IEEE 998-1996 pdf download IEEE Guide for Direct Lightning Stroke Shielding of Substations
1. Overview
1.1 Scope
The scope of this guide is the identification and discussion of design procedures to provide direct stroke shielding ofoutdoor distribution, transmission, and generating plant substations. All known methods of shielding from directstrokes were investigated during the preparation of this guide, and information is provided on two methods found to bewidely used:
The classical empirical method6The electrogeometric model
A third approach, which involves the use of active lightning terminals, is briefly reviewed in clause 6.
This guide does not purport to include all shielding methods that may have been developed. The guide also does notaddress protection from surges entering a substation over power or communication lines or the personnel safety issues
Users of this guide should thoroughly acquaint themselves with all factors that relate to the design of a particularinstallation and use good engineering judgment in the application of the methods given here, particularly with respectto the importance and value of the equipment being protected.
1.2 Purpose
The intent of this guide is to provide design information for the methods historically and typically applied bysubstation designers to minimize direct lightning strokes to equipment and buswork within substations. The generalnature of lightning is discussed in clause 2 and the problems associated with providing protection from direct strikesare described in clause 3. The methods reviewed in this guide for designing a syste of protection are explained inclauses 4 and 5, and sampl calculations are given in annex B to illustrate use of the methods. Clause 7 contains anextensive bibliography for further study of the subject.
1.3 Definitions
The definitions of terms contained in this document are not intended to embrace all legitimate meanings of the termsThey may only be applicable to the subject treated in this document. For additional definitions refer to IEEE Std 1001992 [B44]1.
1.3.1 critical stroke amplitude: The amplitude of the current of the lightning stroke that, upon terminating on thephase conductor, would raise the voltage of the conductor to a level at which flashover is likely.
1.3.2 dart leader: The downward leader of a subsequent stroke of a multiple-stroke lightning flash.1.3.3 effective shielding: That which permits lightning strokes no greater than those of critical amplitude (less designmargin) to reach phase conductors.
1.3.4 electrogeometric model (EGM): A geometrical representation of a facility, that, together with suitableanalytical expressions correlating its dimensions to the curent of the lightning stroke, is capable of predicting if alightning stroke will terminate on the shielding system, the earth, or the element of the facility being protected.
1.3.5 electrogeometric model theory: The theory describing the electrogeometric model together with the relatedquantitative analyses including the correlation between the striking distances to the different elements of the modeand the amplitude of the first return stroke.
1.3.6 ground flash density (GFD): The average number of lightning strokes per unit area per unit time at a particularlocation.
1.3.7 isokeraunic lines: Lines on a map connecting points having the same keraunic level.
1.3.8 keraunic level: The average annual number of thunderstorm days or hours for a given locality. (0) A dailykeraunic level is called a thunderstorm-day and is the average number of days per year in which thunder is heardduring a 24 h period.(2) An hourly keraunic level is called a thunderstorm-hour and is the average number of hours peryear that thunder is heard during a 60 min period.
1.3.9 lightning fash: The complete lightning discharge, most often composed of leaders from a cloud followed by one
or more return strokes.1.3.10 lightning mast: A column or narrow-base structure containing a vertical conductor from its tip to earth, or thatis itself a suitable conductor to earth. Its purpose is to intercept lightning strokes so that they do not terminate onobjects located within its zone of protection.
1.3.11 negative shielding angle: The shielding angle formed when the shield wire is located beyond the area occupiedby the outermost conductors. See also: shielding angle, positive shielding angle.
1.3.12 positive shielding angle: The shielding angle formed when the shield wire is located above and inside of thearea occupied by the outermost conductors. See also: shielding angle, negative shielding angle.
..3.13 rolling sphere method: A simplified technique for applying the electrogeometric theory to the shielding ofsubstations. The technique involves rolling an imaginary sphere of prescribed radius over the surface of a substationThe sphere rolls up and over (and is supported by) lightning masts, shield wires, fences, and other grounded metalobjects intended for lightning shielding. A piece of equipment is protected from a direct stroke if it remains below thecurved surlace of the sphere by virtue of the sphere being elevated by shield wires or other devices. Equipment thattouches the sphere or penetrates its surface is not protected.
IEEE 998-1996 pdf download
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