electric power substations engineering (14)

12 Grounding and 12.1 Lightning Stroke Protection ...12-1 The Design Problem 12.2 Lightning Parameters ...12-2 Strike Distance • Stroke Current Magnitude • Keraunic Level • Ground Flash

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12

Grounding and

12.1 Lightning Stroke Protection 12-1

The Design Problem

12.2 Lightning Parameters 12-2

Strike Distance • Stroke Current Magnitude • Keraunic Level • Ground Flash Density • Lightning Detection Networks

12.3 Empirical Design Methods 12-5

Fixed Angles • Empirical Curves

12.4 The Electrogeometric Model (EGM) 12-7

Whitehead’s EGM • Recent Improvements in the EGM • Criticism of the EGM • A Revised EGM • Application of the EGM by the Rolling Sphere Method • Multiple Shielding Electrodes • Changes in Voltage Level • Minimum Stroke Current • Application of Revised EGM by Mousa and Srivastava Method

12.5 Calculation of Failure Probability 12-1812.6 Active Lightning Terminals 12-20References 12-20

12.1 Lightning Stroke Protection

Substation design involves more than installing apparatus, protective devices, and equipment The nificant monetary investment and required reliable continuous operation of the facility requires detailedattention to preventing surges (transients) from entering the substation facility These surges can beswitching surges, lightning surges on connected transmission lines, or direct strokes to the substationfacility The origin and mechanics of these surges, including lightning, are discussed in detail in Chapter 10

sig-of The Electric Power Engineering Handbook (CRC Press, 2001) This section focuses on the design process

amplitude [less design margin] to reach phase conductors [IEEE Std 998-1996]) against direct lightningstroke in substations

1 A large portion of the text and all of the figures used in the following discussion were prepared by the Direct Stroke Shielding of Substations Working Group of the Substations Committee — IEEE Power Engineering Society, and published as IEEE Std 998-1996, IEEE Guide for Direct Lightning Stroke Shielding of Substations, Institute of Electrical and Electronics Engineers, Inc., 1996 The IEEE disclaims any responsibility or liability resulting from the placement or use in the described manner Information is reprinted with the permission of the IEEE The author has been a member of the working group since 1987.

Robert S Nowell

Georgia Power Company

1703_Frame_C12.fm Page 1 Monday, May 12, 2003 5:50 PM

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12-2 Electric Power Substations Engineering

12.1.1 The Design Problem

The engineer who seeks to design a direct stroke shielding system for a substation or facility must contendwith several elusive factors inherent in lightning phenomena, namely:

• The unpredictable, probabilistic nature of lightning

• The lack of data due to the infrequency of lightning strokes in substations

• The complexity and economics involved in analyzing a system in detail

There is no known method of providing 100% shielding short of enclosing the equipment in a solidmetallic enclosure The uncertainty, complexity, and cost of performing a detailed analysis of a shieldingsystem has historically resulted in simple rules of thumb being utilized in the design of lower voltagefacilities Extra high voltage (EHV) facilities, with their critical and more costly equipment components,usually justify a more sophisticated study to establish the risk vs cost benefit

Because of the above factors, it is suggested that a four-step approach be utilized in the design of aprotection system:

1 Evaluate the importance and value of the facility being protected

2 Investigate the severity and frequency of thunderstorms in the area of the substation facility andthe exposure of the substation

3 Select an appropriate design method consistent with the above evaluation and then lay out anappropriate system of protection

4 Evaluate the effectiveness and cost of the resulting design

The following paragraphs and references will assist the engineer in performing these steps

12.2 Lightning Parameters

12.2.1 Strike Distance

Return stroke current magnitude and strike distance (length of the last stepped leader) are interrelated

A number of equations have been proposed for determining the striking distance The principal onesare as follows:

(12.1)(12.2)(12.3)(12.4)(12.5)where

It may be disconcerting to note that the above equations vary by as much as a factor of 2:1 However,lightning investigators now tend to favor the shorter strike distances given by Equation 12.4 Anderson,

(1987), now feels that Equation 12.4 is more accurate Mousa (1988) also supports this form of theequation The equation may also be stated as follows:

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Grounding and Lightning 12-3

(12.6)

12.2.2 Stroke Current Magnitude

Since the stroke current and striking distance are related, it is of interest to know the distribution ofstroke current magnitudes The median value of strokes to OHGW, conductors, structures, and masts isusually taken to be 31 kA (Anderson, 1987) Anderson (1987) gave the probability that a certain peakcurrent will be exceeded in any stroke as follows:

(12.7)where

P(I) is the probability that the peak current in any stroke will exceed I

Mousa (1989) has shown that a median stroke current of 24 kA for strokes to flat ground producesthe best correlation with available field observations to date Using this median value of stroke current,the probability that a certain peak current will be exceeded in any stroke is given by the following equation:

(12.8)

where the symbols have the same meaning as above

Figure 12.1 is a plot of Equation 12.8, and Figure 12.2 is a plot of the probability that a stroke will bewithin the ranges shown on the abscissa

FIGURE 12.1 Probability of stroke current exceeding abscissa for strokes to flat ground (IEEE Std 998-1996 With permission.)

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12.2.3 Keraunic Level

Keraunic level is defined as the average annual number of thunderstorm days or hours for a given locality

A daily keraunic level is called a thunderstorm-day and is the average number of days per year on whichthunder will be heard during a 24-h period By this definition, it makes no difference how many timesthunder is heard during a 24-h period In other words, if thunder is heard on any one day more thanone time, the day is still classified as one thunder-day (or thunderstorm day) The average annual kerauniclevel for locations in the U.S can be determined by referring to isokeraunic maps on which lines of equalkeraunic level are plotted on a map of the country Figure 12.3 gives the mean annual thunderstorm daysfor the U.S

FIGURE 12.2 Stroke current range probability for strokes to flat ground (IEEE Std 998-1996 With permission.)

FIGURE 12.3 Mean annual thunderstorm days in the U.S (IEEE Std 998-1996 With permission.)

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12.2.4 Ground Flash Density

Ground flash density (GFD) is defined as the average number of strokes per unit area per unit time at aparticular location It is usually assumed that the GFD to earth, a substation, or a transmission ordistribution line is roughly proportional to the keraunic level at the locality If thunderstorm days are to

be used as a basis, it is suggested that the following equation be used (Anderson, 1987):

(12.9)or

(12.10)where

12.2.5 Lightning Detection Networks

A new technology is now being deployed in Canada and the U.S that promises to provide more accurateinformation about ground flash density and lightning stroke characteristics Mapping of lightning flashes

to the earth has been in progress for over a decade in Europe, Africa, Australia, and Asia Now a network

of direction-finding receiving stations has been installed across Canada and the U.S By means of gulation among the stations, and with computer processing of signals, it is possible to pinpoint the location

trian-of each lightning discharge Hundreds trian-of millions trian-of strokes have been detected and plotted to date.Ground flash density maps have already been prepared from this data, but with the variability infrequency and paths taken by thunderstorms from year to year, it will take a number of years to developdata that is statistically significant Some electric utilities are, however, taking advantage of this technology

to detect the approach of thunderstorms and to plot the location of strikes on their system Thisinformation is very useful for dispatching crews to trouble spots and can result in shorter outages thatresult from lightning strikes

12.3 Empirical Design Methods

Two classical design methods have historically been employed to protect substations from direct lightningstrokes:

of the angle alpha that is commonly used is 45° Both 30° and 45° are widely used for angle beta (Samplecalculations for low-voltage and high-voltage substations using fixed angles are given in annex B of IEEEStd 998-1996.)

N k= 0 12 T d

N m= 0 31 T d

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12.3.2 Empirical Curves

From field studies of lightning and laboratory model tests, empirical curves have been developed todetermine the number, position, and height of shielding wires and masts (Wagner et al., 1941; Wagner,1942; Wagner, McCann, Beck, 1941) The curves were developed for shielding failure rates of 0.1, 1.0,5.0, 10, and 15% A failure rate of 0.1% is commonly used in design Figure 12.6 and Figure 12.7 have

referred to as the Wagner method

12.3.2.1 Areas Protected by Lightning Masts

Figure 12.8 and Figure 12.9 illustrate the areas that can be protected by two or more shielding masts(Wagner et al., 1942) If two masts are used to protect an area, the data derived from the empirical curves

represents an approximate limit for a selected exposure rate Any single point falling within the hatched area should have <0.1% exposure Points outside the cross-hatched area will have >0.1% expo-sure Figure 12.8b illustrates this phenomenon for four masts spaced at the distance s as in Figure 12.8a

cross-FIGURE 12.4 Fixed angles for shielding wires (IEEE Std 998-1996 With permission.)

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The protected area can be improved by moving the masts closer together, as illustrated in Figure 12.9

In Figure 12.9a, the protected areas are, at least, as good as the combined areas obtained by superimposingthose of Figure 12.8a In Figure 12.9a, the distance s′ is one half the distance s in Figure 12.8a To estimatethe width of the overlap, x′, first obtain a value of y corresponding to twice the distance s′ between the

greater than 0.1% has been significantly reduced (Sample calculations for low-voltage and high-voltagesubstations using empirical curves are given in annex B of IEEE Std 998-1996.)

12.4 The Electrogeometric Model (EGM)

Shielding systems developed using classical methods (fixed angles and empirical curves) of determiningthe necessary shielding for direct stroke protection of substations have historically provided a fair degree

of protection However, as voltage levels (and therefore structure and conductor heights) have increasedover the years, the classical methods of shielding design have proven less adequate This led to thedevelopment of the electrogeometric model

FIGURE 12.5 Fixed angles for masts (IEEE Std 998-1996 With permission.)

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12.4.1 Whitehead’s EGM

In 1960, Anderson developed a computer program for calculation of transmission line lightning performance

An early version of the EGM was developed in 1963 by Young et al., but continuing research soon led to newmodels One extremely significant research project was performed by Whitehead (1971) Whitehead’s workincluded a theoretical model of a transmission system subject to direct strokes, development of analyticalexpressions pertaining to performance of the line, and supporting field data that verified the theoreticalmodel and analyses The final version of this model was published by Gilman and Whitehead in 1973

12.4.2 Recent Improvements in the EGM

(1972) and his work on lightning strokes to tall structures (1972) Sargent showed that the frequencydistribution of the amplitudes of strokes collected by a structure depends on the structure height as well

as on its type (mast vs wire) In 1976, Mousa extended the application of the EGM (which was developedfor transmission lines) to substation facilities

12.4.3 Criticism of the EGM

Work by Eriksson reported in 1978 and later work by Anderson and Eriksson reported in 1980 revealedapparent discrepancies in the EGM that tended to discredit it Mousa (1988) has shown, however, thatexplanations do exist for the apparent discrepancies, and that many of them can be eliminated by adopting

a revised electrogeometric model Most investigators now accept the EGM as a valid approach fordesigning lightning shielding systems

FIGURE 12.6 Single lightning mast protecting single ring of object — 0.1% exposure Height of mast above protected object, y, as a function of horizontal separation, x, and height of protected object, d (IEEE Std 998-1996 With permission.)

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12.4.4 A Revised EGM

The revised EGM was developed by Mousa and Srivastava (1986; 1988) Two methods of applying the

EGM are the modified version of the rolling sphere method (Lee, 1979; Lee, 1978; Orell, 1988), and the

method given by Mousa and Srivastava (1988; 1991)

The revised EGM model differs from Whitehead’s model in the following respects:

1 The stroke is assumed to arrive in a vertical direction (It has been found that Whitehead’s

assumption of the stroke arriving at random angles is an unnecessary complication [Mousa and

Srivastava, 1988].)

2 The differing striking distances to masts, wires, and the ground plane are taken into consideration

3 A value of 24 kA is used as the median stroke current (Mousa and Srivastava, 1989) This selection

is based on the frequency distribution of the first negative stroke to flat ground This value best

reconciles the EGM with field observations

4 The model is not tied to a specific form of the striking distance equations (Equation 12.1 through

Equation 12.6) Continued research is likely to result in further modification of this equation as

it has in the past The best available estimate of this parameter may be used

12.4.4.1 Description of the Revised EGM

Previously, the concept that the final striking distance is related to the magnitude of the stroke current

was introduced and Equation 12.4 was selected as the best approximation of this relationship A

Equation 12.4 is repeated here with this modification:

FIGURE 12.7 Two lightning masts protecting single object, no overlap — 0.1% exposure Height of mast above

protected object, y, as a function of horizontal separation, s, and height of protected object, d (IEEE Std 998-1996.

With permission.)

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(12.11)or

(12.12)where

S f is the strike distance in feet

plane

for strokes to a lightning mast

Lightning strokes have a wide distribution of current magnitudes, as shown in Figure 12.1 The EGM

theory shows that the protective area of a shield wire or mast depends on the amplitude of the stroke

FIGURE 12.8 Areas protected by multiple masts for point exposures shown in Figure 12.5a with two lightning

masts, 12.5b with four lightning masts (IEEE Std 998-1996 With permission.)

Tiêu đề	Grounding and Lightning
Tác giả	Robert S. Nowell
Trường học	Georgia Power Company
Chuyên ngành	Electric Power Engineering
Thể loại	Thesis
Năm xuất bản	2003
Thành phố	Atlanta

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Số trang	21
Dung lượng	2,02 MB