독립형 교류 마이크로그리드에서 전압품질을 개선한 정확한 전력분배 제어기법

Abstract

With the growth of environmental awareness and concerns about carbon emission, distributed generators (DGs) such as photovoltaics, wind turbines, fuel cells, and microturbines have been extensively used in power distribution system. Microgrid concept has been widely adopted to effectively coordinate the operation of multiple parallel DGs because it can flexibly operate in either grid-connected mode or islanded mode to provide cost-effective operation and more reliable power. In islanded mode, the droop controller has been conventionally adopted to operate the microgrid. However, it is impossible to satisfy the power sharing accuracy and voltage quality requirements with the conventional droop controller. Therefore, this thesis presents advanced control strategies to address these power sharing and power quality issues in islanded microgrids.

Firstly, based on analysis of an islanded microgrid with meshed structure, enhanced power sharing control schemes based on centralized approach or distributed approach are developed to share active and reactive power accurately in a meshed microgrid, with the aid of adaptive regulation of the virtual impedances. The proposed control methods always achieve accurate power sharing even when the microgrid configuration or the load condition is changed. Furthermore, the proposed control strategy can be implemented directly without any knowledge of the detailed microgrid configuration or the required load power measurement, which decreases the complexity and cost of the system.

Next, in order to guarantee accurate power sharing even though nonlinear load is applied intensively into the microgrid system, the thesis presents an enhanced control scheme by adaptively regulating virtual impedances at dominant frequencies. The proposed scheme provides accurate active, reactive, and harmonic power sharing despite variations of microgrid configuration or load condition. Additionally, a simple secondary controller has also been proposed to remove the frequency and voltage magnitude deviations without additional voltage measurement at the point of common coupling (PCC). The complexity and cost of the system are significantly reduced because no information about the detailed microgrid configuration, feeder impedances, and the load powers is needed.

Finally, an enhanced DG virtual impedance controller (VIC) is proposed to provide accurate harmonic power sharing along with voltage harmonic compensation in islanded microgrids. The proposed VIC is developed based on simple integral controllers with two controllable parts and no information about the feeder impedances or load currents. The control performance is theoretically analyzed using a small-signal state-space model to evaluate the system dynamics and stability.

All control strategies are validated through digital simulation using PSIM and experiment with scaled-down microgrid prototypes in the laboratory. The obtained results verify the feasibility and effectiveness of the proposed control schemes. The last part of the thesis draws conclusions and offers future studies of the research.