Lightning and Overvoltage Protection

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Source: STANDARD HANDBOOK FOR ELECTRICAL ENGINEERS

SECTION 27

LIGHTNING AND OVERVOLTAGE PROTECTION
A. P (Sakis) Meliopoulos .
Professor, School of Electrical and Computer Engineering, Georgia Institute of Technology

CONTENTS
27.1 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27-1 27.2 BASIC CONCEPTS AND DEFINITIONS . . . . . . . . . . . . . . .27-2 27.3 MECHANISMS AND CHARACTERISTICS OF LIGHTNING . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27-6 27.4 POWER SYSTEM OVERVOLTAGES . . . . . . . . . . . . . . . . .27-14 27.5 ANALYSIS METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . .27-23 27.6 OVERVOLTAGE PROTECTION DEVICES . . . . . . . . . . . .27-37 27.7 OVERVOLTAGE PROTECTION (INSULATION) COORDINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27-49 27.8 MONTE CARLO SIMULATION–BASED METHODS . . . .27-67 27.9 LIGHTNING ELIMINATION DEVICES . . . . . . . . . . . . . .27-69 ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27-71 BIBLIOGRAPHY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27-72

27.1 INTRODUCTION
Temporary overvoltages in power systems occur for a variety of reasons such as faults, switching, and lightning. By far, the most severe overvoltages result from lightning strokes to the power system. Most likely, lightning overvoltages will be very high, resulting in insulation breakdown of power apparatus with destructive results. It is therefore imperative that power systems be designed in such a way that expected overvoltages be below the withstand capability of power apparatus insulation. Many times, this basic requirement is translated into excessive cost. For this reason, one seeks a compromise in which power systems are designed in such a way that the possibility of destructive failure of power apparatus due to overvoltages is minimized. This procedure is based on coordinating the expected overvoltages and the withstand capability of power apparatus. Two steps are typically involved: (1) proper design of the power system to control and minimize the possible overvoltages and (2) application of overvoltage protective devices. Collectively, the two steps are called overvoltage protection or insulation coordination. The importance of overvoltage protection cannot be emphasized enough. First it affects system reliability, which translates into economics. Traditionally, overvoltage protection methods were guided by the objective to maximize system reliability with reasonable investment cost. In this sense, transient overvoltages which do not lead to interruptions are acceptable and short-duration interruptions are tolerable. Recently, however, with the introduction of sensitive electronic equipment, new concerns have been raised. The issue of power quality is important and it is transforming the practices for overvoltage protection. While the application of overvoltage protection devices is pertinent, more and more emphasis is placed on design procedures to minimize the possible overvoltages and control the sources of disturbances. An attempt has been made in this section to provide a balanced treatment of overvoltage protection in view of present-day concerns.

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LIGHTNING AND OVERVOLTAGE PROTECTION

27-2

SECTION TWENTY-SEVEN

The subject of lightning and overvoltage protection is rather complex. A thorough treatment requires good understanding of many related subjects. First, the mechanisms by which lightning is generated and how its pertinent characteristics are related to power systems must be well understood. Second, the response of power systems to lightning and other causes of overvoltages must be studied. Analysis methods...
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