독립형 교류 마이크로그리드에서 정확한 전력분배와 공통연결점의 고조파전압 보상을 위한 임피던스 제어기법

Abstract

The need for highly reliable and flexible distribution networks is a critical challenge for industrial application. This issue can be addressed by managing and scheduling microgrids, which is the main core of power distribution system. Microgrid is a small-scale distribution system consisting of distributed generation (DG) units, micro turbines, energy storage system, and transformer [1], [2]. A microgrid is linked to the utility grid at the point of common coupling (PCC) and it can transfer active and reactive power to the load. Compared to the traditional transmission system, the microgrid can operate autonomously in the islanding operation to regulate voltage and frequency at the point of common coupling. The islanded mode, according to the IEEE 1547 standard, can provide numerous benefits, including improved reliability and power quality, cost reduction, and auxiliary services [3]. In the islanding operation, it is important to manage the desired power among DG units, and the droop control algorithm has been widely used to operate DG units independently. However, the conventional control method cannot guarantee the power-sharing accuracy and high PCC voltage quality. Therefore, this thesis presents advanced control strategies to address these inaccurate power sharing and voltage distortion issues in the islanded microgrid system. Firstly, based on analysis of an islanded microgrid with complex line impedance structure, an effective coordinated virtual impedance control scheme based on centralized approach is developed to accurately share active and reactive power in a microgrid. In the proposed control strategy, both virtual resistance and virtual inductance are simultaneously tuned to compensate for the mismatched line impedance among units. Even if the microgrid configuration and load condition change, the proposed control approaches still provide proper power sharing among DG units. Furthermore, the proposed method can be implemented directly without any knowledge of the detailed microgrid configuration or the required load power measurement, which increases the system reliability and reduces system complexity. Next, in order to overcome the inaccurate fundamental power sharing and voltage quality issues under nonlinear load conditions, this thesis presents a harmonic compensation control scheme by adaptively regulating resistive-capacitive virtual impedances at the selected harmonic frequencies (3rd, 5th, and 7th). To comply with the practical application, the control system was developed in both a centralized and distributed network. Regardless of microgrid architecture or load circumstances, the proposed scheme properly distributes the active and reactive power. Additionally, a modification of impedance at harmonic frequency has also been proposed to mitigate the PCC voltage distortion. Because no information regarding the detailed microgrid parameters such as line impedances and nonlinear load current is required, the system complexity and expense are considerably reduced. Finally, an impedance‑based harmonics compensation (IBHC) is presented to provide accurate harmonic power sharing along with voltage harmonic compensation for DG units. The proposed IBHC is developed based on simple proportional integral controllers without using a load current sensor. To confirm the proposed control stability and reliability, the microgrid system is theoretically evaluated using a small-signal state-space model. All control approaches are validated using PLECS simulations and experiments using scaled-down laboratory microgrid prototypes. The results show that the proposed control methods are both feasible and effective. The final section of the thesis draws conclusions and suggests future research.