Research On Fault Clearing And Recovery Technology Of LCC-MMC Hybrid HVDC Transmission System

China’s overall power layout shows the trend of west-east power transmission.The future DC transmission is oriented towards extensive and long-distance transmission,and the development and application of high-voltage DC transmission technology has great potential.Compared with the flexible DC transmission system,the hybrid DC transmission system improves its economy and power transmission capacity,and overcomes the shortcomings of traditional HVDC transmission technology which is prone to phase change failure when connecting to weak AC systems and cannot supply power to passive networks.In order to solve the problems that the hybrid DC transmission system cannot be fault self-clearing,blind reclosing and fixed long delay in fault recovery,it is necessary to propose a fault clearing and recovery strategy that predicts the nature of faults,reduces the impact on the system,and improves the reliability of system power supply.This thesis firstly analyzes the adaptability of the Baihetan hybrid cascaded multiterminal HVDC transmission system based on the existing DC transmission system configuration and the DC line fault clearing and recovery strategy without DC circuit breakers,simulates and compares the clearing and recovery effects of different schemes,and determines the scheme of configuring DC circuit breakers at both ends of the DC line at the receiving end of the system.For the identification of fault nature,this thesis analyzes the traveling wave transmission process generated when the DC breaker trips after a fault occurs on the receiving DC line,and proposes an adaptive reclosing method based on the spectral cosine similarity,which can predict the fault nature and identify the arc extinguishing moment by calculating the cosine similarity of the line mode voltage spectrum at both ends of the fault line,and adaptively adjust whether to reclose and the reclosing time to improve the system recovery The success rate of the system is improved.Further,the deep learning method is used to batch simulate the fault sample data,explore the influence of the initialization processing scheme of the input data of the convolutional neural network and the configuration of parameters such as the number of network dimensions on the fault identification effect,and combine with the fast Fourier transform to process the input layer data to obtain a structure optimization algorithm with high accuracy in identifying the nature and type of faults.The method is resistant to fault location,fault type,and transition resistance changes,and has higher recognition accuracy than the fault nature recognition method based on spectral cosine similarity without the process of data synchronization and threshold selection at both ends of the fault line.Finally,a hybrid DC transmission system fault clearing and fast recovery strategy based on hybrid DC circuit breakers is developed.For fault clearing,the clearing method of hybrid DC circuit breaker transfer branch graded opening is proposed,which can reduce the impact on the system in the process of opening the current.The strategy is aimed at DC line faults at the receiving end of the Baihetan project,using a rapid recovery strategy of simultaneously opening circuit breakers at both ends of the faulty line in a graded manner and,after identifying the nature of the fault and waiting for the completion of the de-travelling phase,first reclosing the circuit breaker near the MMC side and then directly reclosing the circuit breaker near the sink bus,and adding the control strategy switching on the MMC side and controlling the commutator PI of the sink bus voltage during the process The controller is set to zero.The simulation compares the six combined strategies and verifies that the proposed strategy has a good effect of reducing the impact of the fault clearing and recovery process and shortening the system recovery time.