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Control of an Islanded Multi-DG Microgrid under Unbalanced and Nonlinear Loads Conditions

Hamzeh, Mohsen | 2013

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 44131 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Mokhtari, Hossein; Karimi, Houshang
  7. Abstract:
  8. Nowadays, the use of distributed generation (DG) units and microgrid systems is mainly limited to their operation in the grid-connected mode. In the grid-connected mode, a microgrid including its loads and DG units is connected to the main grid at the point of common coupling (PCC). In this case, the main grid provides fixed voltage and frequency for the microgird and each DG unit regulates its power components based on the conventional dq-current control strategy. Autonomous (islanded) operation of a microgrid, however, is not as straightforward as its operation in the grid-connected mode, i.e., control and power management of an islanded microgrid is very difficult due to the stochastic nature of the renewable energy sources that act as the prime movers for the DG units. A 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. To regulate the reference voltage, a control strategy is presented. the controller has PR structure and the optimal parameters of this controller are obtained using of PSO algorithm. New droop-characteristics are presented to improve the performance of the microgrid in terms of power sharing, voltage regulation, dynamic and transient responses, reduces the reactive power sharing dependency on real power and system parameters, and smoothes the system’s dynamic and transient responses. This work has developed the modeling, control parameters design, and power-sharing control starting from a simplified small signal model of a single voltage source inverter to a number of interconnected DG units forming a flexible microgrid. The proposed control scheme of each DG unit consists of a multi proportional resonance controller (MPRC) with adjustable resonance frequency, a droop strategy, a harmonic impedance controller (HIC), and a virtual negative-sequence impedance controller (VNSIC). The MPRC and droop strategies are, respectively, used to regulate the load voltage and share the average powers among the DG units. The HIC and VNSIC are proposed to effectively share the harmonic and negative-sequence currents of the nonlinear and unbalanced loads among the DG units. The presented strategy reduces the harmonic and negative-sequence currents in the MV lines and thus, improving the power quality of the MG. The performance of the proposed control scheme is verified by using digital time-domain simulation studies in the PSCAD/EMTDC software environment. Moreover, this thesis presents control of a MV microgrid considering multi-hybrid distributed generation systems (MHGS) and renewable energy resources. To guarantee excellent power management, a modular power conversion system is realized by parallel connection of small MHPCS units. The hybrid system includes fuel cells (FC) as main and supercapacitors (SC) as complementary power sources. The SC energy storage compensates the slow transient response of the FC stack and supports the FC to meet the grid power demand. The proposed control strategy of the MHPCS comprises three control loops; dc-link voltage controller, power management
  9. Keywords:
  10. Microgrid ; Voltage Control ; Nonlinear Load ; Dispersed Generation ; Proportional-Resonance (PR)Controller ; Droop Method ; Unbalance Loads ; Vitual Negative-Sequence Impedance

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