Distributed Control Methods and Cyber Security Issues in Microgrids

Distributed Control Methods and Cyber Security Issues in Microgrids
Author: Wenchao Meng,Xiaoyu Wang,Shichao Liu
Publsiher: Academic Press
Total Pages: 400
Release: 2020-03-20
ISBN 10: 012816946X
ISBN 13: 9780128169469
Language: EN, FR, DE, ES & NL

Distributed Control Methods and Cyber Security Issues in Microgrids Book Review:

Distributed Control and Cyber Security Issues in Microgrids presents a thorough treatment of distributed control methods and cyber security issues for power system researchers and engineers. With the help of mathematical tools, this reference gives a deep understanding of microgrids and new research directions, addressing emerging concepts, methodologies and applications of monitoring, control and protection in smart microgrids with large-scale renewables. With the integration of more distributed or aggregated renewables and the wide utilization of power electronic devices, the smart microgrid is facing new stability and security challenges.

Microgrid Architectures Control and Protection Methods

Microgrid Architectures  Control and Protection Methods
Author: Naser Mahdavi Tabatabaei,Ersan Kabalci,Nicu Bizon
Publsiher: Springer
Total Pages: 781
Release: 2019-08-01
ISBN 10: 3030237230
ISBN 13: 9783030237233
Language: EN, FR, DE, ES & NL

Microgrid Architectures Control and Protection Methods Book Review:

This book presents intuitive explanations of the principles of microgrids, including their structure and operation and their applications. It also discusses the latest research on microgrid control and protection technologies and the essentials of microgrids as well as enhanced communication systems. The book provides solutions to microgrid operation and planning issues using various methodologies including planning and modelling; AC and DC hybrid microgrids; energy storage systems in microgrids; and optimal microgrid operational planning. Written by specialists, it is filled in innovative solutions and research related to microgrid operation, making it a valuable resource for those interested in developing updated approaches in electric power analysis, design and operational strategies. Thanks to its in-depth explanations and clear, three-part structure, it is useful for electrical engineering students, researchers and technicians.

The Energy Internet

The Energy Internet
Author: Wencong Su,Alex Huang
Publsiher: Woodhead Publishing
Total Pages: 398
Release: 2018-10-26
ISBN 10: 0081022158
ISBN 13: 9780081022153
Language: EN, FR, DE, ES & NL

The Energy Internet Book Review:

The Energy Internet: An Open Energy Platform to Transform Legacy Power Systems into Open Innovation and Global Economic Engines is an innovative concept that changes the way people generate, distribute and consume electrical energy. With the potential to transform the infrastructure of the electric grid, the book challenges existing power systems, presenting innovative and pioneering theories and technologies that will challenge existing norms on generation and consumption. Researchers, academics, engineers, consultants and policymakers will gain a thorough understanding of the Energy Internet that includes a thorough dissemination of case studies from the USA, China, Japan, Germany and the U.K. The book's editors provide analysis of various enabling technologies and technical solutions, such as control theory, communication, and the social and economic aspects that are central to obtaining a clear appreciation of the potential of this complex infrastructure. Presents the first complete resource on the innovative concept of the Energy Internet Provides a clear analysis of the architecture of the Energy Internet to ensure an understanding of the technologies behind generating, distributing and consuming electricity in this way Includes a variety of global case studies of real-world implementation and pilot projects to thoroughly demonstrate the theoretical, technological and economic considerations

Modeling Control Estimation and Optimization for Microgrids

Modeling  Control  Estimation  and Optimization for Microgrids
Author: Zhixiong Zhong
Publsiher: CRC Press
Total Pages: 379
Release: 2019-11-27
ISBN 10: 1351032445
ISBN 13: 9781351032445
Language: EN, FR, DE, ES & NL

Modeling Control Estimation and Optimization for Microgrids Book Review:

Due to increasing economic and environmental pressures, small-scale grids have received increasing attention in the last fifteen years. These renewable sources, such as solar PVs, wind turbines, and fuel cells, integrated with grid, have changed the way we live our lives. This book describes microgrid dynamics modeling and nonlinear control issues from introductory to the advanced steps. The book addresses the most relevant challenges in microgrid protection and control including modeling, uncertainty, stability issues, local control, coordination control, power quality, and economic dispatch.

Distributed Energy Resources in Microgrids

Distributed Energy Resources in Microgrids
Author: Rajeev Kumar Chauhan,Kalpana Chauhan
Publsiher: Academic Press
Total Pages: 554
Release: 2019-08-17
ISBN 10: 0128177756
ISBN 13: 9780128177754
Language: EN, FR, DE, ES & NL

Distributed Energy Resources in Microgrids Book Review:

Distributed Energy Resources in Microgrids: Integration, Challenges and Optimization unifies classically unconnected aspects of microgrids by considering them alongside economic analysis and stability testing. In addition, the book presents well-founded mathematical analyses on how to technically and economically optimize microgrids via distributed energy resource integration. Researchers and engineers in the power and energy sector will find this information useful for combined scientific and economical approaches to microgrid integration. Specific sections cover microgrid performance, including key technical elements, such as control design, stability analysis, power quality, reliability and resiliency in microgrid operation. Addresses the challenges related to the integration of renewable energy resources Includes examples of control algorithms adopted during integration Presents detailed methods of optimization to enhance successful integration

Microgrid

Microgrid
Author: Magdi S Mahmoud
Publsiher: Elsevier
Total Pages: 398
Release: 2016-10-24
ISBN 10: 0081012624
ISBN 13: 9780081012628
Language: EN, FR, DE, ES & NL

Microgrid Book Review:

Microgrids: Advanced Control Methods and Renewable Energy System Integration demonstrates the state-of-art of methods and applications of microgrid control, with eleven concise and comprehensive chapters. The first three chapters provide an overview of the control methods of microgrid systems that is followed by a review of distributed control and management strategies for the next generation microgrids. Next, the book identifies future research directions and discusses the hierarchical power sharing control in DC Microgrids. Chapter 4 investigates the demand side management in microgrid control systems from various perspectives, followed by an outline of the operation and controls of the smart microgrids in Chapter 5. Chapter 6 deals with control of low-voltage microgrids with master/slave architecture. The final chapters explain the load-Frequency Controllers for Distributed Power System Generation Units and the issue of robust control design for VSIs, followed by a communication solution denoted as power talk. Finally, in Chapter 11, real-time implementation of distributed control for an autonomous microgrid system is performed. Addresses issues of contemporary interest to practitioners in the power engineering and management fields Focuses on the role of microgrids within the overall power system structure and attempts to clarify the main findings relating to primary and secondary control and management at the microgrid level Provides results from a quantified assessment of benefits from economic, environmental, operational, and social point-of-views Presents the hierarchical control levels manifested in microgrid operations and evaluates the principles and main functions of centralized and decentralized control

Security of Cyber Physical Systems

Security of Cyber Physical Systems
Author: Hadis Karimipour,Pirathayini Srikantha,Hany Farag,Jin Wei-Kocsis
Publsiher: Springer Nature
Total Pages: 323
Release: 2020-08-24
ISBN 10: 3030455416
ISBN 13: 9783030455415
Language: EN, FR, DE, ES & NL

Security of Cyber Physical Systems Book Review:

This book presents a comprehensive overview of security issues in Cyber Physical Systems (CPSs), by analyzing the issues and vulnerabilities in CPSs and examining state of the art security measures. Furthermore, this book proposes various defense strategies including intelligent attack and anomaly detection algorithms. Today’s technology is continually evolving towards interconnectivity among devices. This interconnectivity phenomenon is often referred to as Internet of Things (IoT). IoT technology is used to enhance the performance of systems in many applications. This integration of physical and cyber components within a system is associated with many benefits; these systems are often referred to as Cyber Physical Systems (CPSs). The CPSs and IoT technologies are used in many industries critical to our daily lives. CPSs have the potential to reduce costs, enhance mobility and independence of patients, and reach the body using minimally invasive techniques. Although this interconnectivity of devices can pave the road for immense advancement in technology and automation, the integration of network components into any system increases its vulnerability to cyber threats. Using internet networks to connect devices together creates access points for adversaries. Considering the critical applications of some of these devices, adversaries have the potential of exploiting sensitive data and interrupting the functionality of critical infrastructure. Practitioners working in system security, cyber security & security and privacy will find this book valuable as a reference. Researchers and scientists concentrating on computer systems, large-scale complex systems, and artificial intelligence will also find this book useful as a reference.

Microgrids

Microgrids
Author: Nikos Hatziargyriou
Publsiher: John Wiley & Sons
Total Pages: 340
Release: 2014-03-03
ISBN 10: 1118720687
ISBN 13: 9781118720684
Language: EN, FR, DE, ES & NL

Microgrids Book Review:

Microgrids are the most innovative area in the electric power industry today. Future microgrids could exist as energy-balanced cells within existing power distribution grids or stand-alone power networks within small communities. A definitive presentation on all aspects of microgrids, this text examines the operation of microgrids – their control concepts and advanced architectures including multi-microgrids. It takes a logical approach to overview the purpose and the technical aspects of microgrids, discussing the social, economic and environmental benefits to power system operation. The book also presents microgrid design and control issues, including protection and explaining how to implement centralized and decentralized control strategies. Key features: original, state-of-the-art research material written by internationally respected contributors unique case studies demonstrating success stories from real-world pilot sites from Europe, the Americas, Japan and China examines market and regulatory settings for microgrids, and provides evaluation results under standard test conditions a look to the future – technical solutions to maximize the value of distributed energy along with the principles and criteria for developing commercial and regulatory frameworks for microgrids Offering broad yet balanced coverage, this volume is an entry point to this very topical area of power delivery for electric power engineers familiar with medium and low voltage distribution systems, utility operators in microgrids, power systems researchers and academics. It is also a useful reference for system planners and operators, manufacturers and network operators, government regulators, and postgraduate power systems students. CONTRIBUTORS Thomas Degner Aris Dimeas Alfred Engler Nuno Gil Asier Gil de Muro Guillermo Jiménez-Estévez George Kariniotakis George Korres André Madureira Meiqin Mao Chris Marnay Jose Miguel Yarza Satoshi Morozumi Alexander Oudalov Frank van Overbeeke Rodrigo Palma Behnke Joao Abel Pecas Lopes Fernanda Resende John Romankiewicz Christine Schwaegerl Nikos Soultanis Liang Tao Antonis Tsikalakis

Control and Optimization of Distributed Generation Systems

Control and Optimization of Distributed Generation Systems
Author: Magdi S. Mahmoud,Fouad M. AL-Sunni
Publsiher: Springer
Total Pages: 578
Release: 2015-05-14
ISBN 10: 3319169106
ISBN 13: 9783319169101
Language: EN, FR, DE, ES & NL

Control and Optimization of Distributed Generation Systems Book Review:

This text is an introduction to the use of control in distributed power generation. It shows the reader how reliable control can be achieved so as to realize the potential of small networks of diverse energy sources, either singly or in coordination, for meeting concerns of energy cost, energy security and environmental protection. The book demonstrates how such microgrids, interconnecting groups of generating units and loads within a local area, can be an effective means of balancing electrical supply and demand. It takes advantage of the ability to connect and disconnect microgrids from the main body of the power grid to give flexibility in response to special events, planned or unplanned. In order to capture the main opportunities for expanding the power grid and to present the plethora of associated open problems in control theory Control and Optimization of Distributed Generation Systems is organized to treat three key themes, namely: system architecture and integration; modelling and analysis; and communications and control. Each chapter makes use of examples and simulations and appropriate problems to help the reader study. Tools helpful to the reader in accessing the mathematical analysis presented within the main body of the book are given in an appendix. Control and Optimization of Distributed Generation Systems will enable readers new to the field of distributed power generation and networked control, whether experienced academic migrating from another field or graduate student beginning a research career, to familiarize themselves with the important points of the control and regulation of microgrids. It will also be useful for practising power engineers wishing to keep abreast of changes in power grids necessitated by the diversification of generating methods.

Smart Grid

Smart Grid
Author: K S MANOJ
Publsiher: Notion Press
Total Pages: 422
Release: 2020-01-03
ISBN 10: 1646780000
ISBN 13: 9781646780006
Language: EN, FR, DE, ES & NL

Smart Grid Book Review:

All basic knowledge is provided for the Energy Engineers and the Electrical, Electronics, Computer and Instrumentation Engineering students, who work or wish to work, in Smart Grid and Microgrid area. It benefits them in obtaining essential and required understanding of the Smart Grid, from perceptions to actualisation. The book: • Presents the Smart Grid from abstraction to materialization. • Covers power grid networks, including how they are developed and deployed for power delivery and other Smart Grid services. • Discusses power systems, advanced communications, and required machine learning that define the Smart Grid. • Clearly differentiates the Smart Grid from the traditional power grid as it has been for the last century. • Provides the reader with a fundamental understanding of both physical-cyber -security and computer networking. • Presents the complexity and operational requirements of the evolving Smart Grid to the ICT professional and presents the same for ICT to the energy engineers. • Provides a detailed description of the cyber vulnerabilities and mitigation techniques of the Smart Grid. • Provides essential information for technocrats to make progress in the field and to allow power system engineers to optimize communication systems for the Smart Grid. • Is a suitable material for the undergraduate and post graduate students of electrical engineering to learn the fundamentals of Smart Grid.

Advances in Applied Science Engineering and Technology

Advances in Applied Science  Engineering and Technology
Author: Na Na,Zong Hua Wang,Qing Bin Yang,Yi Qian Wang,Pei Long Xu
Publsiher: Trans Tech Publications Ltd
Total Pages: 986
Release: 2013-06-27
ISBN 10: 3038260991
ISBN 13: 9783038260998
Language: EN, FR, DE, ES & NL

Advances in Applied Science Engineering and Technology Book Review:

Selected, peer reviewed papers from the 2013 International Conference on Applied Science, Engineering and Technology (ICASET 2013), May 19-21, 2013, Qingdao, China

Smart Grid Security

Smart Grid Security
Author: Gilbert N. Sorebo,Michael C. Echols
Publsiher: CRC Press
Total Pages: 328
Release: 2012-02-02
ISBN 10: 146651650X
ISBN 13: 9781466516502
Language: EN, FR, DE, ES & NL

Smart Grid Security Book Review:

The Smart Grid has the potential to revolutionize electricity delivery systems, and the security of its infrastructure is a vital concern not only for cyber-security practitioners, engineers, policy makers, and utility executives, but also for the media and consumers. Smart Grid Security: An End-to-End View of Security in the New Electrical Grid explores the important techniques, challenges, and forces that will shape how we achieve a secure twenty-first century electric grid. Includes a Foreword by Michael Assante, President and CEO, National Board of Information Security Examiners Following an overview of the components of the Smart Grid, the book delves into the evolution of security standards and regulations and examines ways in which the Smart Grid might be regulated. The authors discuss the technical details about how metering technology is being implemented and the likely threats and vulnerabilities that utilities will face. They address the home area network (HAN) and examine distribution and transmission—the foundation for the delivery of electricity, along with distributed generation, micro-grids, and operations. The book explores future concepts—such as energy storage and the use of plug-in electric vehicles (PEVs)—in addition to the concomitant risk for fraud and manipulation with stored energy. Consumer-related issues are discussed as they pertain to emerging ways of receiving and generating energy. The book examines dysfunctions ranging from inadvertent outages to cyber-attack and presents recommendations on how to respond to these incidents. It concludes with speculation of future cyber-security challenges and discusses new ways that the grid can be defended, such as better key management and protection. Written in a style rigorous enough for the practitioner yet accessible to a broad audience, this comprehensive volume covers a topic that is becoming more critical to industry and consumers everywhere.

Exploring Cyber Security Issues and Solutions for Various Components of DC Microgrid System

Exploring Cyber Security Issues and Solutions for Various Components of DC Microgrid System
Author: Sultana Razia Akhter
Publsiher: Anonim
Total Pages: 329
Release: 2018
ISBN 10:
ISBN 13: OCLC:1125015563
Language: EN, FR, DE, ES & NL

Exploring Cyber Security Issues and Solutions for Various Components of DC Microgrid System Book Review:

Nowadays, considering the growing demand for the DC loads and simplified interface with renewable power generation sources, DC microgrids could be cost effective solution for the power supply in small scale area. the supervisory control and data acquisition (SCADA) system maintain the bidirectional power communication through the internet connectivity with the microgrid. However, this intelligent and interactive feature may pose a cyber-security threat to the power grid. this work aims to exploring cyber-security issues and their solutions for the DC microgrid system. To mitigate the adverse effects of various cyber-attacks such as the False Data Injection (FDI) attack, Distributed Denial of Service (DDoS) attack etc., two new techniques based on non-linear and proportional-integral (PI) controllers have been proposed. Simulation results obtained from MATLAB/Simulink software demonstrate the effectiveness of the proposed methods in mitigating the adverse effects of cyber-attacks on the DCMG system performance. .

Dynamics and Control of Electric Transmission and Microgrids

Dynamics and Control of Electric Transmission and Microgrids
Author: K. R. Padiyar,Anil M. Kulkarni
Publsiher: John Wiley & Sons
Total Pages: 504
Release: 2019-02-04
ISBN 10: 1119173388
ISBN 13: 9781119173380
Language: EN, FR, DE, ES & NL

Dynamics and Control of Electric Transmission and Microgrids Book Review:

A guide to the latest developments in grid dynamics and control and highlights the role of transmission and distribution grids Dynamics and Control of Electric Transmission and Microgrids offers a concise and comprehensive review of the most recent developments and research in grid dynamics and control. In addition, the authors present a new style of presentation that highlights the role of transmission and distribution grids that ensure the reliability and quality of electric power supply. The authors — noted experts in the field — offer an introduction to the topic and explore the basic characteristics and operations of the grid. The text also reviews a wealth of vital topics such as FACTS and HVDC Converter controllers, the stability and security issues of the bulk power system, loads which can be viewed as negative generation, the power limits and energy availability when distributed storage is used and much more. This important resource: Puts the focus on the role of transmission and distribution grids that ensure the reliability and quality of electric power supply Includes modeling and control of wind and solar energy generation for secure energy transfer Presents timely coverage of on-line detection of loss of synchronism, wide area measurements and applications, wide-area feedback control systems for power swing damping and microgrids-operation and control Written for students of power system dynamics and control/electrical power industry professionals, Dynamics and Control of Electric Transmission and Microgrids is a comprehensive guide to the recent developments in grid dynamics and control and highlights the role of transmission and distribution grids that ensure the reliability and quality of electric power supply.

Smart Microgrid Control

Smart Microgrid Control
Author: Gerro J Prinsloo
Publsiher: Gerro Prinsloo
Total Pages: 120
Release: 2016-01-12
ISBN 10: 0620686650
ISBN 13: 9780620686655
Language: EN, FR, DE, ES & NL

Smart Microgrid Control Book Review:

In a smart microgrid context, transactive energy is one of the more exciting developments in energy management. The concept of Transactive Energy Management (TEM) was introduced by the GridWise Architecture Council (GWAC) and has become a very important ingredient in modern smart microgrid control. Transactive control (TC) generally includes economic intelligence that use pricing signals to coordinate devices throughout the microgrid system on an economic bargaining or price bargaining principles. In this way, the smart microgrid controller can actually make decisions on energy generation and energy consumption based on time-varying pricing signals. Transactive smart microgrid Control (TsmC) use mathematical algorithms than can respond to dynamic pricing and microgrid energy demand, creating a type of interactive bargaining platform for energy users and appliances to bargain for energy purchases and usage within the microgrid. It can be viewed as a two-way transaction system in which energy can be generated, supplied and directed to where energy required. This book takes an in depth look at intelligent and smart control in a microgrid context. It deals with topics such as stochastic microgrid modelling and control optimization, smart microgrid and supply side modelling, smart microgrid demand side management and control optimization in both autonomous or islanded smart microgrids as well as grid connected microgrids.

Borrego Springs Microgrid Demonstration Project

Borrego Springs Microgrid Demonstration Project
Author: Thomas Bialek,San Diego Gas and Electric Company,Horizon Energy Group
Publsiher: Anonim
Total Pages: 71
Release: 2013
ISBN 10:
ISBN 13: UCSD:31822039255054
Language: EN, FR, DE, ES & NL

Borrego Springs Microgrid Demonstration Project Book Review:

Microgrids Design and Implementation

Microgrids Design and Implementation
Author: Antonio Carlos Zambroni de Souza,Miguel Castilla
Publsiher: Springer
Total Pages: 538
Release: 2018-11-29
ISBN 10: 3319986872
ISBN 13: 9783319986876
Language: EN, FR, DE, ES & NL

Microgrids Design and Implementation Book Review:

This book addresses the emerging trend of smart grids in power systems. It discusses the advent of smart grids and selected technical implications; further, by combining the perspectives of researchers from Europe and South America, the book captures the status quo of and approaches to smart grids in a wide range of countries. It describes the basic concepts, enabling readers to understand the theoretical aspects behind smart grid formation, while also examining current challenges and philosophical discussions. Like the industrial revolution and the birth of the Internet, smart grids are certain to change the way people use electricity. In this regard, a new term – the “prosumer” – is used to describe consumers who may sometimes also be energy producers. This is particularly appealing if we bear in mind that most of the distributed power generation in smart grids does not involve carbon emissions. At first glance, the option of generating their own power could move consumers to leave their current energy provider. Yet the authors argue that doing so is not a wise choice: utilities will play a central role in this new scenario and should not be ignored.

Energy Internet and We Energy

Energy Internet and We Energy
Author: Qiuye Sun
Publsiher: Springer
Total Pages: 325
Release: 2018-07-12
ISBN 10: 9811305234
ISBN 13: 9789811305238
Language: EN, FR, DE, ES & NL

Energy Internet and We Energy Book Review:

This book focuses on the framework and implementation of energy integration systems with energy and smart-control technologies. It describes in detail We-Energy, a novel energy interaction mode based on a cyber-physical-economy-energy model, which can be adopted to solve the problem of energy supply and utilization. It then analyzes the key devices and technologies for developing the Energy Internet, such as converters, energy-conversion devices, system-level connection devices, optimization control strategies, cyber-physical system security, energy-system stability, communication technologies’ operating modes and distributed optimization algorithms, to enable readers to gain a comprehensive understanding of the topic. Lastly, it offers an outlook on the development of the Energy Internet, providing a reference for cross-integration between different disciplines. The book is an indispensable resource for power enterprises, manufacturers in the power-supply industry, and researchers in the field of Energy Internet application. It is also useful for university and college teachers and students seeking to deepen their understanding of the Energy Internet, as well as for readers interested in the Energy Internet correlation techniques.

Microgrid Dynamics and Control

Microgrid Dynamics and Control
Author: Hassan Bevrani,Bruno François,Toshifumi Ise
Publsiher: John Wiley & Sons
Total Pages: 720
Release: 2017-07-18
ISBN 10: 1119263700
ISBN 13: 9781119263708
Language: EN, FR, DE, ES & NL

Microgrid Dynamics and Control Book Review:

This book discusses relevant microgrid technologies in the context of integrating renewable energy and also addresses challenging issues. The authors summarize long term academic and research outcomes and contributions. In addition, this book is influenced by the authors’ practical experiences on microgrids (MGs), electric network monitoring, and control and power electronic systems. A thorough discussion of the basic principles of the MG modeling and operating issues is provided. The MG structure, types, operating modes, modelling, dynamics, and control levels are covered. Recent advances in DC microgrids, virtual synchronousgenerators, MG planning and energy management are examined. The physical constraints and engineering aspects of the MGs are covered, and developed robust and intelligent control strategies are discussed using real time simulations and experimental studies.

Power Systems Resilience

Power Systems Resilience
Author: Naser Mahdavi Tabatabaei,Sajad Najafi Ravadanegh,Nicu Bizon
Publsiher: Springer
Total Pages: 353
Release: 2018-08-16
ISBN 10: 3319944428
ISBN 13: 9783319944425
Language: EN, FR, DE, ES & NL

Power Systems Resilience Book Review:

This book presents intuitive explanations of the principles and applications of power system resiliency, as well as a number of straightforward and practical methods for the impact analysis of risk events on power system operations. It also describes the challenges of modelling, distribution networks, optimal scheduling, multi-stage planning, deliberate attacks, cyber-physical systems and SCADA-based smart grids, and how to overcome these challenges. Further, it highlights the resiliency issues using various methods, including strengthening the system against high impact events with low frequency and the fast recovery of the system properties. A large number of specialists have collaborated to provide innovative solutions and research in power systems resiliency. They discuss the fundamentals and contemporary materials of power systems resiliency, theoretical and practical issues, as well as current issues and methods for controlling the risk attacks and other threats to AC power systems. The book includes theoretical research, significant results, case studies, and practical implementation processes to offer insights into electric power and engineering and energy systems. Showing how systems should respond in case of malicious attacks, and helping readers to decide on the best approaches, this book is essential reading for electrical engineers, researchers and specialists. The book is also useful as a reference for undergraduate and graduate students studying the resiliency and reliability of power systems.