Keywords: multilevel inverters, pulse width modulation, PWM, simulated annealing, genetic algorithms, GAs, Newton-Raphson method, optimisation, harmonic elimination, field programmable gate arrays, FPGA, simulation
Solutions to the harmonic elimination problem in a seven level inverter
Multilevel inverters have recently emerged as very important alternatives in high power applications. Several modulation methods have been applied to multilevel inverters. When utilised at low switching frequencies, existing multilevel carrier-based pulse width modulation (PWM) strategies have no special provisions to render quality output; several lower order harmonics are included in the spectrum. The problem of eliminating selected harmonics in a chosen multilevel inverter is considered in this paper. Given a desired fundamental output voltage, the problem is to find the switching times (angles) that produce the fundamental while not generating specifically chosen harmonics. This paper proposes techniques using simulated annealing (SA), genetic algorithm (GA) and Newton-Raphson method for calculating offline the optimum switching angles of chosen seven-level inverter to eliminate the 5th and 7th harmonics. Solution through SA technique is found to be better. The switching angles provided by the three methods are implemented in a field programmable gate array (FPGA) based firing module for the chosen inverter. The simulation and hardware results are presented and evaluated. They validate the solution developed for the chosen problem.