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

• Published in: IEEE transactions on power electronics 2020-08, Vol.35 (8), p.8712-8725
• Main Authors: Perez-Basante, Angel, Ceballos, Salvador, Konstantinou, Georgios, Pou, Josep, Sanchez-Ruiz, Alain, Lopez, Iraide, de Alegria, Inigo Martinez
• Format: Article
• Language: English
• Subjects: Capacitors; circulating current; Converters; Fourier series; Frequency modulation; Harmonic analysis; Metal matrix composites; modular multilevel converter (MMC); optimization algorithms; Pulse duration modulation; Pulse width modulation; Selective harmonic elimination (SHE); Switches; Voltage control
• ISSN: 0885-8993; 1941-0107
• DOI: 10.1109/TPEL.2020.2964522

Modular multilevel converters (MMCs) require control of the circulating current, i_{\text{circ}}, to improve their operation and efficiency. This is particularly important when low switching frequency modulation techniques, such as selective harmonic elimination pulsewidth modulation (SHE-PWM) are applied. This article 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_{\text{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.