Multilevel Converters: Hardware Design, Advanced Modulation Techniques and Experimental Prototype Validation

During the last two decades, the need of reducing global warming, all forms of pollution and the need of increasing energy efficiency in all fields, is transforming the way the energy in produced, transmitted, distributed and utilized. In this scenario, power electronic converters represent a key element because of their “unique” capability to manipulate and/or transform, and/or control huge amounts of electric energy, according to simple or complex control laws implemented using digital computer device. Renewable energy, smart grids, electric transportation cannot be used without power converters, therefore, modern power converters are those unique devices driving the transition from conventional to clean energy. The existing limits of semiconductor’s technology conflict with the high values of power, voltages and currents required in a large amount of applications and for this reason multilevel converters have been proposed because their allow to overcome technological limits of power devices. The whole Ph.D. activity, summarized in part with this thesis, has been developed in this context and mostly within the frame of the industrial research project titled: “CoMoDes”, by DigiPower Ltd. L’Aquila. The project, while using a methodology typical of applied scientific research, has addressed almost all main aspects of multilevel converters technology: topologies, applications, control, modulation, hardware and has involved the Author of this Ph.D. Thesis in a significant number of activities. The project has dealt with the study, design and realization of a three-phase, 33-level converter with rated phase voltage: 10 kV @ 35 Arms, resulting total power close to 1 MW. Some prototypes of the modular multilevel converter are now available at DigiPower Ltd Lab and utilized as R&D systems in relevant research branches, currently: smart grids and electric transportation. The developed architecture is a cascade H-bridge (CHB) topology and it grants higher flexibility, modularity and fault tolerant characteristics with respect to other CHB-MC. Each H-bridge module is equipped with its own DSP unit, which collects module’s information (current, voltage, error status, temperature etc.) and transmits them to the main control board, the latter equipped with a FPGA, a CPLD and a DSP controllers. The main control board analyzes the state of each H-bridge and applies the required modulation signals to the system according to implemented control and modulation algorithms; it takes immediate decisions in case of incoming fault, thus improving the overall reliability and flexibility of the system. The local DSPs can be optionally programmed in order to implement distributed control algorithms, exploiting the unique true multiprocessing capability. The advanced hardware (HW), combined with innovative fundamental frequency modulations developed in parallel with the HW and with control algorithms, offer high flexibility and efficiency, especially in high power applications; moreover they reduce switching losses and harmonic contents in output waveforms. Therefore, developed system significantly improves the power quality and grants a reduction in the average device switching frequency, while minimizing cost and size of the output power filter. Author’s main contributions to the above project can be found in the following parts: • Hardware design of the power modules • Hardware design of the system control board • Implementation of the innovative modulation techniques for the harmonic content reduction • Debug, tuning, and implementation of the control