| The high proportion of renewable energy via the AC grid over long distances,large-capacity transmission presents problems of poor transmission capacity and low utilization rate of transmission corridors,which has an urgent need for innovation in transmission methods.Three-wire bipolar structure based high voltage direct current(TWBS-HVDC)system based on a modular multilevel converter(MMC)can make full use of the rated capacity of converter station and thermal constraints of line,enhance the transmission capacity of original AC lines,which has become an effective method way to achieve AC to DC and line capacity expansion.A profound analysis of MMC based TWBS-HVDC system operating principle,designing an rational operation control strategy,and accurately grasping transient characteristics of TWBS-HVDC have important theoretical value for future planning and operation of flexible HVDC grids in China.Therefore,this paper has carried out related research around MMC based TWBS-HVDC operation control strategy and DC fault mechanism,and main research works are as follows:(1)The working principle of MMC based TWBS-HVDC is studied.Aiming at the difference between TWBS-HVDC and bipole direct current system in connection ways,topology of TWBS-HVDC system is studied based on topology and operating principles of MMC converters,and operation characteristics of TWBS-HVDC system is analyzed.Considering that TWBS-HVDC and bipolar flexible DC transmission systems have the same port characteristics,Station-level double closed-loop control strategies,interphase circulation suppression strategies,valve-level modulation and voltage balancing control strategies for TWBS-HVDC normal stage operation requirements are designed.(2)An operating period setting method of considering both DC lines thermal stability and capacity expansion of TWBS-HVDC is proposed,and a transition stage control strategy based on the resistance timing switching method is designed.TWBS-HVDC capacity increase principle under different modulation pole resistance ratios is studied,the heat balance equation of the long-term operation of the DC line is established.Considering comprehensively the factors of the temperature rise of the line and the switching loss of the equipment,TWBS-HVDC operating period setting method is proposed;Based on the operation rule of TWBS-HVDC,a buffer resistor and switch are added to modulation pole,a calculation method of transition resistance is proposed,and a timing switching strategy of transition resistance is designed.The simulation results show that the proposed operation period setting method and transition control strategy can ensure the reliable and stable operation of TWBS-HVDC,complete the smooth transition process,and achieve capacity increasing target.(3)A DC side short-circuit fault current calculation method for MMC based TWBS-HVDC system is proposed.the transient equivalent model of TWBS-HVDC system during different DC side faults occur to DC circuit breaker action is established based on MMC transient equivalent circuit.The number of independent circuits and the order of dynamic components in the transient equivalent circuit are taken as the standard,all DC faults are classified into three categories,and the state equations of each kind of fault is analyzed and deduced.The analytical expression of fault current is obtained by solving the eigenvalue and eigenvector of coefficient matrix in the state equation.a TWBS-HVDC system simulation model is established in the MATLAB/Simulink digital simulation platform,and the simulation results validate the effectiveness and accuracy of the short-circuit fault state equation solution method on the DC side of the TWBS-HVDC system proposed in this paper,which can provide a scientific basis for selection of system main circuit parameter and setting of relay protection action time. |
