Research On Capacitor Voltage Balance Control And Optimization Of Modular Multilevel Converter

For the modular multilevel converter(MMC)with a large number of sub-modules,the real-time sorting of traditional capacitor voltage balancing strategy occupies a large number of computing resources of the controller.When MMC is used in the field of high frequency,excessive computation will lead to a large delay in the control loop.This is difficult to meet the requirements of the real-time control system and affects the response characteristics and stability of the system.At the same time,the traditional voltage balancing strategy needs to update the switching state of the sub-module(SM)in real time,resulting in high-frequency switching operations and a sharp increase in the total switching loss of MMC,which affects the service life of the device.To solve the problems,the simplified equivalent model and average value model are established based on the MMC topology and working principle in this paper first.Compared with the average value model,the simplified equivalent model can also reflect the detailed electrical changes outside the system.But the implementation is simpler,which is beneficial to the establishment of the MMC control system.Therefore,on basis of the simplified equivalent model,the MMC system level control,converter level control and valve level control are built.And the traditional capacitor voltage balancing strategy and circulation control control are embedded in the valve control.In order to reduce the computation and improve the performance of of traditional voltage balancing strategy,a capacitor voltage balancing strategy based on the quicksort algorithm is proposed in this paper.Through divide and conquer technology,modular voltage sequences based on comparison and division.The modules put into operation are decided by capacitor energy variation to realize voltage balanced.Time complexity and sort efficiency are deduced and the effect on system performance is analyzed.The adaptability of the strategy under different circumstances is researched.The strategy can greatly reduce the number of sorting,improve the running speed,keeps the same control accuracy of capacitor voltages,and will not influence on the system performance.The DSP controller TMS320F28335 is used to measure execution time of algorithms.The MMC model is set up on the PSCAD/EMTDC and the efficiency and correctness of the voltage balancing strategy based on quicksort are proved.However,the capacitor voltage balancing strategy based on the quicksort algorithm still needs real-time sorting.For this reason,a kind of searching the k value balancing approach is proposed.The voltage of the module between switched-on and switched-off state is determined through the median-of-median-of-five partitioning principle.By comparing all capacitor voltages with the value,the number information of switched-on and switched-off module is obtained instead of sorting all the module voltages in every control cycle.The new approach keeps the same control precision of module voltages with the conventional strategy.At the same time,the detailed selection process of the privot element is given,and the number of comparison foe searching k value in worst case is derived.Furthermore,through setting the maximum module voltage deviation,avoiding the same SM switching on or off repeatedly and changing operating states of SMs as little as possible,the switching operation is reduced significantly.The results of DSP experiment and PSCAD/EMTDC simulation show that the proposed strategy can achieve voltage balance with lower computation,improve the simulation speed,and reduce the switch frequency significantly by setting the maximum voltage deviation allowable value compared with the capacitor voltage balancing strategy based on the quicksort algorithm.