Review on Control of DC Microgrids and Multiple Microgrid Clusters

This paper performs an extensive review on control schemes and architectures applied to dc microgrids (MGs). It covers multilayer hierarchical control schemes, coordinated control strategies, plug-and-play operations, stability and active damping aspects, as well as nonlinear control algorithms. Islanding detection, protection, and MG clusters control are also briefly summarized. All the mentioned issues are discussed with the goal of providing control design guidelines for dc MGs. The future research challenges, from the authors' point of view, are also provided in the final concluding part

Dynamic modeling, stability analysis, and controller design for DC distribution systems

The dc distribution systems or dc microgrids are known to be best suited for integration of renewable energy sources into the current power grid and are considered to be the key enabling technology for the development of future smart grid. Dc microgrids also benefit from better current capabilities of dc power lines, better short circuit protection, and transformer-less conversion of voltage levels, which result in higher efficiency, flexibility, and lower cost. While the idea of using a dc microgrid to interface distributed energy sources and modern loads to the power grid seems appealing at first, several issues must be addressed before this idea can be implemented fully. The configuration, stability, protection, economic operation, active management, and control of future dc microgrids are among the topics of interest for many researchers. The purpose of this dissertation is to investigate the dynamic behavior and stability of a future dc microgrid and to introduce new controller design techniques for the Line Regulating Converters (LRC) in a dc distribution system. Paper I is devoted to dynamic modeling of power converters in a dc distribution system. The terminal characteristics of tightly regulated power converters which are an important factor for stability analysis and controller design are modeled in this paper. Paper II derives the simplified model of a dc distribution system and employs the model for analyzing stability of the system. Paper III introduces two controller design methods for stabilizing the operation of the LRC in presence of downstream constant power loads in a dc distribution system. Paper IV builds upon paper III and introduces another controller design method which uses an external feedback loop between converters to improve performance and stability of the dc grid. --Abstract, page iv

Active Stability Monitoring and Stability Control of DC Microgrids Using Incremental Continuous Injection

Electrified transportation and integration of renewable energy in the electric power grid requires the use of power electronic converters for integrating different forms of power; from ac to dc, dc to ac, dc to dc, etc. Recent trend towards electrifying automobiles, aircraft and ships, and increasing penetration of renewable energy has increased the required power levels and number of the power electronics converters connected together in a dc microgrid system. Stable operation of these interfacing converters for all operating conditions has been a topic of renewed interest in the last couple of decades. Traditionally, dc microgrids have been designed conservatively to handle the worst case conditions. However, increasing power capacity of emerging dc microgrids causes this conservative design to become cost and size prohibitive, and over-designing causes the system to become slow and unable to handle fast loads such as pulsed power loads, radars etc. To reduce the dependency on passives components and to increase system response speed, recent literature proposed techniques using control so that the system may be designed with smaller filters and guaranteed with system stability. Traditional design of dc microgrids extend the existing stability analysis techniques originally developed to analyze stability of cascaded power converters. This proved to be useful in the design stages for systems with duplicated power sources/loads like in solar systems. However, the existing stability analysis methods are not applicable for online evaluation of stability and for control-based stabilization in a dynamic system with reconfiguration and addition/removal of various kinds of sources and loads. This dissertation first develops a general stability criterion which is easily applicable to complex dc microgrids, and highly suitable for online evaluation of stability. Next, an online stability monitoring system is developed based on the new criterion which uses incremental continuous injection by an existing converter interfacing energy storage in the system and continuously evaluates system stability margin. Furthermore, this dissertation develops an active stability control for dc microgrids which utilizes the evaluation of the continuous monitor and provides additional damping without adding any passive filters. The theory and techniques developed in this dissertation are demonstrated on a lab scale 2 kW dc microgrid