Harmonic Suppression Technology Of Power Systems Based On MMC-APF

With the widespread application of non-linear loads in power systems,the problems of power quality degradation and three-phase imbalances in the power grid have followed.Harmonic detection and control based on active power filters(APF)has been a research hotspot.Modular multilevel converter(MMC)has the advantages of high modularity,easy to expand and convenient to increase the voltage level.which has become an effective way for APF to be used in high voltage and large capacity applications.However,the application of MMC in the field of harmonic control is still in its infancy,so it is of great practical significance to study MMC-APF.The thesis focuses on the harmonic control of high-voltage distribution network and follows the process of harmonic management.It discusses the basic principles of the three key links of MMC-APF power grid voltage synchronization,harmonic detection,and current tracking control.and its specific implementation is studied.First of all,a sequence-decoupled resonant(SDR)controller combined with a cascaded double second-order generalized integrator(CDSOGI)was explored to solve the detection misalignment problem of synchronous reference frame phase locked loop under complex working conditions.This method uses the SDR controller to filter the grid voltage for the first time to quickly remove the negative sequence and high-frequency components.Then it uses CDSOGI to filter the former output twice to suppress the DC bias and accelerate the speed of following the grid frequency.Secondly,in order to solve the problem that the traditional ip-iq harmonic detection method is limited by the low-pass filter(LPF),the principle of the adaptive noise cancellation technology is analyzed,and then a method using the modified variable step size least mean square algorithm to replace the LPF is proposed.By modifying the filtering link of the ip-iq method,it will not be affected by the zero-mean random noise and the reference signal when it fully converges,and has higher steady-state accuracy.On this basis,the forgetting weight of historical error is introduced to compensate for the step size update,which optimizes the dynamic performance of the algorithm at the end of convergence.Finally,based on the mathematical model of MMC-APF in the dq coordinate system,a current tracking strategy combining PI control and repetitive control was studied.This method not only has the advantages of fast response of PI control,but also eliminates the steady-state errors in the following process.In addition,due to the particularity of the MMC structure,in order to ensure its normal operation,a circulating current suppression strategy based on quasi-proportional resonance control is adopted,which effectively limits the unique double-frequency negative sequence circulating current of the topology.Then,a voltage equalization method based on hybrid sort is explored to reduce the voltage fluctuation of sub-modules while reducing the IGBT switching loss.According to the above research content,based on the actual engineering data,the simulation model of the MMC-APF system was built using MATLAB/Simulink,and the feasibility and correctness of the proposed harmonic control scheme were verified through comparative simulation.