Research On Key Technologies Of DC Side Fault Current Suppression And Adaptive Restart For MMC-HVDC

Modular Multilevel Converter Based High Voltage DC(MMC-HVDC)is one of the most representative flexible DC transmission systems,with the outstanding advantages of large transmission capacity,strong controllability and high waveform quality.It is widely used in the construction of modern green-smart grid.However,due to the fast response speed and weak damping characteristics,the fault current not only rises rapidly but also has a high peak value when a short-circuit fault occurs on the MMC-HVDC DC side transmission line,which seriously threatens the safe and stable operation of the transmission line and power system equipment.Moreover,if a permanent fault occurs on the line,the traditional automatic reclosing device blindly recloses within the preset action time,causing the line to run in the DC side short-circuit state again and resulting in secondary overcurrent that has a more severe impact on the system.Therefore,research on short-circuit current suppression and adaptive restart technology on the DC side is critical to improve the fault resistance and self-healing ability of flexible DC power grids.Taking the MMC-HVDC overhead transmission line as the research object,the research is carried out from the aspects of the transient characteristics of the DC side short-circuit fault,the fault current suppression scheme,and the adaptive discrimination of the fault property.The main research work and the innovation achievements are as follows:(1)For the double-ended MMC-HVDC,the operating characteristics of the half-bridge sub-module and the full-bridge sub-module are analyzed respectively according to the MMC topology type and working principle.Combined with the characteristic parameters of overhead transmission lines,the equivalent model of MMC and centralized parameters of transmission lines are established.The frequency domain analysis method is used to study the short-circuit fault mechanism of the overhead transmission line on the DC side,revealing the influence of the half-bridge sub-module and the full-bridge sub-module on the transient characteristics of fault current.The above research provides theoretical support for the subsequent proposal of fault current suppression and adaptive restart technology.(2)Aiming at the disadvantages of half-bridge and full-bridge in fault current suppression,corresponding improved MMC-HVDC topologies are proposed respectively.With the technical goal of realizing self-clearing of the half-bridge MMC-HVDC,a fault clearance module including the main branch,charging branch,discharge branch and bleeder branch is designed.The operating principle before and after fault blocking is analyzed,so that it has functions such as limiting the rising speed of fault current,quickly clearing current,and reducing voltage fluctuation.In order to solve the secondary overcurrent problem caused by the restart of the full-bridge MMC-HVDC,the traditional full-bridge sub-module is improved by adding a current-limiting resistor.The resistance-capacitance energy transfer branch is used to reduce the fault current amplitude and accelerate the attenuation.(3)Based on the improved MMC-HVDC topology with fault current suppression capability,the current transient characteristics of overhead transmission lines when the converter station is restarted are analyzed and the adaptive restart scheme is proposed.For the fault clearance module,the current signal is injected into the line through the pre-charge capacitor.The ratio of the peak value of the inflow current at both ends of the fault point is used as the fault property criterion.The introduction of boundary judgment conditions provides decision-making basis for judging the nature of faults,improving the success rate of fault restart of the transmission system.For the improved full-bridge MMC without secondary overcurrent threat,the frequency domain amplitude characteristics of line current in a very short time after the converter station restarts is analyzed.The characteristic waveforms of transient and permanent faults under specific frequency components are extracted to realize adaptive restart and improve the speed of fault identification.(4)In order to realize the comprehensive application of fault current suppression and adaptive restart under high controllability,the operation characteristics of hybrid DC circuit breaker in the stages of fault development and clearance are studied.Hilbert theory is utilized to analyze the transient characteristics of different stages from fault occurrence to recovery.The port network containing virtual capacitors and virtual inductors is constructed based on the high controllable characteristics of flexible DC power grid.A phase Angle synchronous compensation control link based on virtual network port voltage and DC side fault current is designed.Therefore,the amplitude and oscillation of fault overvoltage can be suppressed rapidly by switching port attributes synchronously.Based on the phase compensation control,the DC side recovery voltage is introduced into the decision making of fault identification,and an adaptive restart scheme with high safety was proposed.