Flexible Universal Branch Model for Steady State Operational Analysis and Optimisation of Hybrid AC/DC Grids

Abstract

This thesis presents a new Flexible Universal Branch Model (FUBM) formulation for solving Power Flows, Optimal Power Flow (OPF) and Security Constrained Optimal Power Flow (SCOPF) for hybrid AC/DC grids. The prowess of the new formulation is that it (i) provides a direct link between AC and DC parts of the grid allowing for solving the entire network within a unified frame of reference (not sequentially) and (ii) can realistically model any element within the AC/DC power grid, ranging from conventional AC transmission lines to multiple types of AC/DC interface devices such as Voltage Source Converter (VSC) by introducing additional control variables. The model is formulated in such a way that it does not make a distinction, from a mathematical perspective, between AC and DC elements. Therefore, traditional AC power balance equations can be used to solve a complete AC/DC grid. Moreover, the physical attributes and optional independent variables of the FUBM encapsulate the characteristics and controls of other complex elements in the electrical power grid allowing a flexible analysis of a fully controllable AC/DC grid allowing the formulation to achieve operating points which will be infeasible otherwise. Detailed description of the FUBM is presented and compared to the traditional approaches. In comparison, traditional approaches require several model libraries and specific power balance equations per element as well as the type of grid to achieve the same results as the ones presented with the FUBM. Power flow formulation using FUBM also allows for the introduction of extra levels of control that may be available in a hybrid AC/DC grid. Similarly, the flexible OPF formulation takes advantage of the FUBM multiple control options to properly simulate flexibility of operation in these grids. Finally, the FUBM is implemented for the SCOPF to allow for corrective actions following contingencies for even more flexibility of operation. Post-contingency corrective actions include fast controls available
to the VSCs that are used to adjust the system operating point to withstand even the most severe contingencies. Throughout the thesis, the model was tested against several test systems and solvers. The results clearly show that the FUBM is not only on a par with existing models for steady state analysis but surpasses them in solution accuracy, computational efficiency and scalability for larger system sizes.