Navas Fonseca, Alex Dario
(2022)
A distributed, predictive secondary control for voltage and frequency regulation, economic dispatch and imbalance sharing in isolated microgrids.
PhD thesis, University of Nottingham.
Abstract
Microgrids (MGs) are the cornerstone for a new model of electrical generation and distribution based on renewable resources. However, managing the operation of an MG is a challenging and complex task due to the characteristics of the various types of renewable sources and interactions between different types of generating equipment. In this context, some of the most pressing problems in MGs are associated with guaranteeing a cost-effective operation and the quality of the supply. Therefore, new and more reliable control strategies need to be developed for the management of microgrids. Distributed model predictive control (DMPC) is one of the best solutions for MGs as it can model complex systems and simultaneously address multiple objectives.
Traditionally MGs have been controlled via a three-level hierarchical structure, where each level operates at a different time scale. The primary control level is the fastest and aims to maintain the stability of the MG and ensures correct power sharing. The secondary control level restores the variables modified by the primary control level. The tertiary control level is the slowest and aims for economic dispatch (i.e., aiming for the lowest monetary cost of generated energy) of the MG and correct coordination with the main grid. However, isolated MGs are prone to fast changes in generation and demand whilst having a slow time response at the tertiary control level. The latest research suggests that this control should be performed on a time-scale comparable to that used at the secondary control level. In addition, as the power references sent by the tertiary control level tend to be updated with a slower sample time, the power limits of distributed generators (DGs) can be exceeded.
Therefore, this thesis focuses on the application of DMPC schemes for the secondary control level for ac MGs and hybrid ac/dc microgrids (H-MGs - composed of an ac sub-MG and a dc sub-MG connected through interlinking converters (ILCs)). The main characteristics of the proposed methodologies are the use of novel multi-objective cost functions and prediction models that correctly represent the main dynamics of the DGs and the ILCs (in the case of H-MGs) in the formulation. Three control strategies are proposed that fulfil the main task of the secondary control level (i.e., restoring frequency and voltage). These strategies are able to restore the frequency and voltage to nominal values or within secure bands. The first proposed strategy considers the economic dispatch of DGs in a balanced ac MG. The second strategy achieves the economic dispatch of ac DGs, dc DGs and manages the power transference of ILCs based on an economic criterion in H-MGs. The third strategy manages the sharing of phase imbalance in an unbalanced ac MG. All the proposed strategies include important operating constraints, e.g., power limits due to convertor ratings.
Extensive experimental, real-time simulation and simulation studies validate the proposed DMPC schemes for the most common operating scenarios in MGs, namely, load changes, robustness in the presence of changes in microgrid structure (i.e., disconnection and reconnection of DGs and ILCs (in H-MGs)), and communication link failures and communication delays. Finally, the controllers’ scalability has been investigated, and comparative studies have also been performed to highlight the advantages of the proposed schemes over other reported distributed schemes.
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