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Robust Hierarchical Control of AC Microgrids

Mahdian Dehkordi, Nima | 2016

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 49193 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Sadati, Naser; Hamzeh, Mohsen
  7. Abstract:
  8. New nonlinear voltage and current control strategies based on backstepping control and a high-order sliding mode differentiator for microgrid are proposed. The microgrid consists of multiple distributed generation (DG) units with an arbitrary configuration that can be parametrically uncertain or topologically unknown. The proposed method robustly regulates the microgrid voltages and currents in the presence of parametric uncertainties, unmodeled dynamics, load imbalances, and nonlinear loads with harmonic/interharmonic currents. In contrast to existing methods, the proposed method does not need to know the frequency of harmonic and interharmonic current of microgrid loads that lead to the reduction of the steady-state error of the controllers in the frequency of unknown harmonics and interharmonics. In the next step, we address the robust stability analysis for an islanded microgrid with droop-controlled inverter-based DGs. Due to large load changes, microgrid structure reconfiguration, and higher power demands, the low-frequency (LF) dominant modes of a microgrid stir toward unstable zone, and make the system more oscillatory or even unstable. In this study, a robust two-degree-of-freedom (2DOF) decentralized droop controller, which is the combination of the conventional droop with a robust transient droop function, is utilized for each inverter-based DG unit. Unlike conventional 2DOF droop controllers, a new design procedure is proposed to robustly determine the transient droop gains to effectively damp the LF oscillatory modes of the islanded microgrid irrespective of disturbances, equilibrium point variations, and uncertain parameters of a microgrid. To mitigate the LF power oscillations at different microgrid conditions, inspired by Kharitonov’s stability theorem, a robust D-stability analysis is performed to determine the specific ranges of the transient droop gains to provide a robustness margin for the disturbances, equilibrium point variations, and uncertain parameters of the islanded microgrid. To remove voltage and frequency deviation caused by droop technique, a distributed, robust, finite-time secondary control for both voltage and frequency restoration of an islanded microgrid with droop-controlled inverter-based DGs is proposed. The distributed cooperative secondary control is fully distributed (i.e., uses only the information of neighboring DGs that can communicate with one another through a sparse communication network). In contrast to existing distributed methods that require a detailed model of the system (such as line impedances, loads, other DG units parameters, and even the microgrid configuration, which are practically unknown), the proposed protocols are synthesized by considering the unmodeled dynamics, unknown disturbances and uncertainties in their models. The consensus-based distributed controllers restore the islanded microgrid’s voltage magnitudes and frequency to their reference values for all DGs within finite time, irrespective of parametric uncertainties, unmodeled dynamics, and disturbances, while providing accurate real power sharing. Moreover, the proposed method considers the coupling between the frequency and voltage of the islanded microgrid. Unlike conventional distributed controllers, the proposed approach quickly reaches consensus and exhibits a more accurate robust performance. Finally, the MATLAB/SimPowerSystems toolbox has verified the proposed control strategy’s performance
  9. Keywords:
  10. Adaptive Control ; Robust Control ; Feedback Linearization ; Distributed Generation ; Microgrid ; Backstepping Algorithm ; Fully Distributed Control

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