Design, Control and Characteristics of Multilevel Active NPC Converters for High Power Applications

Active neutral-point-clamped (ANPC) converter is a new family of multilevel technologies, which was originally proposed to solve the drawback of unbalanced power loss distribution among the devices in conventional neutral-point-clamped (NPC) converters. The purpose of this dissertation has been to further develop the design, topology, control and characteristics of ANPC converters for high power applications, such as large wind turbine systems and medium-voltage motor drive systems.;First, the concept of ETO Light NPC and ANPC Power Electronics Building Blocks (PEBB) is proposed. The electrical design and the component physical arrangement are discussed for the PEBBs. The methodologies for power loss calculation and thermal analysis of the PEBBs are presented with detailed analytical equation derivations. The system configurations with the proposed ETO Light PEBBs are identified for multi-MW wind turbine systems with AC and DC transmission, respectively. The thermal performance of the ETO Light NPC PEBB and ANPC PEBB is studied for the 5 MW and the 7 MW wind turbine cases, respectively.;Second, the fault tolerant capability of the 3L-ANPC converter is analyzed and the fault tolerant control strategies are proposed to enable the continuous operating of the converter under any single device open and short failure condition. The requirement on the fault detection time is also discussed. Moreover, the fault tolerant operation of the 3L-ANPC converter under multiple device failure conditions is studied. Simulation and experiment results are provided for verification. Furthermore, the reliability comparison for 3L-NPC and 3L-ANPC converters is investigated. Finally, the control schemes are proposed for the 3L-ANPC converter when it is applied on the generator side of a direct-driven permanent magnet synchronous generator (PMSG) based large wind turbine system, which allows the wind turbine to remain in service and continue providing active power under device failure conditions. Simulation results verify the proposed methods.;Third, a new 9L-ANPC converter is proposed for the next generation PEBB technology for power quality improvement in high power applications. Its operating principles and control, as well as floating capacitor voltage balance schemes are presented in detail. Simulation and experiment results are provided for verification. The proposed 9L-ANPC topology is also compared with other conventional 9L converters and existing 9L-ANPC converters from several aspects. The results indicate that the new 9L-ANPC converter shows better overall performance. Finally, the proposed topology is applied on the grid side of a 6 MW wind turbine system to achieve a filterless grid connection. Simulation results prove that the harmonic limit of the utility standards can be satisfied even without using a grid passive filter, which implies that the cost, efficiency, power density and reliability of the system can be improved.;Last, a simplified space vector based current controller for any general N-level converter is proposed. In this method, the mapping technique between the subhexagon of the N-level converter and the hexagon of the two-level converter is used to identify the proper output voltage vector to suppress harmonic current content in steady state and obtain fast current response in transient state. The proposed current controller does not require the calculation of the back EMF voltage of the load, and the controller complexity is greatly reduced due to the use of very simple lookup tables, thus it can easily be extended to any N-level converter. The proposed current controller is explained and verified by simulation on a five-level ANPC converter for motor drive applications.