Advanced Control Strategies for Voltage Source Converters in Microgrids and Traction Networks

Increasing concerns regarding global warming caused by greenhouse gases, which are mainly generated by conventional energy resources, e.g., fossil fuels, have created significant interest for the research and development in the field of renewable energies. Such interests are also intensified by the finitude availability of conventional energy resources. To take full benefit of renewable energy resources, e.g., wind and solar energy, interfacing power electronics devices are essential, which together with the energy resources form Distributed Generation (DG) units. If properly controlled and coordinated, the optimal and efficient operation of DG units, which are the main building block of rapidly emerging microgrid technologies, can be ensured. In fact, the optimal and efficient operation of any energy conversion systems, e.g., microgrids, traction networks, etc., necessitates some sorts of control strategies. Being structured into two main parts and exploiting two-level Voltage Source Converters (VSCs), this thesis introduces several control strategies in the context of microgrids and electrified traction networks. Although the proposed approaches of this thesis are mainly tailored for two-level VSCs, the methods are equally applicable to other converter technologies. In the first part, adopting an optimization-based loop shaping approach, a vector current control strategy for three-phase grid-tied VSCs is proposed. The proposed control strategy is able to independently regulate the direct and quadrature (dq)-components of the converter currents in a fully decoupled manner and shows very fast dynamic response similar to the existing methods. In order to extend the applicability of the proposed vector control method to single-phase systems, a countermeasure is also proposed. In single-phase systems, to form the orthogonal component of the current needed to create the dq-axes, the converter current is phase-shifted a quarter of a fundamental period. This phase-shift is the reason of strongly coupled dq-axes and oscillatory dynamic response in such systems. To obviate the need for the problematic phase-shifting, adopting a Fictive Axis Emulator (FAE), the orthogonal fictive current is created concurrent to the real one. In such a case, utilizing the proposed decoupled vector control strategy and the FAE, the dq-currents of single-phase converters are also regulated in a fully decoupled manner. Moreover, in this part, using a generalized version of the optimization-based loop shaping approach, three voltage control schemes are proposed for the voltage regulation of islanded microgrids. Since the dedicated loads of islanded microgrids are not fixed, the loop shaping is simultaneously carried out for various operating points of interests, i.e., for various combinations of the load parameters. Two single-stage control strategies and a cascade one are proposed: (i) a single-stage PI-based Multi-Input Multi-Output (MIMO) controller, (ii) a single-stage PI-based MIMO controller in conjunction with resonant terms, which is able to compensate for the adverse impacts of nonlinear loads, and (iii) a cascade PI-based MIMO controller. The cascade control scheme utilizes the proposed decoupled vector control strategy as its inner loop for regulating the converter current. In the second part, this thesis focuses on electrified traction networks and addresses a power quality problem in such networks, i.e., catenary voltage fluctuations. The Active Line-side Converter (ALC) of modern locomotives is utilized as STATic COMpensator (STATCOM) in order to inject reactive power to compensate for the adverse effects of catenary line voltage fluctuations. To determine the proper amount of reactive power, several control strategies belonging to the PI-controllers family are proposed: (i) a P-controller, (ii) a PI-controller, and (iii) a gain-scheduled PI-controller. Among the proposed approaches, the gain-scheduled strategy provides the best performance. The gain-scheduling is performed through identifying the catenary inductance at the connection point of the locomotive to that. The inductance identification is carried out by the injection of harmonic current through the ALC and monitoring its effect on the locomotive voltage. Despite its acceptable performance, the gain-scheduled approach shows several shortcomings. Therefore, utilizing the optimization-based loop shaping technique, a high-order voltage support scheme is also proposed. The proposed high-order scheme does not need any online tuning and/or modification while provides excellent performance for various operating points.

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