Research On Risk Assessment And Converter Station Prevention Methods For Commutation Failure Of LCC-HVDC System

Energy is a vital material foundation for human survival and development,and is closely related to national economy and people’s livelihoods as well as national strategic competitiveness.Currently,the global energy landscape is undergoing profound adjustments,and the energy revolution is imminent.In 2020,China for the first time proposed to achieve the “dual carbon” goal,and the power industry is a key sector in achieving this goal.In response,the Party Central Committee has explicitly stated that a new power system dominated by new energy sources should be built.However,over 80% of China’s clean energy resources are distributed in the western and northern regions,while the electricity load is concentrated in the eastern and central regions.This determines that China must accelerate the construction of ultrahigh voltage backbone grids to provide a channel for the consumption of clean energy.Due to the significant advantages in transmission capacity,transmission distance,and construction cost,Line Commutated Converter based High Voltage Direct Current(LCC-HVDC)has been rapidly developed and widely used in China,playing a key role in serving the construction of new power system dominated by new energy sources.At present,more than 30 UHV/HV DC projects are built in China,and the characteristics as “Strong HVDC and Weak AC system” are prominent,as well as the commutation failure problems.Due to the tight coupling of AC and DC systems,the fault conditions that cause commutation failure in LCC-HVDC are becoming more and more complex.Moreover,the commutation failure will lead to interruption of DC transmission power,causing power oscillations,frequency fluctuations at the sending and receiving ends,and even the disconnection of renewable inverters,which seriously threatens the safe and stable operation of the power grid.Therefore,it is urgent to explore the research on risk assessment and converter station prevention methods for commutation failure of LCC-HVDC system.This article is funded by the National Key Research and Development Program(2021YFB2400900),Science and Technology Innovation Major Project of Hunan Province(2020GK1010),Integration Projects of National Natural Science Foundation of China-State Grid Joint Fund(U2166602),Natural Science Foundation for Excellent Youth Scholars of Hunan Province(2023JJ20012),Innovation Young Talents Program of Changsha Science and Technology Bureau(kq2107005)and Program for Guangdong Introducing Innovative and Entrepreneurial Team(2017ZT07G237).The article is relying on the State Key Laboratory of Electric Power High-Efficient and High-Quality Conversion and the National Electric Power Conversion and Control Engineering Technology Research Center.This article focuses on the assessment method of commutation failure risk,the prevention method of the first and subsequent commutation failure.Relevant research work has formed a comprehensive risk assessment method and prevention method for commutation failure considering multifactor influence,which can provide theoretical basis and practical guidance for the plan of LCC-HVDC and the prevention of commutation failure in actual projects.The main research content and research results include:(1)Aiming at the problems of high computation burden and low accuracy in existing commutation failure risk assessment methods,a critical commutation failure fault level calculation method considering multi-factor influence under single-phaseto-ground and three-phase-to-ground faults is proposed,which considers both calculation efficiency and accuracy.Firstly,the fault commutating voltage under single-line-to-ground and three-phase-to-ground faults is calculated based on symmetrical component method,and the expression for the DC voltage is derived.Then,the DC current is gained based on the DC transmission line model.Secondly,under single-line-to-ground faults,discreate commutation processes are constructed based on the commutation voltage-time area rule to solve the extinction angle.Thus,the commutation failure fault level calculation method considering the fault time and DC current variation is proposed.Then,under three-phase-to-ground faults,the harmonic voltage and the voltage-time area of harmonic voltage is estimated,and the effect of firing angle variation on commutating voltage is studied based on the quasi-state equation of AC/DC systems.Thus,the commutation failure fault level calculation method considering multi-factor influence is proposed.The accuracy of the proposed method is validated by the simulation results.Compared to traditional method,the proposed methods are found to improve the Mean Absolute Error(MAE)of critical commutation failure fault level under single-line-to-ground faults and three-phase-toground faults by 39.2% and 80%,respectively.(2)Aiming at the problems of slow response and low accuracy in existing the first commutation failure mitigation methods,three kinds of methods are proposed based on advancing firing control optimization.(1)A commutation failure mitigation method based on the combination of measurement-type and prediction-type methods is proposed.Firstly,based on the measurement of commutation process beginning and ending time and the corresponding commutating voltage,the provide voltage-time area is calculated,which considers the variation of commutating inductance indirectly.Then,based on the prediction of DC current,the demand voltage-time area is calculated,and the firing angle is predicted at the end of each commutation process.The effectiveness of the proposed method is validated by the simulation results.Compared to CIGRE control,the proposed method is found to improve the commutation failure immunity index(CFII)under single-line-to-ground and three-phase-to-ground faults by 14.1%and 16.1%,respectively.(2)A commutation failure mitigation method based on the DC current prediction is proposed.Based on the similar theory and the prediction of DC current,the extinction angle is calculated,which can predict commutation failure and provide action basis for the control method.Then,considering the effect of the advanced firing angle on the DC current and overlap angle,the firing angle is calculated.The effectiveness of the proposed method is validated by the simulation results.Compared to CIGRE control,the proposed method is found to improve the CFII under single-line-to-ground and three-phase-to-ground faults by 23.4% and 16.8%,respectively.(3)A commutation failure mitigation method based on imaginary commutation process is proposed.Firstly,considering the DC current,commutating voltage amplitude and phase shift,the imaginary commutation process is constructed.Then,the imaginary extinction angle is calculated and sent to the constant extinction angle controller,enabling the original controller to perceive the extinction angle in advance to mitigate commutation failure.The effectiveness of the proposed method is validated by the simulation results.Compared to CIGRE control,the proposed method is found to improve the CFII under single-line-to-ground and three-phase-to-ground faults by 28.4% and 21.8%,respectively.(3)Aiming at the problems of poor commutation failure mitigation performance under single-line-to-ground faults in the existing methods for mitigating the commutation failure based on reactive compensation equipment,a control strategy considers the characteristics of single-line-to-ground faults for STATCOM is proposed.Firstly,the characteristics of commutation failure under single-line-to-ground faults and the commutation failure mitigation performance with different compensation time are analyzed,and on this basis,the optimal compensation timing of STATCOM is explored.When there is no communication between STATCOM and LCC-HVDC,a commutation failure mitigation strategy based on the identification of the fault time for STATCOM is proposed.When there is communication between STATCOM and LCC-HVDC,a commutation failure mitigation strategy based on the coordinate control of STATCOM and LCC-HVDC is proposed.The effectiveness of the proposed methods is validated by the simulation result.Compared to traditional method,the proposed strategies are found to improve the CFII under single-line-to-ground faults by 4.5%and 40.3%,respectively.(4)Aiming at the problems of slow recovery in existing subsequent commutation failure mitigation methods,a strategy for recovery performance improvement of LCCHVDC based on DC current order optimization is proposed.Firstly,based on the Thevenin equivalent parameters,quasi-state equations of LCC-HVDC system and the related constrain of electric and control quantities,the DC current order of maximum active power point is calculated.Then,based on the imaginary commutation process and the boundary conditions of commutation failure,the maximum DC current order that avoid commutation failure is calculated.Lastly,the smaller value of two DC current orders is sent to the control system.The effectiveness of the proposed methods is validated by the simulation results.Compared to the traditional method and other DC current optimization strategies,the proposed strategy is found to mitigate subsequent commutation failure effectively and maintain the AC voltage and DC current within operational boundaries as possible while recover the DC power faster.