Modulation extension control for multilevel converters using triplen harmonic injection with low switching frequency

This paper presents a modulation extension control method for multilevel converters with low switching frequency. The disadvantage of the fundamental frequency switching control method for multilevel converters is its narrow range of modulation indices where solutions exist. To address this problem, a triplen harmonic compensation method is proposed. First, the resultant method and/or Newton climbing method are used to find solutions of the switching angles for the fundamental frequency switching scheme control. Second, a triplen harmonic is injected into the multilevel converters accompanied with the fundamental frequency control signals to reduce the required level number of the DC voltages without changing the fundamental component of the phase voltage. The computational results show that the triplen harmonic method indeed reduces the required DC voltage level number to reduce the hardware cost. A 17-level example was implemented with an 11-level H-bridge multilevel converter and an 8 μs control resolution to demonstrate the triplen harmonic compensation method. The experimental results confirmed the method with this example.