Research On Control And Small Signal Stability Of Three-phase AC Microgrid

Global energy and environment crisis are driving countries all over the world to find alternative energy sources in order to substitute for traditional fossil fuel. Renewable energies such as photovoltaic and wind power are attracting more and more attention in recent years. Appropriate utilization of renewable energies is a beneficial supplement of current fossil fuel, and can be a prospective solution of energy crisis. Microgrid is proposed to integrate all kinds of renewable generation and energy storage elements, supplying stable, reliable and high quality power to customers. Stability is the most important to operate a microgrid. Therefore, small signal modeling is a hot point in the microgrid research fields. Based on the existing research, a microgrid small signal stability analysis method using elements equivalent models is proposed, and this method is applied to analyze the small signal stability of both radial microgrid and meshed microgrid. Simultaneously, voltage harmonics suppression of islanding microgrid with nonlinear loads through the control of inverter’s output impedance is proposed. Furthermore, grid voltage and frequency feedforward is proposed to stablize the power flow between the microgrid and the main grid.Firstly, equivalent models on synchronous reference frame of droop regulated voltage source inverters, loads and line impedance as well as the grid are derived as Thevenin equivalent models, impedance models and voltage source model. Three-phase AC microgrid equivalent model on synchronous reference frame is obtained based on the elements equivalent models, and closed loop power characteristic equations of each elements in the microgrid can be developed. Consequently, eigen values can be solved from the power characteristic equations under certain parameters, and the eigen values distribution can be used to predict the system dynamics and small signal stability. Furthermore, root locus of the system can be obtained by changing a certain parameter when calculating the eigen values, and this method can be used to observe how this parameter influences the system dynamics and small signal stability. Based on this method, both grid connected and islanded microgrid are analyzed using a single bus microgrid as an example, and closed loop power characteristic equation and eigen values are obtained as well as the root locus. Secondly, the proposed microgrid modeling method derives the closed loop power characteristic equations from basic circuit theorems with the equivalent models of the elements on the synchronous reference frame. What’s more, the system matrix scale is related to the number of inverters and independent to the microgrid structure. Therefore, the proposed method is quite appropriate for solving complex-structured microgrid system. Additionally, meshed microgrid is rarely discussed in the current literature. Consequently, a generalized meshed microgrid is proposed, and the modeling method proposed in this dissertation is adopted to analyze the meshed microgrid, develop the closed loop power characteristic equations, and solve the eigen values and root locus. Furthermore, the deduction of other microgrid structures from the proposed generalized meshed microgrid is given as well.Then, the output impedance of droop regulated voltage source inverter is derived, and the impact of the output impedance on the voltage harmonics of islanded microgrid is analyzed. When the microgrid is in islanded mode with nonlinear loads such as diode bridge rectifiers, the harmonic current will lead to harmonic voltage drop across the inverter’s output impedance and finally lead to the output voltage distortion, deteriorating the power quality. A voltage harmonic suppression algorithm which controls the output impedance of the inverter is proposed to improve the islanded microgrid power quality. Furthermore in grid-connected mode, the power injected into the main grid fluctuates along with the grid voltage and frequency, thus the power flow cannot be controlled accurately. Grid voltage and frequency feedforward is proposed in the dissertation to stablize the power flow between the microgrid and the main grid.In this dissertation, the proposed microgrid modeling method can be applied to solve both grid connected and islanded microgrid, and it’s also suitable for complex-structured microgrid. The derived closed loop power characteristic equations and root locus provides a new way of inverter’s control parameter design and microgrid small signal stability analysis.Finally, the design of an RT-LAB based three-phase AC microgrid experimental bench is introduced, including three2kVA voltage source inverters. RT-LAB is employed as the rapid control prototype and can be used to validate the microgrid modeling and voltage harmonic suppression discussed in this dissertation. What’s more, this experimental bench can be used in the research of different structure microgrids with all kinds of loads.