Power management and control of hybrid AC/DC microgrids integrated with renewable energy sources and electric vehicles
thesisposted on 28.03.2022, 19:20 by Md Shamiur Rahman
Microgrids are the building blocks for the next generation power grid, the so-called `Smart Grid'. They facilitate the integration of various distributed-generation (DG) units such as electric vehicles (EV), diverse energy storage systems (ESS), and renewable energy resources (RER) utilizing intelligent forecasting, Information and Communication Technology (ICT) and control infrastructures to achieve active consumer participation, augmented network reliability, reduced expansion cost, and self-healing capabilities. Due to the binary nature of electricity i.e. alternating current (AC) and direct current (DC), microgrids are broadly classiffed as AC microgrids and DC microgrids. However, to comply with the legacy AC system and to interface with the growing DC technologies, lately, coupled AC and DC microgrids or hybrid AC/DC microgrid structures are gaining momentum.The benefit of this structure is that it can accommodate both AC and DC loads and generators instantaneously with minimum power-electronics-based losses. In addition, this structure is suitable for integrating distributed storages such as the emerging electric-vehicle energy-storage systems (EV-ESS) for vehicle-to-grid (V2G) applications. EV storages can effectively improve the overall performance of a hybrid microgrid in terms of voltage and frequency regulation,system stability, active and reactive power support and fault robustness. However, the optimized coordination of EV storages within microgrids is an intricate issue due to their different control and configuration structures along with inadequate standards regarding V2G applications. The centralized and distributedcontrol structures are viable options to coordinate spatially dispersed EV storages within microgrids of different geographical sizes. Consequently, this Ph.D. thesis presents three contributions in the area of microgrid control techniques and V2G application within microgrids. The first contribution of this research is the design and implementation of an improved three-layered centralized coordinated control strategy considering EV availability constraints for three-phase (3P) and DC type EV-ESSs to improve the operation of a hybrid AC/DC microgrid. The first layer of the algorithm ensures DC subgrid management, which includes DC bus voltage regulation and DC power management. The second and third layers are responsible for the AC subgrid management, which includes AC bus voltage and frequency regulation with active and reactive power management. The multi-layered coordination is embedded into the microgrid central controller (MGCC) which controls the interlinking controller in between the AC and DC subgrids as well as the interfacing controllers of the participating EVs and distributed RER. The second contribution of this research is to develop a new need-based distributed coordination strategy (NDCS) for multiple EV storages in an islanded commercial hybrid AC/DC microgrid with extended geographical size. The control capacity of the interlinking converter is enhanced by incorporating combined power-droop and voltage-droop strategies to leverage the coupling of AC and DC voltages. Therefore, the AC bus voltage can be regulated simultaneously by regulating only the DC bus voltage without affecting the power-sharing capabilities of the converter. The NDCS is proposed to coordinate the EV storages to regulate the DC bus voltage. The main objective of the NDCS is to decide whether the coordination of the available EV storages is to be performed in a decentralized or a distributed manner. The mathematical model and the algorithm to deploy NDCS are developed to realize its application to a real system. The final contribution of this research is to establish an optimized distributed controller for coordinating EV storages within microgrids. An optimizer is incorporated with the previously developed distributed controller for EV storages. An economic dispatch problem is solved in real time with the optimizer to minimize the output power-generation cost. The optimizer adjusts the power setpoint for each EV, which ensures proper power management within the microgrid. As a result, a cost-effective distributed V2G operation can be ensured. The hybrid AC/DC microgrid and its extended version are designed in a MATLAB/Simulink environment resembling the microgrid under construction at Griffth University, Australia. Extensive case studies are performed considering real-life solar irradiation, commercial load profiles, EV time delay, and EV plug and-play and fault conditions etc. to validate each proposed control scheme. Additionally, the performance of the controllers is compared with the conventional controllers. The results of the case studies demonstrate the efficacy of the overall system in terms of improved transient response, fault-robustness, scalability, cost effectiveness and reliability.