Control contributions to ac microgrid inverters

This thesis is focused on the microgrid control framework. Specifically, it is concentrated on alternative current microgrids. As a result of the author involvement in different industrial projects concerning microgrids during the last years, the goal of the thesis is to apply and expose the acquired experiences for exploring new control possibilities. The followed methodology is detailed in the next lines. Primarily, the thesis presents two chapters whose purpose is to introduce the concept of operation and to analyse the context and challenges posed by microgrids from a top to bottom approach. The first chapter is oriented in general terms while the second one devotes more efforts in the alternative current microgrids particular case. After identifying the different actors that play an key role in the alternative current microgrids it is disclosed that the inverter becomes a key element. In this sense, the first two chapters allows to pave the way addressing how an inverter installed in an alternative current microgrid can be considered and operated. In this direction, the named grid-tied operation mode reflects the most complex configuration. Considering that the scope of the thesis is focused on voltage source inverters, the grid-tied configuration may not only consider the grid-connected or grid-disconnected operation mode but also the transients between them. Thus, important challenges such as the inverter operation itself, the anti-islanding detection capability according to the behaviour of the inverter and the galvanic isolation restriction in a microgrid framework are dealt. The control of the voltage source inverters operated in an alternative microgrid results crucial. For this reason, a full chapter carries out a study proposing and delving into various type of controllers according to different possibilities exposed in the literature. It is conducted a stability analysis for stationary reference frame resonant current and voltage controllers. The chapter concludes with a new hybrid formulation based on fractional calculus and conventional resonant control. This novel formulation allows to enhance the controllable frequency bandwidth range decreasing inter-harmonic excitations and reducing the controller order in respect with other classical formulations. The fourth and fifth chapters are involved into discussing and presenting different operation options when the inverter is considered as a current or as a voltage source. The fourth chapter poses over how to tackle the problem of transients when a current source grid-tied inverter is assumed while grid-connected operated. Two situations are studied; the flying transference from current to voltage source and how to extend the maximum power point extraction when a back-up system supports the inverter operation. A set of different scenarios conducted in simulation and in a real experimental platform complements the two considered situations. The fifth chapter analyses the operation of the inverter from an opposite perspective. The considered inverter is operated by using a hybrid combination between alternative current droop control strategy when grid-connected and voltage-frequency control when grid-disconnected. In this way it is possible to preserve dynamics capabilities in both operation modes meanwhile the inverter is treated as a voltage source in any case. A predominant resistive model applying virtual resistance is considered for the case of alternative current droop control. For both operation modes an LCL-type coupling filter is assumed, complementing the control strategy with an active damping mechanism to mitigate possible resonances. Then, a three-phase four-wire four-leg inverter is proposed to match better with a proper approximation to an AC microgrid with over-load supervisor when grid-connected and sinusoidal short-circuit proof when grid-disconnected operated