Unit 1 - Transient Electrical Surge and Traveling Waves PDF

EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 UNIT 1 TRANSIENT ELECTRICAL SURGES AND TRAVELLING WAVES INTRODUCTION “In the beginning when God created the heavens and the earth, th...

EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 UNIT 1 TRANSIENT ELECTRICAL SURGES AND TRAVELLING WAVES INTRODUCTION “In the beginning when God created the heavens and the earth, the earth was a formless void and darkness covered the face of the deep, while a wind from God swept over the face of the waters. Then God said, Let there be light; and there was light.” – Genesis 1:1-3 Welcome to Unit I of Power System Protection 2! This unit delves into the world of electrical surges, a significant threat to the reliable operation of power systems. Our primary focus will be on understanding the phenomenon of lightning strikes as a major source of these surges. Source: Protect switching power supplies against voltage surges (schukat.com) We'll begin by exploring the nature of lightning, its characteristics, and the various types of lightning strikes that can impact power lines. We'll delve into the mechanism of a lightning discharge, gaining insight into the physical processes involved. Page 1 of 42  EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 Next, we'll shift our focus to travelling waves. These waves, generated by lightning strikes or switching operations within the power system, travel along transmission lines like ripples on a pond. Understanding their propagation is crucial, as these waves carry surge energy that can damage equipment. This unit will equip you with the knowledge to analyze the behavior of these waves. We'll explore concepts like wave reflection at the end of lines, the impact of impedance changes and line junctions, and the influence of wave shape and line length. By the end of Unit I, you'll possess a solid foundation for understanding the nature and propagation of electrical surges caused by lightning and other internal factors within the power system. This knowledge will pave the way for exploring effective mitigation strategies in the subsequent units. LEARNING OUTCOMES By the time the students finish this unit, they will have demonstrated the ability to; 1. Explain the fundamental concepts of electrical surges in power systems, including their origin and potential consequences. (Knowledge, Comprehension) 2. Describe the characteristics of lightning strikes, including different types and their impact on power lines. (Knowledge, Comprehension) 3. Illustrate the mechanism of lightning discharge, outlining the physical processes involved. (Knowledge, Application) 4. Analyze the propagation of surge voltages as travelling waves on transmission lines. (Analysis, Problem-Solving) 5. Explain the concept of wave reflection at the end of a transmission line and its implications for surge propagation. (Analysis, Comprehension) 6. Describe the influence of impedance changes and line junctions on travelling waves. (Analysis, Comprehension) Page 2 of 42  EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 7. Evaluate the impact of wave shape and line length on the characteristics of travelling waves. (Analysis, Problem-Solving) 8. Identify the potential sources of internal overvoltages within a power system besides lightning strikes. (Knowledge, Comprehension) LESSON I LIGHTNING PHENOMENA IN POWER SYSTEM “For just as lightning flashes and lights up the sky from one side to the other, so will the Son of Man be in his day” – Luke 17:24 In the realm of electrical engineering and structural safety, few phenomena command as much awe and respect as lightning. Its sheer power and unpredictable nature pose significant risks to both lives and property. Thus, understanding the principles of lightning protection is paramount for safeguarding structures against its destructive force. Lightning is a natural electrical discharge that occurs during thunderstorms when electrically charged regions in the atmosphere equalize. With bolts capable of reaching temperatures hotter than the sun's surface and delivering billions of joules of energy, lightning strikes pose severe hazards to buildings, infrastructure, and the surrounding environment. Principles of lightning protection are rooted in redirecting the immense energy of lightning strikes away from vulnerable structures, mitigating the risk of damage, fire, or injury. This involves a multifaceted approach encompassing both passive and active protection measures. Passive protection measures focus on dissipating and diverting lightning's electrical energy to minimize its impact on structures. Components such as lightning rods, grounding systems, and surge protection devices form Page 3 of 42  EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 the backbone of passive protection strategies, ensuring the safe dissipation of lightning's energy into the ground. In addition to passive measures, active protection systems employ advanced technologies to detect, intercept, and counteract lightning strikes in real-time. These systems often include early-warning sensors, lightning detection networks, and lightning suppression devices, offering enhanced defense against the destructive potential of lightning. The development and implementation of effective lightning protection systems are guided by stringent regulatory standards and industry best practices. Compliance with the Philippine Electrical Code and other local and international standards such as NFPA 780 (National Fire Protection Association) and IEC 62305 (International Electrotechnical Commission) ensures that structures are adequately protected against lightning-related hazards. Effective lightning protection requires close integration with the design and construction of buildings, industrial facilities, and critical infrastructure. From lightning conductor placement to the selection of conductive materials, every aspect of structural design plays a crucial role in enhancing the resilience of structures to lightning strikes. As technology continues to advance, so too do the tools and techniques available for lightning protection. Innovations in materials science, sensor technology, and predictive analytics offer new avenues for enhancing the reliability and effectiveness of lightning protection systems. Ultimately, an informed and proactive approach to lightning protection is essential for mitigating risks and minimizing the impact of lightning-related incidents. Educating stakeholders, raising awareness of lightning safety practices, and fostering a culture of preparedness are vital components of comprehensive lightning protection strategies. Page 4 of 42  EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 In exploring the principles of lightning protection, we embark on a journey to safeguard lives, property, and infrastructure against one of nature's most formidable forces. By embracing a holistic approach that combines passive measures, active systems, regulatory compliance, and technological innovation, we endeavor to build a more resilient and lightning-safe world. LEARNING OBJECTIVES By the time the students finish the lesson, learners of this course are expected to: 1. Define a lightning strike and explain its key characteristics, such as peak current, voltage, and duration. (Knowledge, Comprehension) 2. Differentiate between various types of lightning strikes (e.g., cloud- to-ground, intra-cloud) and describe their specific effects on power systems. (Knowledge, Analysis) 3. Illustrate the mechanism of a lightning discharge, outlining the stages involved from leader formation to channel development. (Knowledge, Application) 4. Explain the principles behind lightning protection strategies commonly employed in power systems, including air termination systems, grounding networks, and surge protection devices. (Knowledge, Comprehension) 5. Evaluate the effectiveness of different lightning protection measures in mitigating surge damage. (Analysis, Evaluation) ENERGIZED YOUR MIND This section will measure the level of your understanding regarding lightning phenomenon, how it form, and the common safety measures taken during lightning storm. Page 5 of 42  EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 Activity 1. Individual Reflection (5 minutes): 1.1 List down 3-5 words that describe lightning. 1.2 Based on your current understanding, briefly explain how lightning is formed. 1.3 What safety measures do you typically take during a lightning storm? Activity 2. Group Discussion (10 minutes): Instruction: Divide into small groups (3 – 5 each group) Task: Share and discuss your responses with your respective group members. Everyone is encourage to identify any similarities or differences in understanding in activity 1. Then, discuss the reasoning behind their safety measures during lightning storms. NAVIGATE TO THE SOURCE OF NEW KNOWLEDGE AND SKILLS We'll dissect the mechanism of a lightning discharge, uncovering the fascinating stages involved from leader formation to the establishment of a conductive channel and explore various lightning protection strategies employed to safeguard power systems, including air termination systems, grounding networks, and surge protection devices. 1.1 What is Lightning Strike? An electric discharge between cloud and earth, between clouds or between the charge centres of the same cloud is known as lightning. Lightning is a huge spark and takes place when clouds are charged to such a high potential (+ve or -ve) with respect to earth or a neighboring cloud that the dielectric strength of neighboring medium (air) is destroyed. There are several theories which exist to explain how the cloud to be charge. The most accepted one is that during the uprush of warm moist air from earth between the air and the tiny particles of water causes the building up of Page 6 of 42  EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 charges. When drops of water are formed, the larger drops become positively charged and the smaller drops become negatively charged. When the drops of water accumulate, they form clouds, and hence cloud may possess either a positive or a negative charge, depending upon the charge of drops of water they contain. The charge on a cloud may become so great that it may discharge to another cloud or to earth and we call this discharge as lightning. The thunder, which accompanies lightning, is because lightning suddenly heats up the air, thereby causing it to expand. The surrounding air pushes the expanded air back and forth causing the wave motion of air, which we recognize as thunder. 1.2 Mechanism of Lightning Discharge When a charged cloud passes over the earth, it induces equal and opposite charge on the earth below. Fig. 24.4 shows a negatively charged cloud inducing a positive charge on the earth below it. As the charge acquired by the cloud increases, the potential between cloud and earth increases and, therefore, gradient in the air increases. When the potential gradient is sufficient (5 kV/cm to 10 kV/cm) to break down the surrounding air, the lightning stroke starts. The stroke mechanism is as under: (i) As soon as the air near the cloud breaks down, a streamer called leader streamer or pilot streamer starts from the cloud towards the earth and Page 7 of 42  EE ELECTIVE 2: POWER SYSTEM PROTECTION 2 carries charge with it as shown in Fig. 24.4 (i). The leader streamer will continue its journey towards earth as long as the cloud, from which it originates feeds enough charge to it to maintain gradient at the tip of leader streamer above the strength of air. If this gradient is not maintained, the leader streamer stops and the charge is dissipated without the formation of a complete stroke. In other words, the leader streamer will not reach the earth. Fig. 24.4 (i) shows the leader streamer being unable to reach the earth as gradient at its end cloud not be maintained above the strength of air. It may be noted that current in the leader streamer is low (