Circulating Current Control for Modular Multilevel Converters with (N+1) Selective Harmonic Elimination - PWM

Modular multilevel converters (MMCs) require control of the circulating current, i_circ, to improve their operation and efficiency. This is particularly important when low switching frequency modulation techniques, such as selective harmonic elimination (SHE-PWM) are applied. This work provides a novel method to control the circulating current along with (N+1) SHE-PWM. Unlike the case of (2N+1) SHE-PWM, explicit redundant levels are not available and, therefore, different modulation indexes, m_1 and m_2, are employed in the upper and lower arms to obtain the desired modulation index m_a. Unlike previous (N+1) circulating current methods, the distances between m_a, m_1 and m_2 remain constant to not disturb the phase output voltage, with an interchange of m_1 and m_2 between the arms used to follow the desired i_circ. The control adjusts the dc component of the circulating current and the energy stored in the SMs to their references, while maintaining the energy balance between the upper and lower arms. Simulation tests and experimental results, obtained from a single-phase laboratory prototype MMC, validate the proposed control technique.