Foreword

The idea of microgrids is not new. However, as new technologies are coming into existence to harvest renewable energy as well as more efficient electricity production methods coupled with the flexibility of power electronics; a new industry is developing to promote these technologies and organize them into microgrids for extracting the maximum benefits for owners and the power grid. More than 15 years ago, the Department of Energy has sponsored early research that laid the foundations for microgrids and explored the benefits. One key aspect is the ability and promise to address environmental concerns that have been growing in recent years. Today the microgrid concept has exploded to include a variety of architectures of energy resources into a coordinated energy entity that its value is much greater than the individual components. As a result the complexity of microgrids has increased. It is in this environment of evolution of microgrids that the present book is very welcome. It is written in a way that provides valuable information for specialist as well as non-specialists.

Chapter 1 provides a well thought view of the microgrid concept from the various forms of implementation to the potential economic, environmental and technical benefits. It identifies the role of microgrids in altering the distribution system as we know it today and at the same time elaborates on the formation of microgrids as an organized entity interfaced to distribution systems. In a refreshingly simple way identifies the enabling technologies for microgrids, that is power electronics, communications, renewable resources. It discusses in simple terms the ability of microgrids to minimize green house gases, help the power grid with load balancing and voltage control and assist power markets. While it is recognized that participation of the microgrids in power markets is limited by their size, it discusses possible ways that microgrids can market their assets via aggregators and opens the field for other innovations.

The book addresses two of the great challenges of microgrids: control and protection. Four chapters are devoted to these complex problems, three on control (Chapters 2, 3 and 5) and one on protection (Chapter 4). The multiplicity of control issues and their complexity is elaborated in a clear and concise manner. Since microgrids comprise many resources that are interfaced via power electronics, the book presents the organization of the control problems in a hierarchical architecture that consists of local controllers that control specific resources, their operation and their protection as well as outer loop controllers that perform load-generation management, islanding operation as well as the interaction with up-stream controllers including power system control centers. It provides a good overview of approaches as well as the role of state estimation in controlling and operating a microgrid. In addition to conventional control methods, recent intelligent control approaches are also discussed. The specific issues and challenges of microgrid control are clearly elaborated. As an example, because the microgrid typically comprises many inverters connecting various resources to the microgrid it is possible to trigger oscillations due to inverter control interactions. Methods for solving these issues are clearly discussed in an easy to follow way. It is recognized that multiple microgrids can exist in a system and the issue of controlling and coordinating all the microgrids is very important from the point of view of managing the microsources as well as providing services to the power grid by coordinating all the resources. The services can be any of the ancillary services that are typically provided by large systems: frequency control, voltage control, power balance, capacity reserves. The hierarchies involved in the control and operation of multi-microgrid systems is eloquently presented as a hierarchical control problem.

Protection of microgrids is a challenging problem due to the fact that microgrid resources provide limited fault currents. Detection of faults in microgrids is problematic at best because the grid side fault current contribution may be very high while the contribution from microsources is limited. Present protection schemes and functions are not reliable for microgrids. The book describes clever methods for providing adequate protection functions such as adaptive protection schemes, addition of components that will provide temporarily high fault currents to enable the operation of protective relays, increasing inverter capacity and therefore fault current contribution. While the book provides some solutions it also makes it clear that there is much more work that needs to be done to reliably protect microgrids.

The basic approaches in designing, controlling and protecting microgrids are nicely complimented by a long list of microgrid projects around the globe that provide a picture of the evolution of microgrid design and lessons learned. Specific microgrid projects in Europe, United States, Japan, China and Chile are described and discussed. These projects provide an amazing insight into the lessons learned, challenges faced and issues resolved and issues outstanding. The examples span small capacity microgrids as well as some very large microgrids; grid-connected microgrids as well as stand-alone or island microgrids. The information provided is extremely useful and enables appreciation of the challenges as well as the rewards of these systems.

Finally, the last chapter elaborates on the technical, economic, environmental and social benefits of microgrids. The discussion is qualitative as well as quantitative. While the quantitative analysis is very much dependent upon specific areas and other conditions, the qualitative discussion is applicable to microgrids anywhere in the globe. Indirectly, this discussion makes the case for microgrids comprising mostly renewable energy resources as a big component in solving the environmental, economic and social issues that are facing a society that relies more and more in electric energy. The technical issues are solvable for transforming distribution systems into a distributed microgrid. The work presented in this book will be a fundamental reference toward the promotion and proliferation of microgrids and the accompanied deployment of renewable resources.

This book is a must read resource for anyone interested in the design and operation of microgrids and the integration of renewable resources into the power grid.

Sakis Meliopoulos

Georgia Power Distinguished Professor

School of Electrical and Computer Engineering

Georgia Institute of Technology

Atlanta, Georgia