Characteristics Analysis And Countermeasures Study Of Converter Faults In HVDC Systems

High voltage direct current(HVDC)transmission system has been widely used in long-distance large-capacity power transmission scenarios in China due to its high economy,interconnectivity,and rapid controllability.Converter is the most concerned from the perspective of equipment safety in the HVDC transmission system,and it's also the key equipment that determines the fault ride-through capability of the HVDC transmission system,which makes the control and protection system of the converter area a challenge.This paper focuses on the characteristic analysis and countermeasures study of converter faults of HVDC transmission system.The study involves two aspects: one is fault characteristics analysis and relay setting calculation,which can improve the ability to deal with the short-circuit faults of the converter area and keep equipment safe.The other is the commutation failure suppression and recovery,which helps to improve the ability of HVDC transmission systems to deal with commutation failures and fault ride-through.The main works and achievements of the paper are summarized as follows.Firstly,a method of converter area fault characteristics analysis and relay setting calculation is proposed.All possible factors affecting the electrical quantities at the protection measuring point are listed based on the protection operation principle.Fault characteristics are theoretical analyzed by changing individual factor one by one and fixing other factors.Factors that have an obvious effect on relay setting are filtered out.The most stringent operating condition for setting is quickly determined with a small amount of simulation based on the foregoing analysis results.A reasonable relay setting is determined based on the most stringent operating condition.The detailed process of fault characteristics analysis and relay setting is displayed by taking DC differential protection,the main protection of converter area,as an example which shows the effectiveness of the proposed method.Secondly,a fault time selection method suitable for converter area relay setting verification is proposed.Selecting the fault occurrence time in natural intervals,the transient process of electrical quantities at the protection measuring point is theoretical analyzed from the perspective of protection principle.The periods with consistent changing process of electrical quantities are classified and merged,while the periods during which it is unable to make a clear judgment on changes in electrical quantities are determined.In the merged period,the fault occurrence time is intensively or sparsely selected according to needs,and transient simulation is performed to verify the relay setting.The simulation results show that the method can greatly reduce the simulation workload under the premise of ensuring the verification accuracy.Thirdly,a commutation failure suppression strategy based on superconducting fault current limiter(SFCL)is proposed.Possible installation locations of SFCLs are given according to the influencing factors of commutation failure.Three indicators are proposed to evaluate the suppression effect of SFCLs with different installation locations on the commutation failure and the best installation location for SFCLs is recommended.The theory and simulation derivation methods of the required resistance value when SFCLs are installed in different positions are given.In addition,an improved structure is presented to ensure fast recovery of a SFCL so that the system can be restored quickly when the fault disappears.The simulation results verify the rationality of the selected installation location and verify the correctness of the SFCL resistance setting method.Fourthly,a constant extinction area based control strategy for commutation failure mitigation in hybrid dual-infeed HVDC system is proposed.In a hybrid dual-infeed HVDC transmission system consisting of line commutated converter HVDC(LCC-HVDC)and voltage source converter HVDC(VSC-HVDC),an AC voltage compensation derived from the constant extinction area principle is added to the reactive power control loop of the VSC-HVDC to improve the reactive support capability to the inverter AC system after a fault occurs.Dynamic power limit regulation is put forward to ensure the priority of the transmission of reactive power,and also to reduce the amplitude reduction of active power as possible.The simulation result shows that the proposed strategy can effectively improve the ability of suppressing commutation failures of LCC-HVDC.Finally,a commutation failure suppression strategy based on multi-angle prediction in hybrid dual-infeed HVDC system is proposed.Combining the advantages of two prediction methods,constant extinction area prediction and zero-sequence detection and ?? transform prediction,we obtain the extinction angle reference.Then,the required reactive power that is calculated by the difference between the extinction angle reference and the setting value is fed back to the VSC control loop to adjust the reactive power that VSC generates after fault occurs,and thus to improve the immune ability to commutation failure of LCC.The simulation result shows that the proposed strategy doesn't depend on the real-time measurement of extinction angle and has a good suppression effect on commutation failure of LCC-HVDC.