Author: Wen, Zihan
Title: Study of hierarchical control of microgrids
Advisors: Chan, K. W. Kevin (EE)
Degree: M.Sc.
Year: 2020
Subject: Microgrids (Smart power grids) -- Automatci control
Electric power systems -- Control
Hong Kong Polytechnic University -- Dissertations
Department: Department of Electrical Engineering
Pages: vii, 54 pages : color illustrations
Language: English
Abstract: The increasing greenhouse gas emission partly caused by fossil energy generation with the increasing demand for electricity, and the decreasing tendency of fossil sources have led to an upheaval of thinking about what future grids could look like. To alleviate the above problem, renewable sources become an integral part of the energy mix of electricity. One of the promising technologies is the concept of a microgrid, roughly defined as a cluster of wind turbines, solar PVs, energy storage and local loads. Microgrids have ability to manage local power balance and continue to provide power service to end-users in case of utility grid failures. The fundamental problem becomes how to safely and economically operate a microgrid. Therefore, this dissertation conduct investigation on the hierarchical control of microgrids. Hierarchical control generally includes three layers, i.e., primary control, secondary control, and tertiary control. Each control layer aims to achieve different objectives to ensure the reliable and stable operation of microgrids.
Based on the current status and dynamics of research on microgrids at this stage, this thesis mainly introduces the concepts of Distributed Generation (DG), the background of microgrid and its definition and characteristics, and hierarchical control of microgrids including some specific methods of each layer.
The control of the microgrid mainly lies in the control of the inverters. The research focus of the multi-inverter parallel microgrid is mainly focused on the rational distribution of load power, frequency and voltage stability, and harmonic suppression. Aiming at these problems, the thesis firstly analyzes the droop control. After analyzing the power distribution mechanism of parallel inverter in low voltage microgrid, the disadvantages of conventional droop control are pointed out as follows: the strict matching of the droop coefficient can only distribute the active power in proportion. Due to the influence of line impedance, it is difficult to accurately distribute reactive power. This method will result in a steady-state deviation of the system frequency and common AC bus voltage. In order to solve these problems, the dissertation establishes a hierarchical control for an islanded microgrid. In the primary control, the method of virtual impedance configuration is used to improve the reactive power distribution performance in the conventional droop control.
However, there is still the problem of reactive power allocation error relying on the primary control only, so the secondary control of reactive power adjustment is introduced. At the same time, the frequency and voltage deviation problems caused in the primary control are compensated and restored in the secondary control to improve the power supply quality of the system.
Finally, through the Matlab/Simulink software platform to simulate two micro-source systems under different working conditions, comparison and analysis with the simulation results of the conventional droop method, the effectiveness and accuracy of the hierarchical control are verified.
Rights: All rights reserved
Access: restricted access

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