The student Diego Alexis Aragon Sotelo obtained an OUTSTANDING CUM LAUDE qualification
The student Diego Alexis Aragon Sotelo obtained an OUTSTANDING CUM LAUDE qualification
The student Diego Alexis Aragon Sotelo obtained an OUTSTANDING CUM LAUDE qualification
Thesis title: Control methods and stability analysis of electricity networks strongly dominated by electronic power converters
Court:
- Chairmanship: Oriol Gomis-Bellmunt (Universitat Politecnica de Catalunya)
- Vocal: Jef Beerten (KU Leuven)
- Vocal: Adolfo Anta (AIT Austrian Institute of Technology)
- Vocal: Maider Santos Mugica (TECNALIA)
- Secretary: Gonzalo Abad Biain (Mondragon Unibertsitatea)
Abstract:
Contemporary society faces significant energy challenges due to the accumulation of greenhouse gases in the atmosphere. This scenario has been driven mainly by the use of fossil fuels for energy generation, population growth and exponential technological development. In response, the scientific community has focused its efforts on reducing dependence on fossil fuels in the energy matrix. In this direction, new non-conventional energy sources, such as wind, solar photovoltaic, and geothermal, have been integrated into the power system.
The integration of such renewables into the electricity system is progressively replacing synchronous machine-based generation, resulting in a transformation of the electrical system. The reason is that most non-conventional renewables are connected to the grid via electronic converters, eliminating the rotating components of the system. Converters have the advantage of controlling the bi-directional power flow between the grid and the energy source, thereby reducing power losses and thus providing precise control of the non-conventional renewable sources. Conversely, the increasing integration of such renewables is causing new challenges for the system operation by manipulating the various system dynamics. Consequently, this transformation in the power system is leading to a reduction of the total mechanical inertia supplied by the rotating elements of the synchronous machines. Likewise, the fast converter dynamics means that the classical small and large signal stability studies based on phasor models do not represent the true behaviour of the converter-dominated networks. The reason is that the dynamics of the inverters interact with the grid’s passive elements, i.e., transmission lines and loads— endangering the system’s stability.
Inspired by these challenges, the main objective of this thesis is to analyse the stability and control of the electrical networks strictly dominated by the converters, ensuring their correct operation in terms of transient response and steady state, and avoiding adverse interactions between the converter and the grid. To achieve this objective, initially, a methodology for analysing small- and large-signal stability through accurate models of the inverter-dominated networks using electromagnetic models (EMT) is discussed. Subsequently, the grid-supporting control strategy called "Second-order filter-based inertia emulation (SOFIE)" is proposed to provide frequency support employing primary control, inertia emulation and oscillation damping. The SOFIE control is compared against the classical inertia emulation control in grid-supporting converters, demonstrating that the developed control solves several stability problems caused by the adverse interactions between the inverter controls and the LC resonances of the grid. Besides, a new control technique for hybrid ac/dc grids is proposed to improve the inertial response, primary control and damping of oscillations on both sides of the converter. It is worth mentioning that each of the control strategies developed in this thesis have been carefully designed to improve the stability limits of the power system,as well as to increase the number of converter services in the grid.
Among other contributions of this thesis, the small-signal stability based comparison of the various grid-forming control approaches under different network conditions is also highlighted. This class of converters demonstrate the need to improve the damping of power oscillations to increase the stability limits in stiff grids.