| dc.description.abstract | This thesis explores the challenges and opportunities associated with integrating offshore wind turbines into a geographically compact power-intensive isolated electric grid that is currently supplied by gas-powered synchronous generators. To explore the best power and energy characteristics of different energy storage devices, a hybrid energy storage system is evaluated as the means of mitigating the negative effects of wind intermittency. In this hybrid system, batteries are employed as fast energy storage devices providing inertial and primary frequency control reserves, whereas a pair of electrolyzers and fuel cells are used as slower secondary reserve providers. These storage devices are coupled via dc/dc converters to a common dc link which, in turn, is connected to the isolated grid’s main ac highvoltage busbar through an active front-end converter. Large converter-interfaced flexible loads in the isolated grid are also expected to act as primary power reserve providers.
In addition to frequency control problems introduced by non-synchronous intermittent renewable energy sources, the power electronic converters of wind turbines, flexible loads, and energy storage systems tend to operate as constant power devices. This is known to cause stability issues in micro and large grids. Within this context, three goals have been defined for this PhD research: (1) identify and address causes of converter-induced instabilities, (2) propose robust control strategies to exploit efficiently existing and new assets added to the grid, and (3) assess and validate experimentally in the laboratory the proposed control strategies. These goals led to five research questions, namely: (1) which phenomena may lead to converter-driven instabilities and how can one mitigate them, (2) how can one build a robust control structure for regulating frequency in the time scale of seconds to minutes and for providing reactive or voltage support, (3) how can one share fast power reserves among traditional generators and converter-interfaced sources in the time scales of seconds to minutes, (4) what are the consequences of shifting primary frequency control reserves from slower synchronous generation to faster converter-interfaced ones, and (5) how can one properly reproduce the phenomena under study in a reduced-scale laboratory setup.
The pursuit for answers to these research questions led to a number of contributions to the scientific literature, which were presented at academic conferences and published in journals. Among them, the five main contributions compiled in this thesis are as follows: (1) a set of control structures for energy storage systems for providing inertial, primary, and secondary power reserves, (2) identification and analysis of the root cause of an oscillation phenomenon occurring in power converters operating with dual rotating reference frames, (3) the description of a sequence separation method applied to the direct and quadrature axes of the rotating reference frames, (4) the expansion of a requirement for frequency reserves stemming from interconnected systems and application of this extended concept to an isolated grid dominated by constant power loads, and (5) a method for matching preexisting reduced-scale laboratory converters to specific characteristics of real-life large power converters.
In conclusion, this thesis sheds light on the complex dynamics and interactions that arise when non-synchronous converter-interfaced renewable energy sources are connected to existing power-intensive isolated electric grids previously dominated by traditional synchronous generation. | en_US |
