Research On Fault Current Suppression Technologies Of VSC-HVDC Grid Based On Half-bridge MMC

The voltage source converter high voltage direct current(VSC-HVDC)transmission technology based on modular multilevel converter(MMC)overcomes the defect of line commutation converter(LCC)HVDC transmission technology that cannot directly access and transmit wind and solar renewable energy.Due to the reverse distribution of energy bases and load centers in China,the VSC-HVDC transmission technology based on overhead lines is the main technical means to realize large-scale development and longdistance transmission of new energy.The VSC-HVDC projects have developed rapidly in China.In 2020,the Zhangbei ±500k V VSC-HVDC grid project and the Kunliulong ±800k V VSC-HVDC transmission project had been put into operation.The VSC-HVDC grid has the characteristic of low damping.When a short-circuit fault occurs in the DC line,the fault current would rise sharply to a very high overcurrent level within a few milliseconds,seriously endangering the safety of DC system and equipments.In order to suppress the short-circuit fault current of the VSC-HVDC grid based on the halfbridge MMC,Zhangbei project adopted high-speed and large-capacity DC circuit breakers(DCCB)to clear the fault.The DCCBs configured in Zhangbei project must have the ability to break the fault current of 25 k A within 3 ms.These strict technical requirements lead to the high cost of DCCD,which affects the economy of the VSC-HVDC grid.In view of the shortcomings of the existing fault current suppression technology of the VSC-HVDC grid,this paper systematically studies a variety of fault current suppression technologies of the VSC-HVDC grid based on half-bridge MMC,and proposes the active current-limiting control technology suitable for the VSC-HVDC grid based on half-bridge MMC.The active current-limiting control technology makes full use of the control ability of the half-bridge MMC to suppress the DC short-circuit fault current,reducing the technical requirements of the DC circuit breaker and the overall construction cost of the DC grid.The main work of this paper is as follows:1.Aiming at the passive fault isolation technology of the VSC-HVDC grid,a reclosing mechanical DC circuit breaker is proposed,which can use the charging and discharging between capacitors to have the reclosing capability.The topology and operation principle of this DCCB are analyzed.The parameter design method of the internal components in the DCCB is studied.Taking Zhangbei project as an example,the typical parameters are designed.Finally,the simulations of multiple working conditions in DC voltage source and four-terminal DC grid are carried out to verify the various functions of the DCCB.2.An active current-limiting control technology of half-bridge MMC based on arm voltage emergency-droop control is proposed.It is demonstrated that the half-bridge MMC has the control capability of suppressing fault current.It is suitable for application scenarios with short fault detection delay.The principle of half-bridge MMC active current-limiting control technology and its coordination strategy with DCCB are studied.The calculation equations of the fault current under the active current-limiting control is analyzed and deduced.It is proved that reducing the fault detection delay can enhance the effect of fault current suppression.Finally,the arm voltage emergency-droop control is simulated in the four-terminal DC grid model based on the half-bridge MMC to verify its function and adaptability to high-resistance faults.3.For the application scenarios with long fault detection delay,an active currentlimiting control technology of half-bridge MMC basd on two-stage current-limiting is proposed to optimize the current-limiting effect at the initial stage of the DC short-circuit fault.The improve method of the modulation scheme of the arm voltage is studied.The structure and principle of the two-stage current-limiting control are analyzed.The calculation equations of the DC fault current and arm currents of the half-bridge MMC are deduced,and the calculation equations of the DC fault current and arm currents under the two-stage current-limiting control are verified.Finally,the effectiveness of the two-stage current-limiting control is verified by simulation in the four-terminal VSC-HVDC grid based on the half-bridge MMC.4.An active current-limiting control technology of half-bridge based on non-switching dynamic-amplitude-limiting is proposed.That realizes the non-delay start of active currentlimiting control after fault,reducing the requirements of DC grid on the action speed of DCCB.The principle of the dynamic-amplitude-limiting control and its coordination strategy with DCCB are studied.The fault current equations of the converter under the dynamic-amplitude-limiting control is deduced and verified.It is proved that the increase of the parameters of the current-limiting controller is conducive to fault current suppression.The selection scheme of the parameters of the dynamic-amplitude-limiting controller is studied.Finally,the simulations in the four-terminal VSC-HVDC grid verify that the dynamic-amplitude-limiting control can further suppress the fault current,and evaluates its adaptability to the non-fault disturbance and its impact on the accuracy of fault detection.5.For the application scenarios of strong AC system,an improved half-bridge MMC topology with full-bridge sub-modules in series on the AC side is proposed.The methods of half-bridge MMC for suppressing the feeding current of strong AC system are studied.The topology and control strategy of the improved half-bridge MMC are analyzed.The parameter design method of the key components in this converter is studied,and an example of parameter design is given.The function of this converter topology to suppress DC fault current and AC fault current is verified by the steady-state and transient simulations in a two-terminal DC system.Finally,the technical performance and economy of this topology are compared with typical MMC topology.