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Robust Control of an Islanded Microgrid Consisting of Parallel Connection of Multiple DG Units

Babazadeh, Maryam | 2011

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 42135 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Karimi, Houshang
  7. Abstract:
  8. A robust decentralized control strategy for the islanded operation of a microgrid is proposed in this thesis. The microgrid consists of the parallel connection of several electronically-coupled distributed generation (DG) units. All DG units are connected to a point of common coupling (PCC) where a passive load is also connected. It is shown that the islanded microgrid can be modeled by an interconnected composite system comprising equal number of subsystems (to the DGs). Moreover, it is shown that the overall islanded microgrid can be controlled by only the local controllers about the individual subsystems. In this case, one of the DGs, referred to as the Master DG, is responsible for voltage and frequency control, and the remaining DGs, referred to as the Slave DGs, utilize the dq-current control strategy to regulate their own power components. The microgrid model is parametrically uncertain as the load parameters can vary within the prespecified limits. However, all the load parameters can be included in the dynamic model of the Master DG subsystem. To achieve robust performance in presence of parametric uncertainties, a Linear Fractional Representation (LFR) of the Master subsystem is first obtained. Then, a robust controller is designed using the -synthesis. To enhance the robustness of the control system, an alternative approach in the context of real convex polytopic uncertainties is presented based on an improved bounded real lemma. The designed controller demonstrates the excellent robust performance for the microgrid system with a wide range of load perturbations. Finally, a new robust control strategy is presented for the general case of unknown load model, i.e., the load is assumed to be parametrically and topologically unknown. Therefore, there are unmodeled dynamics in the microgrid model. In this case, the control system is characterized by a two-degree-of-freedom (2DOF) control system consisting of a feedback and a feedforward controller. The optimal solution of the 2DOF control strategy is obtained using the linear matrix inequalities (LMIs). Simulation case studies carried out in ATLAB/SimPowerSystems Toolbox show that the proposed control scheme is highly robust against the uncertainties in the load dynamics.
  9. Keywords:
  10. Dispersed Generation ; Islanding Operation ; Decentralized Control ; Linear Matrix Inequality (LMI) ; Polytopic System

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