Research On SHEPWM Technology Of Seven-level Inverter Based On SA-PSO Algorithm

The multilevel inverter has attracted more and more attention in high-voltage and high-power applications because of its advantages of small Total Harmonic Distortion(THD)of output waveform,low voltage stress of devices and low Electromagnetic Interference(EMI)of system.Selective harmonic elimination pulse width modulation(SHEPWM)technology has accurate harmonic elimination ability,can optimize the output with low switching frequency,and is widely used in multi-level inverter control.Various intelligent algorithms have become effective means to solve the mathematical model of SHEPWM technology.On the basis of introducing the classical multilevel inverter topology and its control strategy,this paper studies the realization of SHEPWM technology with cascaded seven-level inverter as the control object.The harmonic elimination model of SHEPWM technology with 1/4 period symmetry is established.For solving the harmonic elimination model,the performances of Newton iterative method,homotopy algorithm and Particle Swarm Optimization(PSO)algorithm with simple principle and high efficiency in solving nonlinear harmonic elimination equations are compared.According to the independent variable characteristics of PSO algorithm and harmonic elimination model,an effective independent variable initialization method is proposed.The results show that Newton iterative method is highly dependent on the initial value,and the accuracy of homotopy algorithm is not enough.The Newton iterative method modified by homotopy algorithm has strong convergence ability and improved accuracy,but it still cannot reach the accuracy of particle swarm optimization.In order to further enhance the global optimization ability and improve the solution accuracy of the algorithm,the Simulated Annealing(SA)algorithm is introduced,and the iterative process of PSO algorithm is modified by using the probability jump characteristic of SA algorithm.The optimized SA-PSO algorithm and PSO algorithm are applied to the simulation analysis of cascaded seven-level inverter.It can be seen that SA-PSO algorithm has improved global optimization ability and accuracy on the basis of keeping PSO algorithm simple in principle and fast in convergence speed.According to the 1/2 period symmetry of the output phase voltage of multilevel inverter,the mathematical model of harmonic elimination technology with half-period odd symmetry and half-period mirror symmetry of multilevel inverter is established.According to the characteristics of harmonic elimination model under two symmetrical modes,SA-PSO algorithm is used to solve the harmonic elimination switching angle,which is applied to the simulation model control of half-period symmetric SHEPWM technology of multilevel inverter.The simulation results show that,The harmonic elimination technology can achieve ideal harmonic elimination effect under the condition of half-cycle odd symmetry of inverter voltage and half-cycle mirror symmetry.The THD value of inverter output voltage is slightly higher under the condition of half-cycle mirror symmetry of inverter voltage,but the residual amount of specific harmonic to be eliminated is lower,and the harmonic elimination equations have infinite solutions,and the harmonic phase is controllable.In practical application,the symmetrical mode of inverter voltage can be chosen flexibly.