Advanced Control of Power Converters: Modular Multilevel Converter
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- Institutt for elkraftteknikk 
Original versionDefence 8 December
Multilevel converters can reach medium-voltage operation increasing the efficiency of high-power applications. Among the available multilevel converter topologies, the invention of Modular Multilevel Converter (MMC) has gained considerable attention of academia and industry in recent times. There are attractive advantages of this converter in terms of modularity, high-quality waveforms, low device rating, fault tolerant capacity and voltage scalability. Moreover, the main advantage compared to cascaded multilevel converters is the lack of an input transformer which results in a reduction of cooling requirements, size, and cost. The converter concept has already been introduced in the market for high voltage dc transmission (HVDC) applications. A considerable research is going on presently for using this topology for dc-ac power conversion e.g. for ac drives. The main focus of this research work will be based on identifying and solving some of the challenges of medium and high voltage converters for drives and power grid. Reliability and fail safe operation of these converters are primary requirements. Apart from this, condition monitoring and health diagnosis of the converter including the load is also essential. This thesis studies mathematical models to predict the behaviour of the converter under steady state and dynamic conditions. The obtained models can be useful to know the transient response inside the converter during faulty conditions and make it possible to design the components for this converter for abnormal operations. Exploring the fault operation of cells and bypassing in case of a fault have been investigated by simulation and laboratory experiments. Moreover, in MMC, there is a strong harmonic content in the arm current that essentially controls the dynamic behavior of the converter. In this project an explicit derivation of the equations in the frequency domain has been developed based on the switching functions concept. The salient features of this analysis are (i), how the second order harmonic in the circulating current influences capacitor voltage ripples, and (ii) to find the most critical parameters related to capacitor voltage fluctuations. Pulse width modulation techniques are widely employed for the synthesis of ac voltages at the terminals of a voltage sourced converters. The new level-shifted PWM method presented in the thesis offers reduced switching frequency of the cells, no need to calculate the duty cycles based on the reference waveforms as well as easy real-time implementation in a DSP. This research also contributes toward introducing circuit arrangements to start-up the converter from a de-energized condition without adding auxiliary voltage source. Some modification in the existing circuitry for dc-ac power conversion is also explored where a transformer has been introduced inside the MMC. The proposed circuit helps to reduce the voltage rating of the power devices and the capacitors to half when compared to the conventional MMC. Additionally, the transformer also helps in limiting the circulating current. An improved decoupling current controller is proposed that seeks to tackle some drawbacks of the conventional d-q current controller, while keeping the successful reference tracking and disturbance rejection. One of the specified objectives of this work is to develop and implement an efficient and robust capacitor voltage observer. The observer will work as a software sensor working in parallel with the hardware measurement part. It could be used for condition monitoring for capacitors, predictive control, fault detection and parameter estimation. All concepts and proposed approaches in this thesis have been analyzed and validated through simulations and experimental results, as relevant issues are discussed. Finally, this thesis addresses the software and hardware design of two laboratory setups for four-level MMC-based inverter as part of a project. These setups can be used effectively to test the above mentioned ideas as well as enable them for future practical use. The control circuit will be built based on DSP and FPGA platforms which are utilized to produce the gate control signals.