Power and DC link voltage control considerations for indirect AC/AC Modular Multilevel Converters

A highly interesting application field of the Modular Multilevel Converter is the interconnection of the European three-phase 50 Hz industrial grid with the single-phase 16 2/3 Hz railway supply. Contrary to existing technologies, the utilization of a bulky and costly transformer on the low frequency side as well as additional filtering components on the AC lines or DC link are avoided. However, the inherent converter dynamics still pose challenges in the design of control systems, which should ensure safe operation as well as high performance in steady state and transient conditions. This paper investigates an effective way to control the power as well as the DC link voltage in such a Modular Multilevel Converter-based network interface. The additional control objectives, such as branch energy stabilization as well as sub-module capacitor voltage balancing are maintained, regardless of the aforementioned externally controlled magnitudes. The DC link second harmonic power fluctuation, which is a property of the single-phase side, is also compensated by means of proper circulating current control. The analyzed concepts are evaluated by means of simulation results, using a discrete-time model of a real power application.