Preface

The integration of recent and emerging energy technologies in the existing electric grid requires modifications in several aspects of the grid, including its architecture, protection, operation, and control. Micro-grid provides a solution for integrating distributed energy resources (DERs) such as renewable energy generation, energy storage systems, electric vehicles, controllable loads, etc. and delivers flexibility, security, and reliability by operating in both grid-connected and isolated modes.

This book provides an overview of micro-grid solutions, applications, and implementations. State-of-the-art methods for micro-grid operation, optimization, and control are presented. Distributed energy resources and their interactions in micro-grids are also studied. In addition, micro-grid designs and architectures are covered, as are micro-grid protection strategies and schemes for different operation modes. This book presents a new model for urban development using micro-grid features and technologies. Furthermore, IEEE Standards for AC and DC micro-grids are investigated.

The book is organized in three sections and six chapters.

Section 1 includes three chapters covering micro-grid implementations for different applications, while Section 2 includes a chapter covering optimization and control of micro-grids for power quality issues.

Finally, Section 3 includes two chapters covering protection of micro-grids with DERs.

Chapter 1 develops a novel model for urban development and design based on micro-grid solutions. Several technologies such as conventional fossil-fueled and steam-powered generation, renewable wind and solar systems, and energy storage devices are included in the model with the objective of optimizing the building design to maximize such assets. A commercial tower with real-world data is studied to demonstrate the economic and environmental advantages of the proposed development model over traditional developments.

The model is based on trigeneration of power. Conventional generators produce electricity and heat at the first stage, which is used to power a steam turbine to produce more electricity at the second stage. The third stage of trigeneration is to use the remaining exhaust heat to provide building heat, hot water, and air conditioning. Renewable solar and wind power supplement the system. The design includes energy storage systems to help balance the power over peak and off-peak load periods.

The study demonstrates that the proposed model balances load and generation capability on a large scale, allowing the construction of large numbers of buildings to accommodate increasing populations with essentially no impact on the existing power distribution infrastructure. It is also environmentally sustainable, producing fewer emissions than a traditional development on the same scale. The economic and environmental advantages of this development model provide an opportunity to promote general welfare.

Chapter 2 presents a model for the development of micro-grids in remote areas. The model includes diesel generators and renewable wind and solar. A real-time simulation is performed to evaluate different scenarios for micro-grid operation, and to determine the best scenario enabling the micro-grid to operate in an island mode without the need to connect to the main grid.

Lack of commercially available electronic devices and systems, and their required standards and regulations, is a major challenge that hinders the rapid development of residential DC micro-grids. Chapter 3 studies the existing standards applicable to DC micro-grids and establishes the main technical requirements and characteristics for the implementation of DC systems designed for residential applications. Advantages and challenges of low-voltage DC distribution systems, particularly when integrating renewable energy sources along with storage systems, are discussed in the chapter. In addition, several topologies are presented for residential DC micro-grids to address the challenges.

Chapter 4 evaluates micro-grid power quality under steady-state grid-connected operation. An energy management method is proposed to optimize generation, storage, and load scheduling to resolve the power quality issues of the micro-grid. A regulation loop between a mixed-integer linear programming optimization and harmonic load flow is used to adjust the power generation, storage, and consumption in response to the electricity price of the grid while meeting the power quality standards. A test micro-grid is used to evaluate the proposed method for different scenarios. The results demonstrate the efficiency of the designed energy management.

Increased penetration of DERs has complicated the protection of micro-grids. The conventional protection schemes are mainly designed for radial distribution systems and pose serious challenges when applied to mesh-connected distribution systems with DERs.

Chapter 5 provides a comprehensive analysis of micro-grid protection systems. Several methods, including directional over current relays, distance relays, differential protection, voltage-based protection, transformation method, harmonic method, and adaptive protection schemes, are presented. These methods are evaluated for their effective implementation with minimal changes in the existing infrastructure.

Chapter 6 proposes a novel differential protection scheme based on current sensors for micro-grids. The proposed method is modeled in Simulink/Matlab and evaluated for several different faults, including single line-to-ground, line-to-line, double line-toground, three-line, and three line-to-ground faults. Simulation results demonstrate the efficiency of the proposed differential protection over distance relay protection due to its reduced number of input data and computational complexity. It is concluded that differential protection is the preferred solution for micro-grid applications.

> **Dr. Mahmoud Ghofrani** University of Washington Bothell, USA

Section 1

Applications and

Implementations

1

Section 1
