Control strategies formenhanced dynamic response in hybrid ac/dc microgids considering enery storage integration

Abstract

Abstract The goal of minimizing the fossil fuels dependency in the electrical sector, has triggered the search for alternatives to conventional generation systems based on renewable distributed generation. Those necessities have led to the definition of the microgrid concept, that has been presented in three main topologies during the last years: AC, DC and hybrid AC/DC microgrids. However, the penetration of power electronic converters, renewable energies, reduced size generators, and tightly regulated loads, involve new challenges in this kind of grids, that can compromise their reliability and robustness. This thesis is focused on the dynamic control of hybrid AC/DC microgrids by promoting new strategies for an improved dynamic response. The proposals in this work includes solutions for the control of grid-tied converters, the primary control of hybrid AC/DC microgrids with high penetration of power electronic converters and constant power loads, paying special attention to the role of interlinking converters, the compensation for the lack of inertia that characterize this kind of grids, and provide tools to address the delay and latency in frequency/phase estimation and the estimation of grid impedance, useful for its application in the converter/primary control, active power compensation, protections or power quality. The research includes several contributions and proposals in the fields of grid-tied converter control and hybrid AC/DC microgrids: 1) An observer-based current control for LCL in grid-tied 3-phase converters is designed for avoiding the grid-side current sensors. 2) An online grid impedance estimation method is developed based on pulsed signal injection. The method presents a reduced computational burden and total harmonic distortion when compared to other alternatives, being valid for different applications in the field of microgrids (islanding detection, fault location, adaptive control...) 3) The dynamic voltage control of 3-phase AC and DC converters is extensively analyzed under a high penetration of constant power loads. Their adverse effects in conventional feedback voltage control based on proportional-integral regulators is analyzed, evincing the benefits of a novel alternative quadratic-based voltage control, not used before in AC applications. 4) An active power compensator in 3-phase AC microgrids based on an energy storage system, suitable for improving the transient response for the frequency and voltage magnitude by providing virtual inertia or virtual capacitance respectively. An enhanced Luenberger observer-based frequency drift compensator is devised, able to operate continuously or only during transients and overcome the issue of frequency estimation delay. The solution allows a reduction of the compensator phase lag, improving the initial transient compensation when compared with previous proposed methods. 5) A predictive sequence estimation method based on the sliding Goertzel transformation, that improves the dynamics of frequency and phase estimation under highly distorted grid conditions. The estimator can be employed for different AC applications (synchronization, fundamental control, harmonic compensation...) The proposed method can estimate the grid phase, the positive and negative sequences and the harmonics components with an improved dynamic response when compared to state-of-the-art alternatives. 6) A decentralized control scheme is presented for the regulation of DC grids based on DC virtual generators combined with DC P/V droop control. The proposed method allows to adjust the contribution of each unit to the overall inertia and to the steady state power sharing, extending the virtual synchronous generator concept to DC applications. 7) Two different primary control strategies are defined for a hybrid AC/DC microgrid with integrated energy storage systems, composed by a DC bus and several 3-phase AC subgrids, one based on voltage-controlled grid-forming master-slave in both DC and AC sides, and the other based on DC virtual generators and virtual synchronous generators. 8) Two enhanced dynamic control alternatives for hybrid AC/DC microgrids, exploring the cooperative active power sharing between the DC and the AC subgrids. The first one is based on an adaptive power sharing algorithm and the second on a novel hybrid AC/DC virtual generator scheme. Both solutions aim for a reduction in the dependence from the utility grid and improve the response in the DC subgrid under low inertia conditions. The proposed control strategies have been verified both by simulation and by experimental tests. Practical validation of the conducted work has been carried out in a hybrid AC/DC lab microgrid developed in the facilities of the LEMUR group in the University of Oviedo, Spain, and in the facilities belonging to the PEMC group in The University of Nottingham, UK. El creciente interés por minimizar la dependencia de combustibles fósiles en el sector eléctrico ha desencadenado la búsqueda de alternativas de generación basadas en energías renovables. Este objetivo ha llevado a la definición del concepto de microrred, que se ha presentado principalmente en tres topologías durante los últimos años: microrredes AC, DC e híbridas AC/DC. Sin embargo, la alta penetración de convertidores electrónicos de potencia, generación renovable, y cargas electrónicas, implican nuevos retos en este tipo de redes, que pueden comprometer su fiabilidad y robustez.