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Electric System Overvoltage Protection

Lee Layton, P.E.

Course Outline

Beginning with a general discussion of lightning, this course covers the theory of what causes overvoltages on an electric distribution power system.  The electrical characteristics of lightning are covered and well as the importance of ground flash density.

The different types of arresters are reviewed including the design, operation, and electrical protection characteristics.  Methods to design and protect an electric distribution line to minimize the impacts of lightning are covered including structure design considerations.  Protection of underground electrical lines is also covered. 

The impact of grounding on overvoltage is discussed including a brief discussion of ferroresonant overvoltages.

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

After taking this course you should,

Intended Audience

This course is intended for electrical engineers, mechanical engineers, and others who want to more about overvoltage protection on electric distribution systems.

Benefit to Attendees

Taking this course will give you a better understanding of how lightning and other overvoltage situations impact an electric distribution system and how a system can be designed and operated to minimize the impact.

Course Introduction

Overvoltage situations occur everyday on electric distribution power systems.  The overvoltages may be the result of external conditions such as lightning or from internal conditions such as from switching surges and ferroresonance.  These overvoltages may breakdown equipment insulation causing equipment damage and possibly failure and often result in disruption in service to customers.

The most common cause of distribution overvoltages is lightning strikes from electrical storms in the atmosphere each year.  In the United States alone there are over 30 million cloud to ground lightning strikes, so the potential for damage to an electric distribution system is significant.  Lightning may cause an insulation breakdown in a single transformer, which results in only few homes losing power, or the lightning strike may flashover an insulator on a three-phase line interrupting power to thousands of homes.  Usually the damage from a single lightning strike to an electric distribution system is not a sufficient economic loss, but when the total number of strikes is considered, the annual economic impact is in the millions of dollars, not to mention the interruption in electrical service that occurs.

In comparison to lightning, switching surges and ferroresonance have only a minor economic or service impact on an electric distribution system.  Transient switching surges are not much of an issue with distribution systems and will not be discussed here, but, we will briefly look at how ferroresonance may cause an overvoltage condition.

To analyze overvoltages, we must first understand something about how lightning occurs and how it impacts the electric system.  The first section of this course describes how lightning occurs, the characteristics of lightning, and how to calculate ground flash densities.  Next, we will look at the design and operation of surge arresters.  Subsequent sections include information about electrical line design methodologies to minimize the impacts of lightning, equipment protection strategies, and the impacts of grounding on lightning protection.

Course Content

This course content is in the following PDF document:

Electric System Overvoltage Protection

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Course Summary

Whether the cause is a lightning surge or ferroresonance, overvoltages on electric utility distribution systems are disruptive events that frequently damage equipment and interrupt the flow of power. 

The impact of lightning surges can be minimized by the proper application of surge arresters, shielding, and structure design.  Careful attention must be paid to BIL coordination to ensure all components work together to minimize the impacts of lightning surges.  Minimizing overvoltages from ferroresonance requires attention to underground cable lengths and switching procedures.


Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.