Modelling and simulation of an insular grid with the presence of High Voltage Direct Current

Insular electrical grids, usually isolated from larger mainland grids, are more prone to having stability issues. These are usually weak and have limited resources in regards to power sources, which adds up with the fact that this type of grids usually lack access to the electricity market. High Voltage Direct Current (HVDC) technology presents the possibility of long distance overseas connections that alleviate the effects of these issues. Use of HVDC technology is on the rise, its main focus being long distance transport of power. Its effciency and the stability it provides to grids, paired with the aforementioned need for overseas connections and off-shore wind generation have helped it become as common as it is nowadays. The aim of this thesis is to study the behaviour of an insular grid with the presence of HVDC and the needed power converter for the HVDC connection. The behaviour of the system with and without the converter's frequency support will be analyzed. A case based on the Majorcan 220kV grid (Balearic Islands, Spain) with an HVDC connection representing a link with the mainland is used to analyze the behaviour of an insular grid with and without the frequency support. HVDC presence in the grid is represented by a converter modelled as seen in ”Active and Reactive Power Control of Grid Connected Distributed Generation Systems” [1]. The tools used for modelling and simulating are MatLab® 2015b and PSCAD 4.6 EDU. MatLab® is used for running steady state simulations of the grid's behaviour and for initial modelling of the VSC converter. For the first, the MATPOWER X.Y package is used. In regards to the former, the Simulink environment is used. PSCAD is used for dynamic analysis of the grid, with and without the power converter's grid frequency support. Comparison of the MatLab® 2015b and PSCAD 4.6 EDU simulations show that the dynamic model behaves as it is expected from it. There are no relevant differences from the dynamic and the static models' results. The study of the dynamic behaviour of the system in front of a fault shows that the HVDC connection's grid frequency support alleviates the negative impact that said events have on the grid's frequency. This is done while also reducing the amount of power that needs to be generated with the limited local resources

Efficient Computational Methodologies for Multi-Objective Optimization of Distributed Energy Resources (DER) Inverters

The paralleling of power converters connected to the grid for power-sharing is a widely used technique. In this context, the design framework for a low-cost, lightweight, compact, and high-performance optimum configuration is an open research problem. This thesis proposes an innovative Multi-Objective Hierarchical Optimization Design Framework (MO-HO-DF) for an Alternating Current (AC) grid interface with N interleaved H-bridges, each with M parallel ``to-be-determined'' switches, connected through coupling inductances (Lf). A total of eight Figures of Merit (FOMs) were identified for the design framework optimization. A rigorous model of the power electronic system is presented. Next, a highly computationally efficient algorithm for the estimation of the required frequency modulation ratio (mf) to meet current harmonic performance requirements for any given configuration is proposed. Then, the concept and implementation of the algorithm are presented for the MO-HO-DF. The effectiveness of the design optimization framework is demonstrated by comparing it to a base case solution. Finally, the design calculations are validated via Piecewise Linear Electrical Circuit Simulation (PLECS) software with manufacturer-provided Three-Dimensional (3D) power semiconductor models that include thermal modelling. In particular, when an H-bridge is interfaced with a single-phase grid, it requires controllers to regulate the voltages and currents in the system. In this context, the static optimization of controllers responsible for Direct Current (DC) bus voltage regulation and AC regulation, considering time-domain and frequency-domain behaviours, is an open research problem. Firstly, this thesis proposes a method to obtain FOMs with the use of inbuilt functions in MATLAB software. Then for the Type-II Proportional+Integral (PI) controller, a single-variable two-objective convex optimization is proposed. Next, for the Proportional+Multi-Resonant (PMR) controller, three-variable five-objective convex optimization is proposed. The design of the PMR controller is a multi-variable problem that can inherit the principles of a hierarchical framework and leverage the effect of a design variable on the final optimization result. Thus, the work on PMR controller design optimization is extended to a three-level hierarchical design framework and evaluates all six possible paths for optimization. Finally, enhanced macro-model-based MATLAB simulation results are provided to verify the performance of controller designs and generate statistical insights