Transient Droop Control Strategy for Parallel Operation of Distributed Energy Resources in an Islanded Microgrid

Abstract

Future electric grid will evolve from the current centralized and radial model toward a more distributed one. In recent years, distributed generation (DG) units have been playing an important role in electric generation due to their promising advantages in reducing air pollution, improving power system efficiency, and relieving stress on power transmission and delivery systems. Despite the increased penetration of DG systems, the application of individual DG system always has its limitation such as high cost/W, limited capacity and reliability, and safety concerns. A better way to utilize the emerging potential of DG is to take a system approach viewing generation and associated loads as a subsystem called a “microgrid”. Forming an electric island, the microgrid can work autonomously following a disturbance. In the islanded microgrid, micro sources are responsible for maintaining the voltage and the frequency of the microgrid system within their specified limits and sharing the load between the generators in a stable manner. However, a robust and stable operation of a microgrid depends on a robust control scheme of the microgrid sources. The most common technique to control microgrid sources is based on conventional droop characteristics. Although the conventional frequency/voltage droop technique properly shares a common active load, the reactive power sharing accuracy can be strongly affected by system parameter and active power control. In addition, frequency variations of different sources in transient mode can cause poor active power sharing. To override the above-mentioned problems, a novel frequency/voltage droop scheme is proposed in this thesis. The proposed scheme improves the performance of the microgrid in terms of power sharing and voltage regulation and smooths the system’s dynamic and transient responses. This work has developed the modeling, control parameters design, and power-sharing control starting from a single voltage source inverter to a number of interconnected DG units forming a flexible microgrid. Specifically, this thesis presents: • A control-oriented modeling based on active and reactive power analysis. • A control synthesis based on enhanced droop control technique. • A small signal stability study to give guidelines for properly adjusting the control system parameters according to the desired dynamic response.