimission technology is the foundation of building a flexible DC grid,the probability of occurrence of DC line faults is higher compared with cables when overhead lines are used for transmission.In the mean time,the low damping characteristics of the MMC-MTDC system lead to rapid development of the faults,which will damage electronic devices in the system and affect the safe and stable operation of the system.Therefore,the analytic calculation method of DC line fault characteristics and fault current of MMC-MTDC system has great theoretical and engineering guidance significance for the suppression of fault current.and protection design.Firstly,based on modular multilevel converter(MMC)topology,basic working principle and basic control strategy,this thesis applies the controller suitable for unbalanced grid voltage,the third harmonic modulation technology and the quick sorting algorithm of capacitor voltage to improve performance.In the same time the MMC bridge arm is simplified according to Thevenin equivalent.Based on the system-level control strategy of DC voltage deviation control,the electromagnetic transient simulation model of the four-terminal MMC-MTDC system based on the half-bridge MMC is established in the PSCAD/EMTDC with Zhangbei engineering parameters,which established the model basis for subsequent analysis.Secondly,in the case of a DC side fault of a single-ended system,the internal energy exchange process is studied and the fault mechanism is analyzed.The DC side fault current analysis method is proposed and its effectiveness is verified by simulation.At the same time,a pole-to-pole fault of the DC line of the four-terminal MMC-MTDC system is taken as an example,the theoretical analysis of the DC line fault characteristics of the MMC-MTDC system are proceeded.Starting from the discharge circuit of each converter station to the fault point,the complex coupling relationship between the converter stations in the MMC-MTDC system is revealed and the cause of the coupling relationship is analyzed.The essence of the fault current is pointed out,which laies the foundation of DC line fault current calautation in MMC-MTDC system.Thirdly,based on the nature of the fault current and its influencing factors,the two ends of the fault line near the valve side are regarded as two-ports,the relationship between the fault current and the voltage on both sides of the two-ports is analyzed,and the theoretical basis of the fault current calculation method in MMC-MTDC system is obtained.Then based on the idea that the voltage characteristics of the two-ports do not change much,the discharge effect of the converter station in the MMC-MTDC system is equivalent by the topology reduction method of the two ends equivalent and the topology split.Thus,the analytical calculation method of DC line pole-to-pole fault current is obtained and its effectiveness is verified by comparison with simulation results.Finally,based on the theoretical basis of the fault current calculation method in MMC-MTDC system,the DC line pole-to-ground fault in the MMC-MTDC system is studied,it is pointed out that there is still a coupling relationship between the systems on both sides of the fault point after the topology is simplified.Through analysis of the coupling relationship and the corresponding decoupling method,the analytical calculation method of the DC line pole-to-ground fault current is obtained.The comparisons between the calculation results and the simulation results prove the feasibility of the proposed method.