Investigation of Different Balancing Methods for Modular 3-level T-type Voltage Source Converters with Distributed DC-link Capacitors

With the increasing relevance of regenerative power generation and more power electronic converters connected to the grid, the requirements on grid current quality increase. Since a 3-level power converter has a higher quality output waveform compared to a 2-level converter, it is a popular power converter topology for wind power and photovoltaic systems. However, the 3-level power converter has a split dc-link. Regarding the control, the main challenge is to balance the two capacitor voltages. In this paper different methods to solve the balancing problem of the neutral-point potential of the three-level T-type neutral-point-clamped (TNPC) voltage source converters (VSC) are investigated. The converter is used as an active front end (AFE) converter, which is composed of three single phase cells with distributed dc-link capacitors. Based on a simulation model of a TNPC, the different balancing methods are analyzed and compared. Furthermore, a new balancing strategy is proposed. In order to verify the investigated balancing methods, a three phase converter with newly designed intelligent single phase printed circuit board (PCB) cells has been developed, built and assembled.