Offshore hybrid power plants with wind energy, photovoltaic and energy storage

Abstract

Large centralized conventional power plants, although robust and reliable, have numerous disadvantages. The use of distributed generation has increased to fulfill the growing energy demand, and to reduce the distance between generation and consumption. Consequently, hybrid energy systems that combine several renewable energy sources are flowering, mainly in grid-connected configurations. The presence of this distributed generation in the electric grid changes its control and operation significantly. Therefore, special attention should be paid to the study of hybrid systems composed of wind turbines (WTs) and photovoltaic solar panels, due to their intermittent and fluctuation characteristics. Currently, a means of improving the connection of such systems to the electricity grid is to combine different energy sources with energy storage systems. Due to the development of storage technologies, they can be used in numerous applications, such as frequency regulation, system stabilization, reduction of transmission losses, increased reliability, load regulation or support for grid services, among others.The hybrid power generation systems used to date are mostly small-scale, with a rated power of tens or hundreds of kW. Another relevant aspect is that most hybrid power systems are located onshore. Nevertheless, a considerable growth of large-scale offshore wind farms is noticeable currently in Europe, mainly due to advances in WTs and foundation structures, which have improved their economic conditions and contributed to the implementation of offshore plants. It is expected that the installed capacity will continue to increase, since the European Union aims at reaching about 100 GW of offshore wind capacity by 2030. In this thesis, a detailed study evidencing this growth has been carried out. The most significant characteristics of 57 plants installed in Europe with a rated power above 150 MW and fully commissioned until 2019, as well as 11 plants authorized or under construction, are studied in detail, drawing relevant conclusions from the data collected. The results show the trends on WT size and capacity, turbine model, distance to shore, water depth, investment cost, type of foundation, transmission technology, and voltage array systems among others. This thesis gathers the latest information about the topic, deducing future trends from the evaluation of offshore wind farms fully commissioned, authorized or under construction.Furthermore, this thesis evaluates the performance of a hybrid power plant consisting of a WT, a solar photovoltaic (PV) power plant, and an energy storage system that share the same grid connection point. This hybrid power plant has a rated power in the range of several MW. Typically, permanent magnet synchronous generator-based WTs present a two-stage power converter topology based on a DC/DC boost converter and voltage source inverter. In the thesis, this configuration is substituted by a quasi-Z-source inverter, which is an attractive solution for boosting and converting the voltage from DC to AC in a single stage. A switched dynamic model of the quasi-Z-source inverter (including the modelling of all switches and firing pulses) is not recommended for steady-state stability studies, long-term simulations, or large electric power systems. For such studies, two averaged dynamic models are proposed in this thesis. Both models present the same control system as the switched dynamic model, except for the generation of the firing pulses, which is not necessary in the averaged models. The two models proposed are evaluated and compared with the switched dynamic model. Both proposed averaged models can substitute the switched dynamic model with satisfactory accuracy in terms of time-domain response in steady-state stability studies. In addition, two voltage sources emulating the terminals of C1 and C2 are added to enable the integration of PV and BES, respectively. The grid-connected hybrid power plant under study consists of a 1.5 MW WT, a 402 kW PV generator connected to the capacitor C2, and a 532 kW lithium-ion battery connected to capacitor C1, making a total 2.44 MW for the rated power of the hybrid plant. Moreover, a load of 1.2 MVA shares a common connection point with the hybrid power plant.After choosing the most suitable configuration for the connection of the WT, the photovoltaic power plant, and the energy storage system; a second objective of the thesis is the development of control strategies for the energy sources and their converters. Different control strategies are implemented and evaluated to regulate active and reactive power, and voltage levels. Finally, a third objective is the design of a supervisory control system for the hybrid power plant. This system must be able to manage and coordinate the energy flow between the devices in the hybrid system. For this purpose, some of the variables considered by the supervisory control system are the generation set point established by the grid operator, the instantaneous production of WTs and the photovoltaic panels, and the state of charge of the battery. Different control strategies have been developed for a proper energy management, and an adequate regulation of the electric parameters of the hybrid plant internally and at the point of connection to grid. Using widely recognized models for the components of the hybrid power plant, it has been possible to represent the behaviour of the system and to evaluate the original designs of this thesis through simulation under different operating conditions (changes in wind speed, solar radiation, or electricity generation of the grid, etc.). The simulation results have shown the adequate performance of the hybrid power plant through the control strategies implemented, which coordinate the renewable energy sources, the battery and the load using impedance source converters.