A Study on Control Framework and Coordinated Droop Control Method for Enhancement of Scalability in DC Microgrids

Abstract

In this dissertation, a study on control framework and coordinated droop control method for enhancement of scalability in direct current microgrid (DC MG) was presented. DC MG is a complex system composed of various subsystems. The subsystems constituting the DC MG are independently managed and operated, operated with the same purpose, distributed locally, and can be added or reduced in DC MG. That is, the DC MG can be regarded as one entity composed of various systems. Accordingly, an attempt has been made to interpret the DC MG in terms of the system of systems (SoS). From a SoS point of view, the DC MG is scalable. The increase in renewable energy sources or users requires an increase in the scalability. Therefore, various studies have been conducted to improve the scalability of DC MG. Researches for increase interoperability have been carried out to improve scalability through the unification of communication. Researches on control structures based on the control hierarchy also have been carried out. However, the conventional control structure generally presents various requirements such as the control and communication function to the controller of the subsystem. In order to reflect such a requirement, it is inevitable to modify at a lower control level of local controller of subsystem, which requires a lot of time and economic effort. It substantially hinders the scalability of the DC MG. Therefore, in this dissertation, the control framework is presented for increasing the scalability of the DC MG by classifying the controller according to the control purpose and object. Through the proposed framework, the subsystems can be constructed in DC MG by performing only basic local variable control. That is, because it has flexibility to cope with conventional control structures based on control hierarchy, it is possible to increase the scalability of the DC MG which is composed of various control structures. In addition to the control framework, the study on the control method to increase the scalability of the DC MG is needed. In the presence of a higher control entity such as a microgrid central controller (MGCC), it is possible to monitor the global information of the MG and to achieve optimal management of the DC MG. However, the scalability is low because the operation of all subsystems is predetermined by the MGCC and requires promised data transmission to each other. Therefore, various coordinated control methods have been studied to overcome these disadvantages. However, conventional coordinated control methods have drawbacks such as the occurrence of voltage deviation or weak energy management due to the absence of global information. Therefore, in this dissertation, the coordinated droop control method that can restore the voltage deviation and perform energy management by defining a new variable that determines the operation band by reflecting the energy state of DC MG is presented. In order to analyze the dynamic characteristics of the proposed framework and control method, small signal analysis was performed. Furthermore, to verify the validity of the proposed method, various experiments were performed using lab-scale DC MG, and the control framework and coordinated droop control method for enhancement of scalability in DC MG were verified.