Power quality management for grid-connected microgrid

• Main Author: Wang, Yizhu
• Other Authors: Tang Yi
• Format: Thesis-Master by Coursework
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Engineering::Electrical and electronic engineering
• Online Access: https://hdl.handle.net/10356/168256
• Institution: Nanyang Technological University

Distributed generation technology has been rapidly developed to enable large-scale development and utilization of renewable energy sources, since the global energy crisis as well as the environmental degradation has become more and more serious today. Microgrid is an effective way to coordinate the conflicts between distributed generation and the main power grid, ultimately improving the efficiency of distributed generation supply, and it has become a hot topic of research. Operating in both grid-connected and islanded modes, microgrids offer enhanced power supply reliability to loads. Within a microgrid, the majority of distributed energy sources and energy storage systems utilize inverters as their interface circuits. As a critical component of a microgrid generation system, microgrid inverters feature topologies similar to those found in power quality compensation devices, and their flexible and varied control strategies provide a convenient means for controlling the electric power quality within microgrids. Due to the asymmetry of the impedance of the microgrid lines and the presence of randomly distributed single-phase loads, the phenomenon of unbalanced output voltage from microgrid inverters is particularly common. Additionally, the influence of factors such as the connected nonlinear load, resonance, and faults often results in abnormal conditions such as voltage distortion in the microgrid. Research on parallel control strategies and power quality management methods for microgrid inverters under nonlinear and unbalanced conditions can not only ensure the safe and reliable operation of microgrid inverters, but also proactively improve the power quality of the microgrid. This has significant theoretical research value and practical significance for constructing microgrids with high power quality and reliability. This article focuses on the three-phase LCL-type grid-connected inverter and investigates the compensation of specific order harmonics in the presence of non-linear unbalanced loads carried by the grid. Specifically, the Delayed Signal Cancellation (DSC) and Second Order Generalized Integrator (SOGI) are combined to extract the harmonic components of the load current quickly and accurately, which are used as the input current reference for the control system to achieve selective harmonic compensation. This design can exclude small high-frequency noise from the reference current, thereby improving the efficiency of filtering. In terms of control system design, there is no strong coupling term since for the grid-connected inverter, the computational model in the two-phase stationary frame $alpha$-axis and $beta$-axis is absolutely symmetric and independent of each other. Based on this, an inverter current control strategy is proposed, and a current proportional resonant controller is designed to achieve zero steady-state error control of the current, eliminating the need for traditional PI control strategies involving synchronous rotating coordinate transformation and decoupling operations, simplifying controller design. Finally, in MATLAB/Simulink, a simulation model of the microgrid inverter was built, and the effectiveness of the aforementioned control method was validated by simulation.