| In order to achieve a large-scale optimization of energy allocation,our country is actively promoting the implementation of the “West-East Power Transmission”project.Line Commutated Converter based High Voltage Direct Current(LCC-HVDC)transmission has developed rapidly in our country in recent years due to its technical advantages such as large transmission capacity and long transmission distance,becoming the important technical support for the “West-East Power Transmission”project.With the trend of DC transmission moving towards higher voltage levels,greater transmission capacity,and more compact transmission structure,the coupling between AC and DC systems,and between the sending and receiving ends is becoming tighter,the commutation failure of the single-circuit DC transmission system has an increasingly serious impact on the regional power grid,and the fault conditions that cause commutation failure are becoming more and more complicated.As a result,the existing commutation failure prevention strategies are no longer fully applicable.It is urgent to explore the mechanism of commutation failure under different types of fault conditions,and form a more complete commutation failure mechanism and a more comprehensive commutation failure prevention technology.This article is funded by the National Natural Science Foundation of China Youth Science Foundation Project “Research on the Mechanism of Commutation Failure of HVDC Transmission System Containing Multiple FACTS Devices and Its Suppression Methods”[51907058],National Key Research and Development Program“Renewable Energy Power Generation Base DC Transmission System Stability Control Technology”[2017YFB0902000],State Grid Science and Technology Project“Construction of Physical Simulation Platform for UHVDC Transmission in Jiuhu Project and Research on Commutation Failure Mechanism and Prevention Measures”[SGTYHT/17-JS-201].This article focuses on the mechanism and prevention of commutation failure caused by the interaction of LCC-HVDC multiple controllers,the mechanism and prevention of commutation failure caused by the coupling of HVDC multiple DC transmission lines,the mechanism and prevention of commutation failure caused by the AC fault of sending and receiving ends,the LCC-HVDC physical simulation platform and the physical simulation of commutation failure.Relevant research work has formed a comprehensive prevention control technology for commutation failure considering the internal and external influence factors of LCC-HVDC,and the physical simulation of the commutation failure phenomenon has been realized,which can provide theoretical basis and practical guidance for the prevention of commutation failure in actual projects.The main research content and research results include:(1)Aiming at the problem of the interaction of various controllers in the LCC-HVDC system,the mechanism of commutation failure caused by the interaction of multiple controllers and its suppression methods are studied.First,according to the basic control structure of the LCC-HVDC system,the mathematical description method of each controller is given.Based on the description function of each controller,the dynamic interaction process between multiple controllers under fault conditions is analyzed,and the motion trajectory during fault and recovery period is given,and then the influence of each controller's self-characteristics on system recovery is analyzed.Then,the following factors affecting the commutation failure caused by improper interaction of multiple controllers are studied: the duration of the fault,the severity of the fault,and the strength of the receiving end AC system.Finally,a prevention method based on improved constant extinction angle control is proposed,and the simulation results verify the accuracy of the revealed commutation failure mechanism and the effectiveness of the proposed prevention method.(2)Aiming at the problem of the coupling effect of each DC transmission line of LCC-HVDC,the mechanism of the commutation failure caused by the coupling effect of the transmission lines and its suppression method are studied.First,based on the mathematical equation of a single uniform transmission line,a mathematical model of multiple DC transmission lines on the same tower is established.Based on the established line model,the influencing factors of the line coupling effect are analyzed,and the electromagnetic coupling mechanism between different lines under fault conditions is studied.Then,the influence mechanism of line coupling effect on commutation failure is revealed,and the influence of different operation modes and different line structures on the commutation failure caused by the line coupling effect are further explored.Finally,based on the above theoretical analysis,a prevention method for commutation failure caused by line coupling effect in single and double-circuit DC systems is proposed,and its effectiveness is proved by corresponding simulation experiments.(3)Aiming at the external AC faults of LCC-HVDC system,the mechanism and suppression method of commutation failure caused by the faults of sending and receiving ends are studied.Firstly,it analyzes the common fault types of the sending end and receiving end AC systems and their influence characteristics.Secondly,based on the influence characteristics of various faults and the quasi-steady state equation of the LCC-HVDC system,the mechanism of commutation failure caused by various AC faults at the sending end and the receiving end is analyzed respectively,and the prevention methods for commutation failure caused by sending end faults or receiving end faults are studied respectively.Then,the prevention and control methods that only consider the sending end faults or receiving end faults are integrated and simplified to form a comprehensive improved control strategy that considers the faults of both ends.Finally,the simulation results show that the proposed method can effectively reduce the risk of commutation failure caused by external AC faults.(4)Aiming at the physical simulation problem of LCC-HVDC commutation failure,the construction scheme of the low-voltage DC transmission physical simulation platform and the commutation failure simulation scheme are studied,which mainly include the system connection scheme,valve group topology,DC control strategy,fault simulation method,and fault protection methods.A 1600V/100 k W unipolar double twelve-pulse DC transmission physical simulation platform is developed,and the design ideas of the platform in terms of system layout,hardware and software are introduced in detail.The commutation failure phenomenon caused by various types of faults is simulated through the developed platform,and the commutation failure prevention experiment based on the proposed improved control strategy is also carried out.The experimental results show that the overall design of the platform is reasonable,the developed platform has good dynamic and steady-state characteristics,and can safely and reliably simulate the commutation failure phenomenon caused by multiple types of faults;the proposed prevention strategy can reduce the probability of commutation failure to a certain extent. |
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