Research On Fault Ride-Through Control Method Of Multi-Terminal Flexible HVDC Transmission System

When a multi-terminal flexible DC transmission system based on a modular multilevel converter is connected to the grid,If the system fails,it will prevent the normal transmission of power and thus threaten its safety,so how to ensure its co ntinued operation in the event of a system failure has become a hot topic of research today.The thesis investigates the fault ride-through control method for flexible DC transmission systems from the perspective of ensuring the safety of system operation.The main research elements are as follows :Firstly,The topologies of two-level voltage source converters and modular multilevel converters are presented and their advantages,disadvantages and application scenarios are compared and analysed.The working principle of modular multilevel converters is briefly described,and the grid connection methods of mul ti-terminal flexible DC transmission systems are studied and compared with the characteristics of multi-terminal flexible DC transmission systems.Secondly,the mathematical model of single-ended MMC is analysed based on the working principle and operating characteristics of MMC,and the mathematical model of multi-ended MMC-MTDC system is further derived.In addition,the control strategy of the flexible DC transmission system is described,and the inner and outer loop control of the converter station is i llustrated and the corresponding controller is designed.The coordinated control method of the MMC-MTDC system has also been investigated,taking into account the flexibility of power transmission.Then,.a fault current suppression method based on the unscented Kalman filtering algorithm is proposed for symmetrical faults occurring on the AC side of the flexible DC transmission system.By analyzing the effect of the equivalent model of the MMC-MTDC system on the short-circuit current under symmetrical faul t,it can be seen that when the converter station adopts constant reactive power control and sets the reactive power to zero,the q-axis current of the converter station remains zero during the fault.The unscented Kalman filtering algorithm is used to tra ck the d-axis fault currents,dividing the tracked currents into limit currents and uncontrolled cu rrents,and differentiating the uncontrolled currents from the d-axis reference currents to suppress the fault currents.The simulation results show that the control method can effectively suppress the fault current and ensure the continued operation of the co nverter station during the fault.Finally,a control method combining negative sequence current suppression and DC voltage fluctuation suppression is inv estigated for asymmetric faults occurring on the AC side of flexible DC transmission systems.By deriving the AC side expression of the MMC under asymmetric faults,the corresponding negative sequence current controller is designed to achieve the suppressi on of negative sequence currents.Since the MMC bridge arm voltage fluctuates under asymmetric faults,a mathematical e xpression for the MMC DC voltage fluctuation is derived.A active disturbance rejection controller has been introduced to control the DC voltage in response to the diametrical pulsations generated by asymmetric faults,Effective suppression of DC voltage fluctuations is achieved by adding the DC voltage diphasic component to the extended state observer of the active disturbance rejection controller.