Investigation of the Harmonic Optimization Approaches in the New Modular Multilevel Converters

Modular multilevel converter (MMC) is a new type of multilevel converter. Similar to cascaded multilevel converters, each arm consists of several submodules and each submodule includes one separate DC source in parallel. Based on the analysis of its structure and working mechanism, this paper presents two harmonic optimization techniques, known as optimized harmonic stepped-waveform (OHSW) technique and optimal minimization of THD (OMTHD) technique. Then, principles and algorithms of the two methods are discussed in detail. According to a conclusion of the sampling PWM control theory ´the same impulse and different pulse shapes added to the links with the inertia, the effect is basically the same´, a new approach to accessing the initial values of Newton-Raphson is developed to solve the relevant nonlinear transcendental equations, which accelerates convergence of the roots. Finally, their advantages and disadvantages are clarified and compared via simulation experiment, verifying their effectiveness in MMC.