Eliminating the specific harmonics especially low-order harmonics of the output voltage of 9-level inverter using SHE-PWM control scheme is investigated. Harmonic minimization is the intricate optimization problems because the nonlinear transcendental equations have multiple local optima. Increasing the degrees of freedom in the suggested method means that the number of switching angles increases. The suggested method is able to eliminate high number of undesired harmonics. As the number of switching angles increases, using either traditional iterative techniques or resultant theory method gets useless. In this paper to overcome this problem the imperialist competitive algorithm (ICA) is used. Also a DC-DC converter is used to enhance the SHE performance in the range for which the conventional SHE methods do not have any solution. Experimental and simulation results of a 9-level inverter show that the proposed method effectively minimizes a large number of particular harmonics so the total harmonics distortion of output voltage will be lower. 1. Introduction THE developments of FACTS devices [1, 2], medium voltage drives [3, 4], and various types of renewable energy resources [5, 6] have given great opportunities for the implementation of medium- and high-power inverters. The main problem with these applications is the frequency constraint of the pulse width modulation (PWM) which are limited by switching losses and electromagnetic interferences which is the results of high . Thus, to overcome the mentioned problems, selective-harmonic-elimination- (SHE-) based optimal pulse width modulation (OPWM) are proposed which are able to reduce the switching frequency and the total harmonic distortion of output voltage [7]. A typical multilevel inverter utilizes several DC voltage sources to provide a stepwise waveform in output voltage which makes a great development on output voltage THD while the output waveform approaches nearly sinusoidal waveform [8]. Related to the inverter circuit topologies the dc sources can be interconnected or isolated [7]. Because of the intricacy of the problem, in most studies on the SHE methods for multilevel inverters, it is assumed that only one switching angle per each voltage level is defined and the dc voltage sources are balanced (equal to each other). But in practical applications, depending on the output waveform and operation scheme of the inverter, the dc sources could be unbalanced or several switchings per each level are involved [9–13]. SHE method is a modulation strategy whose goal is to determine the proper
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