SESP11907

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SCECO-EAST ENGINEERING STANDARD SES-P-119.07

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Saudi Consolidated Electric Companyin the Eastern Province

PAGE NO. 07: 2 of 15[SES11905/FSP:as] 0799

TABLE OF CONTENTS

1.0 SCOPE

2.0 GENERAL

2.1 Function of Shielding System2.2 Factors Affecting Shielding Requirements2.3 Determination of Shielding Requirements

3.0 METHODS OF SHIELDING

3.1 Shielding Masts3.2 Overhead Shield Wires

4.0 ZONE OF PROTECTION

5.0 HEIGHT AND LOCATION OF SHIELDING

6.0 SHIELDING MATERIALS

6.1 Shielding Masts6.2 Shield Wires

7.0 SHIELD SYSTEM GROUNDING

8.0 EXAMPLE : USE OF WORKING CURVES

9.0 BIBLIOGRAPHY

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1.0 SCOPE

This Standard covers the criteria and method of applying shielding to minimize exposure ofsubstations and substation equipment to direct lightning strokes.

2.0 GENERAL

2.1 Function of Shielding System

The function of a shielding system is to intercept, conduct and dissipate to ground alightning discharge which might otherwise strike a vulnerable part of the substation orsubstation equipment.

2.2 Factors Affecting Shielding Requirements

The degree of shielding required for adequate protection of the energized equipmentwithin the substation is determined by several factors.

2.2.1 The exposure of the substation to direct lightning strokes is dependent uponthe isokeraunic level (IKL) or number of storm days per year for the region.

2.2.2 The susceptibility of an object to a lightning stroke increases with the heightof the object.

2.2.3 The larger the substation area the greater the possibility the equipment toreceive a direct stroke. This relationship applies not only to an individualsubstation but to the total area of exposure of a power system having manysubstations.

2.2.4 Since no shielding system is perfect, the acceptable risk probability of alightning stroke should also be a factor.

2.3 Determination of Shielding Requirements

Statistical and probabilistic methods are used to determine the degree and type ofshielding required in a given region.

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2.3.1 The isokeraunic level (IKL) for a given region is based on the frequency ofoccurrence of thunderstorms. The average number of storm days (days onwhich thunder could be heard) to be expected each year in different parts ofthe region is used to prepare IKL charts. Data available on weather conditionsin Saudi Arabia indicates that an IKL of 10 is a reasonable figure.

2.3.2 Experimental data has been used to establish the frequency of lightningstrokes to transmission lines. Analysis of this data has indicated that lightningstrokes will be drawn to a transmission line, because of its height, from aneffective lateral distance on each side of the line, on the average of 3.5 timesthe structure height.

Other statistical data has indicated that for a substation with width (W), length(L) and height (H) ranging from 18 to 30 meters, the total number of strokes(P) per year to the substation is expressed by ( )1 :

P K W H L H( )( . )( . )

( )18 30 2 22 3 5 2 3 5

1609=

+ +

where K2 = 9.5 for heights between 18 and 30 meters with isokeraunic level of25 to 40, and K2 = 5.8 for height between 18 and 30 meters with anisokeraunic level of 20.

The above equation indicates that the number of direct strokes to anunshielded substation of dimensions of 30 meters by 30 meters isapproximately 0.22 per year (or one every four and a half years) in an areawhere the isokeraunic level is in the range of 25 to 40, or approximately 0.13per year (or one every eight years) in area where the isokeraunic level is 20.

When considering a single unshielded substation in Saudi Arabia where theisokeraunic level is 10, one direct lightning stroke to the energized equipmentis likely to occur every eight to ten years. Studies relating desirability ofperfect shielding to the cost of shielding have indicated that a 0.1 percentexposure is a valid design criteria that would practically eliminate thepossibility of direct strokes to a substation. Percent exposure for a height of18 to 30 meters is defined as follows :

Unshielded station : one direct stroke every 8 to 10 years

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Shielded station :

Ten percent exposure one direct stoke every 80 to 100 years

One percent exposure one direct stroke every 800 to 1000 years

One-tenth of one one direct stroke every 8,000 to 10,000 yearspercent exposure

2.3.3 Figures 07-1 and 07-2 present working curves and examples for determiningshielding mast heights and/or overhead shield wire locations to achieveeffective shielding.

3.0 METHODS OF SHIELDING

Effective shielding of substations may be achieved through the use of shielding masts oroverhead shield wires or a combination of both. Shield wires can be used only between theterminal tower and the gantry structure. All other areas inside the substation shall be protectedby a combination of gantry peaks and shielding masts.

3.1 Shielding Masts

Shielding masts can be used for all types of substations to provide protection againstdirect lightning strokes. They are particularly useful in large substations and those oflow-profile design. They are less susceptible to mechanical failures and are preferredfor substation installations.

Shielding masts shall be mounted on top of self supporting steel/wood poles orlatticed-type towers and shall be electrically bonded to the substation ground grid.

3.2 Overhead Shield Wires

Overhead ground wires or shield wires are often used to provide protection againstdirect strokes. They can be supported by the line dead-end or terminal structures andextend over the substation up to the gantry structure.

Since these shield wires are located above the substation buses and equipment,possible breakage shall be considered in the design to avoid outage of and/or damageto these buses and equipment.

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A complete overhead ground or shield wire system shall include protection foroverhead lines entering and leaving the substation. In areas not employingtransmission line shielding, substation shield wire systems shall be extended at least800 meters away from the substation to limit the exposure of the phase conductors todirect strokes near the substation. For adequate protection, the transmission lineoverhead shield wire systems shall be directly connected to the substation shield wiresystem, if there is any, and electrically bonded to the substation ground grid.

4.0 ZONE OF PROTECTION

The zone of protection of a shielding system is the volume of space inside which building andequipment are considered adequately protected by the system. A shielding system allowingnot more than 0.1 percent of the total predicted number of lightning strokes to terminate onprotected equipment is considered adequate for most situations. For a single shielding mast,the zone of protection can be described as a cone with its apex at the highest point of theshielding mast and with a protective angle between the side of the cone and the shielding mast.For a single shield wire, the zone of protection is a wedge. When multiple shielding masts orwires are used, the zones of protection of each overlap to provide complete coverage.

Experience shows that a shielding mast or wire cannot be relied upon to provide completeprotection within any particular zone, but it can be stated that the protection afforded by ashielding conductor increases as the assumed protective angle decreases.

Where the substation control building is not located within the zone of protection of theshielding system, lightning protection mast or rod shall also be provided on top of thesubstation building in accordance with the requirements specified in Chapter 4 and 7 ofANSI/IEEE 141 or NFPA 780, "The Installation of Lightning Protection System".

5.0 HEIGHT AND LOCATION OF SHIELDING

The curves in Figure 07-1 were drawn using extensive laboratory test data to show theconfiguration of masts or overhead shield wires necessary to reduce an object's exposure to 0.1percent. The curves are plotted to show the height (y) of the shielding masts or shield wiresabove the protected object as a function of the horizontal separation (x) and the height (d) ofthe protected object.

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For an exposed structure with a single prominent projection or several projections in a limitedregion, the dotted curves in Figure 07-1(a) show the necessary configuration of a singleprotecting mast. If the energized parts to be shielded are generally distributed at a givenheight, the necessary protective configuration of the mast, based on the most remote object, isgiven by the unbroken curves. The configuration for a single ground wire is given in Figure07-1(b). The dotted-line and the full-line curves of Figure 07-1(c) apply to two masts and twohorizontal ground wires, respectively.

Figures 07-2(a) through 07-2(d) illustrate the area that can be protected by two or moreshielding masts. For given values of "d" and "y", values of the separation distance "s" andradii "x" can be determined from Figures 07-1(c) and 07-1(a), respectively, which give anexposure of 0.1 percent. Any single point falling on the locus "b" shown in Figure 07-2(a) willhave an exposure of approximately 0.1 percent, while any point falling within the cross-hatched area will have less exposure than 0.1 percent. For rectangular substations, thearrangement of two masts in Figure 07-2(a) will leave some points with a higher exposure thandesired. If the distance between the masts is decreased, the resulting protected area will stillbe at least equal to the sum of the areas shown in Figure 07-2(a). For example, if the distancebetween masts in Figure 07-2(a) is halved, the resulting protected area will be larger,approximately equal to that shown in Figure 07-2(b).

On this basis, the approximate width of the overlap between masts can be obtained by takingthe value of "y" from Figure 07-1(c) which corresponds to twice the actual distance betweenthe masts. The width of overlap then equals the value of "x", obtained from Figure 07-1(a),that correspond to this "y". This gives a conservative width of the substation that can beprotected by two masts.

The protected areas for three masts located at the points of an equilateral triangle or for fourmasts located at the points of a square are shown in Figures 07-2(c) and 07-2(d). The height ofthe shielding mast should be chosen so that the "b" points provide 0.1 percent exposure asobtained from Figure 07-1(c) for the mid-point between two masts. The "x" radii are obtainedfrom the data for a single mast shown in Figure 07-1(a).

6.0 SHIELDING MATERIALS

6.1 Shielding Masts

The size of shielding masts shall be determined primarily by the mechanical strengthrequirements. Although the amplitude of the lightning current wave may be very high,its duration is so short that the thermal effect on a lightning protective system isusually negligible.

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6.1.1 When structural steel dead-end towers are used to support shielding masts,the shielding mast shall be galvanized steel pipe with a minimum size of 38mm diameter and 3050 mm length .

6.1.2 When wooden poles are used as support, the shielding mast shall begalvanized ground rod of 13 mm diameter and 600 mm length .

6.1.3 The shielding mast or lightning rod for the substation building shall be copper-clad ground rod of 13 mm diameter and 600 mm length .

6.1.4 When lighting metal poles are used as shielding masts, the minimum size shallbe determined by the mechanical strength requirements.

6.2 Shield Wires

The shield wire material specification shall be in accordance with the requirement of10-SMSS-4, in coordination with the material of the overhead transmission line shieldwires.

6.2.1 When it is necessary to use wire shielding in a substation, the wire shall behigh strength galvanized steel wire unless the available size is inadequate forcurrent capacities or the steel wire is susceptible to corrosion in any particularapplication.

6.2.2 Aluminum-clad steel wire shall be used only when it is required for its highercurrent capacity near large power sources or if it is found necessary in thecoastal and contaminated areas where the wet salt spray is present in theatmosphere.

7.0 SHIELD SYSTEM GROUNDING

A shielding system cannot effectively protect substation equipment unless adequatelygrounded. Multiple low impedance connections from the shielding system to the substationground grid are essential. It is beneficial to use at least two separate connections to ensurecontinuity and reliability. Whenever non-conducting supports are used, separate groundconductors to establish a direct connection shall be installed from the shielding system to thesubstation ground grid.

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SCECO-East substations are provided with ground grids in accordance with the requirementsof SES-P-119.10.

7.1 Shielding masts supported by steel structure shall be electrically bonded to thestructure which is solidly grounded to the substation ground grid.

7.2 Overhead ground or shield wires terminated at steel structures shall be electricallybonded to the structure which is solidly grounded to the substation ground grid. Themechanical dead-end hardware shall not be used as the electrical path for conductinglightning surges or fault currents to the dead-end structure (see Figure 07-4).

7.3 When the support structure is constructed of wood or other non-conducting material, acontinuous electrical path shall be made between the overhead shielding mast or wireand the station ground grid with a down conductor.

7.4 Down Conductor

7.4.1 The down conductor of a shielding mast or wire system shall be stranded baresoft drawn copper. The size of down conductor shall be 70mm (2/0 AWG).

7.4.2 No bend of a down conductor shall form an included angle of less than 90nor shall it have a radius of bend less than 200 mm.

7.4.3 Down conductors shall be securely attached to their support structures atintervals not exceeding 2,000 mm. The fastener material shall be of suchnature that there will be no serious tendency towards electrolytic corrosion.

7.4.4 Down conductors are not required for electrically continuous metal structures,but do require ground terminals. Connections to steel structures shall be madeon cleaned areas of the steel framework with bolted connectors.

7.4.5 Down conductors shall be guarded/protected to prevent physical damage ordisplacement for a minimum distance of 2400 mm above ground level.

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8.0 EXAMPLE : USE OF WORKING CURVES

Figure 07-3 shows a simple substation layout illustrating the use of the curves of Figure 07-1and configurations of Figure 07-2. A busbar and equipment area of 16,917 mm x 39,930 mmand 6,450 mm above ground is adjacent to the power transformer area of 12,793 mm x 39,930mm that is a maximum of 6,450 mm above ground. Assume that locating shielding masts atpoints 1, 2, 3, and 4 is a reasonable solution. In order to protect the area that is 6,450 mmabove grade, the critical "b" points must be located.

First, consider the high voltage structure area. This area is 16,917 mm x 39,930 mm. Thecritical "b" point therefore, lies along the 39,930 mm side. From Figure 07-1(c) with "d" =6,450 mm and "s" =39,930 mm, "y" " must be at least 5,000 mm.

Shielding masts 5,000 mm higher than the 6,450 mm energized equipment or 11,450 mmabove grade are required at positions 1, 2, 3, and 4. However, the shielding masts at positions1 and 2 are located at the top of the gantry structures. Therefore, the height of the shieldingmasts will be 15,230 mm + 3,050 mm = 18,280 mm. The actual "y" will be 18,280 mm - 6,450mm = 11,830 mm, then the actual "x" will be 18,000 mm from Figure 07-1(a). The disconnectswitches are only 16,917 mm away from the shielding masts and so well protected or shielded.

For the shielding masts at points 3 and 4 with a height of 11,450 mm, the value of "x" fromFigure 07-1(a) is only 6,000 mm and this means that the power transformers are not protected.Then the shielding of the power transformer and lightning arrester at area 12,793 mm x 39,930mm, that is 6,450 mm above ground, requires additional height of shielding masts.

Second, consider the application of masts at locations 3 and 4 to protect the entire 28,710 mmx 39,930 mm area. The critical "b" point is midway between masts 3 and 4. Figure 07-2(b)indicates that the effective distance between masts 3 and 4 must be such that the radius "x" istwice the distance of 12,793 mm.

From Figure 7-1(a) for "d" = 6,450 mm and "x" = 25,586 mm, the height "y" of masts 3 and 4must be 15,000 mm above the protected equipment or 21,450 mm above grade. The protectedarea due to this type of installation will include the 6,450 mm high voltage power transformersand station service transformers located 10,668 mm north of masts 3 and 4.

Third, 9.5 mm diameter lightning protection masts or rods shall also be provided on top of thesubstation building with a maximum distance of 15,000 mm between two masts, connectedtogether and bonded to the substation grounding grid in accordance with the requirementsspecified in NFPA 780.

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FIGURE 07-1 : CONFIGURATION OF SHIELDING OBJECT TOPROTECTED OBJECT FOR 0.10 PERCENT EXPOSURE

DWG # SE-1190701.00.00

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DWG # SE-1190702.00.00

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DWG # SE-1190703.00.00

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DWG # SE-1190704.00.00

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9.0 BIBLIOGRAPHY

1. ANSI/IEEE 141 : "Recommended Practice for Electric Power Distribution for IndustrialPlants".

2. Khalil Denno, "High Voltage Engineering in Power Systems", CRC Press, Inc., USA,1992.

3. NFPA 780 : "The Installation of Lightning Protection System", 1995 Edition.

4. REA Bulletin 65-1, "Design Guide for Rural Substations", 1978.

5. Westinghouse Electric Corporation, "Electrical Transmission and Distribution ReferenceBook", Fourth Edition, Tenth Printing, Pennsylvania, USA, 1964.

TITLETABLE OF CONTENTS1.0 SCOPE2.0 GENERAL3.0 METHODS OF SHIELDING4.0 ZONE OF PROTECTION5.0 HEIGHT AND LOCATION OF SHIELDING6.0 SHIELDING MATERIALS7.0 SHIELD SYSTEM GROUNDING8.0 EXAMPLE: USE OF WORKING CURVES9.0 BIBLIOGRAPHYgo back to MAIN MENU

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