Research On Harmonic Mitigation Strategy Of Cascaded Multilevel Inverter

In recent years,with the rapid development of power electronics technology,a large number of power electronic equipments have caused serious harmonic pollution,which poses a great threat to the security of power systems.In order to reduce the harmonics in power grid effectively,experts apply the specific harmonic optimization PWM technology into the inverter to ensure the power and energy injected in the grid can reach high quality.However,in the current application,there are still some problems have to be solved,such as solving complex nonlinear transcendental mathematical models,which costs a lot of time off-line calculation,the output voltages don’t meet the power quality standards and the power mismatching of the asymmetric cascade multilevel inverter.This paper studies to solve these problems,the main work is as follows:Firstly,in view of the problem that the speed of solving nonlinear transcendental mathematical models of the harmonic mitigation is slow,an improved particle swarm optimization（IPSO）algorithm is proposed.The algorithm mainly improves the iteration speed and search range of the particle swarm optimization parameters——inertia weight and learning factors.At the same time,the principle of the growth function is introduced to improve the motion equations.The multi-objective optimization problem with inequality constraints is transformed into a single objective optimization problem by the penalty factors,so as to implify the solution process and improve the convergence speed of iteration.The algorithm proposed in this paper is faster and more accurate than the traditional algorithm in solving nonlinear transcendental equations.Secondly,in order to solve the problem that specific harmonic elimination technology can only eliminate the specific harmonics and the total harmonic distortion（THD）is still large,an improved method of specific harmonic mitigation-PWM is proposed.According to the mathematical model of asymmetric cascade H-bridge（ACHB）multilevel inverter,the variable DC bus voltage is taken as the degree of freedom,combined with the specific harmonic mitigation technology,the switching angles in the fundamental wave is optimized to realize the optimal combination of switching angles and voltages.The output voltages is similar to sine wave by the angle controlled inverter.All of the odd harmonics less than 50^th order and the total harmonic distortion of voltages can meet the power quality standards,which could simplify the design of the filters.Thirdly,an improved power matching control strategy for asymmetric specific harmonic mitigation-PWM is proposed to solve the power mismatching problems of asymmetric cascade multilevel inverter.Based on the traditional harmonic mitigation strategy,a new harmonic mitigation mathematical model is proposed to solve the problem of uneven load in power units and the power matching of H-bridge cells is realized in one cycle.Meanwhile,the THD of output line-line voltage is reduced and the harmonic mitigation effect is improved.Finally,based on the theoretical research,the simulation model and the platform of cascade H-bridge multilevel inverter are built in this paper.The performance of the proposed method is tested,which verifies the correctness and effectiveness of the proposed strategy.