SES-Shield, an Efficient Software Package for the Analysis of Optimum Shielding against Direct Lightning Strikes to Substations and Transmission Lines

Introduction

The new SES-Shield software package is aimed at providing optimum solutions for the protection of transmission lines and substations against direct lightning strikes. SES-shield is based on the most recent electrogeometric and the rolling sphere theories and methods. It is the most recent addition to the line of software packages developed by SES.

The main purpose of this software package is to optimize the location and configuration of shield wires and masts in order to prevent the exposure of energized conductors, busses and equipment to direct lightning strikes.

The lightning is one of the most spectacular natural phenomena. Since Benjamin Franklin showed, two hundred years ago, that this phenomenon was a gigantic electric discharge, many researchers have conducted detailed studies on charge formation in clouds and lightning flashes.

Since these early discoveries by Benjamin Franklin, it is widely accepted that lightning is a transfer of electric, positive or negative, charges of one area of a cloud to another, or between clouds and ground. The major difficulty resides in how to protect people and assets effectively against this phenomenon.

In order to satisfy this fundamental protection objective, several techniques were developed. One of the most rigorous is the electrogeometric model. This model is based on the following evidences:

•    The existence of a relation between the charge of the leader and the field on the ground.
•    The existence of a relation between  the charge of the leader and the return stroke current magnitude

Based on the above observations, it has been possible to establish an analytical expression of the distance between the leader and the object on the ground. This distance is called the “striking distance” and is defined as follows:

where

S                 is the striking distance in m

I                  is the return stroke current magnitude in kA

According to the electrogeometric model, the lightning impact point is determined by the object on the ground which will be the first seen at a striking distance S from the tip of the step leader. To protect various structures against lightning, the electrogeometric model is implemented using the rolling sphere method.

The principle of this method is then extremely simple. Everything evolves as if the point of the leader was surrounded by a rolling sphere. This sphere has a radius equal to the striking distance, centered at the tip of the leader. As a result, this rolling sphere precedes the step leader during its random trajectory towers earth.

When the step leader approaches the ground, the first object that will touch the sphere, will determine the point of impact of the lightning. This leads to the following process :

Ø       If during its descent, the sphere comes in contact with the shielding system first without touching even one of the objects to protect, then the shielding protection is successful.

Ø       If during its descent to ground, the sphere touches with of the objects that requires protection before touching the shielding system, then the shielding system failed and should, therefore, be modified until such undesirable contacts are eliminated.

SES-Shield is based on the various research studied undertaken these last decades for the protection against direct lightning strikes to substations. It uses the rolling sphere method to optimize the system of protection efficiently. The principle also applies to transmission lines.

For example, SES-Shield is particularly suitable to analyze protection against a direct lightning strike on substations composed of parallel bays. Figure 1-1 shows a substation bay along which shield wires (dotted red lines) and masts (points a1,…, a_n and c1,… c_n) are installed. Indeed, SES-Shield can carry out shielding analysis using a protection by shield wires or by masts.

Figure1-1: Example of a bay that could be studied.

SES-SHIELD

This software package is composed of one main interface with its menu bar, buttons bar, input data zone, graphic zone, display report zone and results display zone.

The input data zone makes it possible to specify at the same time the electrical and geometrical parameters that describe fully the system.

The graphic zone allows to display the position of the phase conductors and the system of protection.

The display report zone displays a report text file.

The results display zone regroups all computed values and all relevant information used to define the optimal configuration and location of the system of protection. This is a sub window that appears after computation.

The Type of analysis section allows you to select between Substation and Transmission line analysis.

Figure 1-2: Main interface of SES-Shield

The Menu Bar

SES-Shield menu bar consists of three main menus that allow you access to the following tasks :

• Generate a study report: create, load and print a report
• Define advanced computation parameters
• Get online help

Figure 1-3 : Menus in SES Shield

File Menu

The File menu allows you to start a new analysis and then generate, load and print reports. From this menu it is possible to:

• Make a new analysis
• Open a selected report
• Create a report using the simple name "SH_Report.txt"
• Create a report under a specific file name
• Print the last generated report

The create and print report menus are not available until one of the following conditions are met:

• An analysis has been carried out during the current session
• An existing report has been loaded

The report is a text file that is created in the software installation folder. This file can be viewed using a standard text editor integrated into SES-Shield. The "Open…" menu loads the file. When the open file dialog box appears, text files (*.txt) are automatically filtered.

Figure 1-4 : Descriptive of file menu

Options Menu

The Options menu offers three possible choices:

• Define a working language : English or French
• Define the system of units : Metric or British (Imperial)
• set advanced options for specific studies

The "Language" menu defines the working language. The default working language is English. A check mark is placed in front of the title of the current language (options | Languages). SES-Shield switches from one language to another by reading a text file called english.txt to get an English interface and francais.txt to get the interface in French. The two files are structured and must follow a logical sequence:

• Name of the software package
• Elements of the Menu bar
• Elements of the "Intermediate values" zone
• Elements of the "Critical values" zone
• Element of the "Substation" tab
• Elements of the "conductors" tab
• Elements of the "Checked points" tab
• Elements of the "Advanced options" dialog box

It is very important to keep this sequence if you wish to customize the interface of SES-Shield.

The Units menu defines the system of units. Two types of units are accepted: Metric or British (Imperial). A check mark is placed in front of the selected system. When you wish to change the system of units, you will be asked if you want to convert all values to the selected system of units.

The Advanced menu opens a dialog box that defines relations that will be used to compute the striking distance, the type of system (AC or DC) and the gradient corona coefficient. We will discuss those parameters later in this chapter.

Figure 1-5 : Options menu in SES-Shield

Help Menu

This menu consists of three submenus items:

• Get help on SES-Shield input command syntax
• Get online help on the software package
• Get information and help from SES web site

The Commands menu option provides help on how to use the software in command mode. The main functionalities of SES-Shield have been regrouped into commands in order to facilitate editing and scripting of the project input data file. Those commands are generated by the software in order to preserve the specified data in the current session.

It is possible to get help on how to use the software by selecting the Contents menu. It is then possible to better understand the software functions as well as the way in which it can be parameterized.

The internet web site of SES abounds in useful information. Click on the Support menu to access SES home page.

Figure 1-6 : Help menu in SES-Shield

Input Data for a Substation Analysis

The input date zone is made up of three tabs that defines the:

• Data and parameters of the studied substation
• Characteristics of the phase conductors to protect
• Coordinates of points to be checked regarding exposure to strokes arriving on the sides of the substation

The electric and geometric parameters that the software computes are:

• The critical current
• The striking distance
• The super-elevation
• The separation

Substation Characteristics

This tab consists of the following input fields:

• Phase to phase voltage of the substation
• Mid-span height of phase conductors
• Height of the bus at tower support points

The Phase to phase voltage of the substation corresponds to the RMS voltage between the phases of the substation. This value is necessary to take into account the highest voltage that the insulators must hold permanently and constitutes an important value for the  computation of the critical current.

The Mid-span height of phase conductors defines the vertical position of phase conductors at the middle of the span in the substation. This value is required to compute the radius of the corona effect.

SES-shield computes the vertical distance between phase conductors and shield wires, or masts. This distance is called super-elevation. This value represents the vertical separation between the phase conductors or buses and the location where the shield wires, or masts, will have to be installed, to guarantee an optimal protection.

Figure 1-7: Optimal position of the system of protection based on the  super-elevation distance

Figure 1-8 : Substation characteristics

The substation tab includes a picture zone illustrating the values that must be defined.

Figure 1-9: Illustration zone of substation tab.

The advanced button defines the choice of the system which will be used as a system of protection.

The protection can be by shield wires or by masts. In the case of a protection by shield wires, it is necessary to define the width of the studied bay. This width also corresponds to the distance between two shield wires and it is used to compute the value of the super-elevation. Note that the protection by shield wires is the default protection.

In the case of a protection by masts, it is necessary to define the dimensions of the rectangle formed by the four masts of protection. It is the width between two masts and the depth between two masts. An appropriate illustration is displayed each time the type of protection is changed.

Figure 1-10 shows the type of protection by shield wires. In this case, the fields corresponding to a protection by masts are not enabled.

Figure 1-10: Type of protection.

Conductors Parameters

The Conductors tab is used to define the geometrical characteristics of the phase conductors, namely the:

• Length of insulators string
• Number of conductors in the bundle
• Diameter of conductors
• Spacing between conductors
• Basic impulse level of insulators (BIL) in the case of a bus bar supported by post-insulators.

The withstand voltage of insulators is an important parameter in the analysis. This value defines the highest level of voltage that is possible to apply on the insulators. In SES-Shield, this value can be defined by an empirical relation which is a function of the length of the insulators, or by the level of insulation of the insulators as defined by a BIL value. In the first case the Length of insulators string must be defined. The BIL value can be specified when the Bus supported by post-insulators option is selected. Figure 1‑11 illustrates this choice.

The radius due to the corona effect is usually given for one equivalent conductor only. But, phase conductors are usually assembled in bundle (series of two, three or four phase conductors) and the spacing between them is maintained by a spacer. Thus, it is necessary to reduce the bundle to an equivalent conductor in order to compute the equivalent corona radius when the bundle contains more than one conductor. Consequently, the number of conductors supported by the spacer, the spacing between conductors and the diameter of the conductors must be defined in their corresponding fields, i.e., Number N of conductors, Spacing E between conductors and Diameter D of conductors.

Note that if the number of conductors is equal to one then the field spacing E between conductors is not enabled.

Figure 1-11 : Input zone of conductors’ characteristics

Points to be Checked

The third and last tab used in SES-Shield is used to check if the protection of substation equipment is correctly done when lightning strikes the sides of the substation

Various points representing the vulnerable location of the equipment can be checked to determine if the protection is appropriate. These points are placed at a certain height above ground and at a specific horizontal separation from the system of protection.

Figure 1-12 shows the horizontal separation BETA ( ) of the equipment point to be checked.

Figure 1-12: Setting for the computation of the horizontal separation

SES-Shield computes the value PHI ( ) and checks if the condition  is valid or not:

Ø       If  then the equipment placed at the ( ,H) coordinates is adequately protected

Ø       Otherwise the equipment placed at the ( ,H) coordinates is not protected

The value  must be defined according to the following rules:

Ø       Negative if the point is within the system of protection zone

Ø       Positive if the point is outside the outermost system of protection zone

Finally, there is an illustration zone which changes depending on the number of selected points.

Figure 1-13 : Points to be checked for lightning surges striking the sides of the substation.

Printed Values for Substation Analysis

The Computed values zone displays simultaneously some intermediate values and all critical values. This zone is divided in two parts:

• Intermediate values
• Critical values

Two non-editable Rich-Text boxes are used. The two zones are automatically cleared when a new analysis is about to be carried out by selecting the New item from the File menu.

The Intermediate values zone displays the values that have been used to compute the critical values. They consist of the:

• Withstand voltage of the insulators. This value is used to compute the stroke current
• Equivalent radius of conductors’ bundle
• Radius due to the corona effect
• Surge impedance value that is a function of the corona radius and the equivalent radius of conductors’ bundle

Figure 1-14 : Intermediate values.

The Critical values zone displays the following results:

• The stroke current which determines the striking distance
• The striking distance that is used to compute the super-elevation and separation values
• The super-elevation that determines the vertical distance between the phase conductors and the shield wires or masts.
• The total height where the shield wires or masts must be placed

Figure 1-15: Critical values

Lightning Specification for Substation in SES Shield

Various options are available to quickly customize the interface and the parameters or elements that must be considered in the computations. The striking distance Options dialog box appears when Striking distance is chosen from the tree view. The possible parameters are:

• Assign a name to the substation under study
• Define type of system
• Choose the striking distance relation
• Select the limit of the corona gradient if the default value is not adequate

Figure 1-16:Lightning specification dialog box

The corona gradient value E0 influences largely the computation of the corona radius. This value belongs to the Other Parameters that can be modified. However, various researches undertaken in the lightning field recommend the use 1500 kV/m as limit of corona gradient.

In SES-shield you can choose the relation that calculates the striking distance. Three relations are proposed. The relation of Mousa (relation adopted by the IEEE standard) is the relation by default.

In order to differentiate various analysis, it is recommended that you use the field Name of studied substation to assign a name to the substation. This name appears in the generated report.

Report Analysis

The report is an important element in SES-Shield because it regroups in the same document the :

• Characteristics of the substation as input data
• Intermediate values as output results

The conclusion to the analysis for checked points regarding exposure to strokes arriving on the sides of the substation are also included in the report.

Figure 1-17 : Edition of one analysis report in order to visualize computations.

Content of the Report File

The report generated in SES-Shield is composed of four parts:

• Headline
• Characteristics of the studied substation
• Results of intermediate computations
• Design of the system of protection

Headline’s report: it contains information about the software version used, the name of the studied substation (or name of the study) and the date of the analysis.

Characteristics of the studied substation: reflects all the values which were specified by the user. Thus, we can find the phase to phase voltage, the various heights of phase conductors and all the characteristics of the conductors (number of conductors, diameter, etc…).

Intermediate results: all values which were used in the computation are displayed. These values are used to design the system of protection and to validate the optimal position of this system. It is in this part that the values of the corona radius and the number of iterations needed to compute this radius appears. Important information, such as the surge impedance, the critical current and the striking distance, are also displayed in this heading.

Design of the system of protection: The optimal height of the system of protection as well as the separation distance is displayed in Design of the system. This part also describes the type of protection (shield wires or masts) and displays either the distance between two shield wires, or the width and the depth of the rectangle formed by the four masts.

The value of the separation is given for each point that must be checked. Those points were specified in the computation tab. The value of  is followed by one of the two following sentences:

·         Equipment placed at the point ( ,H) is shielded

·         Equipment place at the point ( ,H) is not shielded

Example of a Substation Shielding Analysis

This following example uses SES-Shield to analyze a substation consisting of one switch-yard, made of similar parallel bays. The natural choice for a system of protection would be a set of parallel wires located at the boundaries of each bay (represented by red dashed lines in Figure 18). We want to analyze a bay of width L for three levels of voltage,138kV, 230kV and 500kV respectively. The studied bay is shown Figure 1‑18 and Figure 1-19.

The first analysis that is carried out defines the optimal position of the system of protection when a lightning strikes between two shield wires, or at the top of the zone which is defined by the four masts.

Figure 1-18: Top view of a studied bay

The second analysis checks the possible exposure of the equipment when lightning strikes the sides of the substation. This is represented by the points of separation BETA, noted B, on the side view of the bay for various heights H.

Figure 1-19: Side view of the studied bay

Characteristics to set

The following table shows the values that must be specified in SES-Shield. Each section of the table represents one tab in the software.

Table 1-1: Values for substation characteristics

Analysis of the Results

The three following tables summarize the results obtained for the analysis of the superelevation and the analysis of the separation, for the three phase to phase voltage cases of the substation. SES-Shield computes all following quantities when the Process button is pressed.

Table 1-2: Intermediate computed values

Examination of Table 1-2 reveals that the striking distance increases with increase of the nominal voltage of the substation.

Table 1-3: Analysis of the superelevation and the separation for three voltage levels cases.

Concerning shielding against strokes arriving on the sides of the substation, the maximum allowable values are positive. This means that the shielded object is allowed to be located outside the bay. For all wires located inside the bay (negative value), effective shielding exists for the top bus in all cases.

Figure 1-20 shows graphically the position of the protection system from the superelevation computation.

Figure 1-20: Display of the position of the protection system

Transmission line analysis in SES-Shield

Transmission lines are subjected to two types of electric constraints:

·         The constraints whose origin is internal to the network (temporary over-voltages, switching surges, etc…)

·         The constraints external to the network and to the studied transmission line such as lightning, solar flares and the resulting geo-magnetically induced currents, nuclear electromagnetic pulses, etc.

This section focuses on the lightning imposed stresses.

Many factors play an important role in a transmission line route selection:

·         Power system considerations dictate where the transmission line should begin and end

·         Economic considerations require the line to be as short as possible to minimize construction costs and electrical losses

·         Other environmental and ecological constraints dictate where and how transmission lines may be built

Even with these restrictions, there are still various means that a transmission-line designer can use to enhance the performance of a transmission line. One of the effective and relatively easy ways to improve the lightning performance of a transmission line is by selecting a low-lightning activity route, if possible. When two routes with similar soil characteristics are being compared, the route through a region with lower density of severe flashes will have fewer outages. With more detailed maps averaged over enough time, the designer may select a route with a minimum exposure to lightning. Lightning location systems and flash counters provide detailed Ground Flash Density (GFD).

Regions through which transmission lines must be routed are characterized by their keraunic level, or isokeraunic level, as it is usually called. This keraunic level represents the average number of thunder-days per year in this region. This is the average number of days per year during which thunder will be heard for a 24-hour period. For simplicity, it has usually been assumed that the number of flashes to earth or transmission lines in a specific region is proportional to the keraunic level in that region. A large number of relations between flash counts to earth and the keraunic level have been proposed during the last few decades.

Shielding failures are another flashover mechanism that must be considered since they affect the performance of transmission lines. These failures occur when a flash misses the shield wires or transmission line structures and terminates directly on the phase conductors. These shielding failures should be avoided or at least minimized. This is where SES-Shield can help. SES-Shield computes the optimal position of transmission line shield wires for an effective shielding of the phase conductor. SES-Shield also computes the minimum and the maximum stroke currents that may cause a shielding failure or a flashover respectively.

Figure 1-21 shows a simplified model of the postulated shielding failure mechanism for one shield wire and one phase conductor above a horizontal earth. In this figure, three flashes of equal current magnitude are shown nearing the line. As a flash approaches within a certain critical distance, S, of the earth and the line conductors various outcomes can occur. In this case, the tip of the flash or leader is influenced (i.e., aware) by the presence of objects within this striking distance S. The flash will jump (short) the distance, S, to make contact with the conductor at this striking distance.

Figure 1-21: Incomplete shielding – width X_S is uncovered – Stroke B jumps to the phase conductor

In Figure 1-21, Flash A may make its final jump only to the shield wires because anywhere on the arc OP, the distance to the phase conductor, Pc, exceeds S. Flash C may jump only the distance, bS, to the earth because anywhere on the line QR the distance to the phase conductor is too large. The coefficient b is a value larger than one that accounts for the strong likelihood that the final strike distance to the horizontal ground plane, with its widespread attractive effects, will be significantly different from the strike distance to a wire suspended above the plane.

Note that Flash B, as soon as it reaches the arc PQ, may jump only to the phase conductor, Pc, because the distances to the shield wire and earth will exceed the corresponding strike distance. For verticals flashes, the width X_s then establishes the uncovered area of the earth in which flashes that generally would reach the ground, contact the phase conductor instead.

To achieve an effective shielding condition a designer would usually hold the phase conductor fixed and move the shield wire horizontally until the unprotected width, X_s, is reduced to zero. For optimal shielding, if the X_P coordinate of the phase conductor is taken as zero, the X_G coordinate of the shield wire (or offset distance) is then:

In this case, the effective shield angle becomes:

Note that for positive shield angles, X_G will be negative because the shield wire will be to the left of the phase conductor.

Figure 1-22: Effective shielding – Unprotected width X_s is reduced to zero

Input Data Required for a Transmission Line Analysis

The input date zone is made up of three main windows that define the:

·         Length of span and mid-span sag of the transmission Line

·         Various data and parameters of the transmission line

·         Characteristics of the shield wires

Length of Span and Mid-Span Length

The Span and Sag Values window consists of two tabs where the following values are defined and displayed:

·         The span length

·         The mid-span Sag

Figure 1-23: Sag and Span Values

The Span length is the average length of transmission line between two consecutive towers. This value is used to compute the time a lightning stroke takes to travel along the transmission line from one tower to the next in order to account for reflections of the surge at towers.

The Mid-Span Sag corresponds to the maximum vertical distance, in a span of a transmission line, between the conductor location and a straight line passing through the two points that support the conductor at the insulator strings. This value is required to compute the average height of the transmission line.

Characteristics of the Transmission Line

This window (see Figure 1-24) is used to define the geometrical and electrical characteristics of the transmission line, namely the:

·         Number of conductors in the bundle

·         Diameter of the conductors

·         Spacing between conductors

·         Conductor definition

The radius due to the corona effect is also computed when a transmission line analysis is carried out. For the reasons given earlier, SES-Shield computes first the equivalent radius of the conductors’ bundle and then the corona radius. Consequently, the number of conductors supported by the spacer, the spacing between conductors and the diameter of the conductors must be defined in their corresponding fields, i.e., Number N of conductors, Spacing E between conductors and Diameter D of conductors

The Conductor Definition consists of a table where the following specific information can to be entered:

·         The position of the center of the phase conductors’ bundle: X coordinate and height of the bundle

·         The length of the insulator strings installed on the line

·         The voltage of the line

The last field in the table is a check box that identifies the most exposed conductors that require a shielding analysis.

Figure 1-24 shows this table. One row corresponds to one phase conductor. If you wish to analyze that the conductor is correctly shielded, then it must be checked.

The position of conductors is required in order to verify if they are correctly shielded. SES-Shield uses the average height of the conductor to be protected and is based on this value to establish the horizontal position of the shield wire which protects the conductor. Based on the height and X coordinate, SES-Shield computes the actual and the required angle between the phase conductors and shield wires in order to achieve an effective shielding. If the difference between these two angles is positive, then SES-Shield establishes the uncovered width.

For the same reasons given in the Conductor Parameters window used for substation analysis, the length of insulator strings is required to compute the withstand voltage. Thereafter, the withstand voltage is used to compute the critical current.

Figure 1-24: Characteristics of the transmission line phase conductors

Characteristics of the Shield Wires

Shield wires are used to provide a means of intercepting the lightning flash and reduce the Shielding Failure Rate to an acceptable level. The characteristics are specified in the shield wires window accessible by clicking on the transmission line portion of the tree-view. The following information must be entered:

·         Diameter of shield wires

·         Mid-span sag of shield wires

·         Number of shield wires that will be used

·         Coordinates X and Y of the shield wires

The diameter of the shield wires is required to compute the surge impedance of each shield wire.

SES-Shield determines the actual and the effective angle between phase conductors and shield wires. In order to compute these two angles and also the horizontal position of the shield wires, SES-Shield uses average height values which is the total height at the support points minus two-third of the sag. Consequently, the Mid-Span Sag of the shield wires must be defined.

It is possible to define up to three shield wires using the Number of shield wire value. For each specified shield wire The X and Y coordinates input fields are automatically enabled.

Figure 1-25 shows the window where all values mentioned above can be specified.

Figure 1-25: Shield wires specification

Transmission Line analysis Report

The report consists of zones that display simultaneously the intermediate values and the critical values. The report window is divided in two parts:

·         Intermediate Values

·         Critical Values

The Intermediate Values zone displays the necessary values that have been used to compute the critical values. They consist of the:

·         Ground flash density

·         Number of flashes to the line

·         Beta coefficient also named beta factor.

This Beta coefficient accounts for the strong likelihood that the final strike distance to the horizontal ground, with its widespread attractive effects, will be significantly different from the strike distance to a wire suspended above the ground plane.

Figure 1-26: Intermediate values for transmission line analysis

The critical values zone displays the following results:

·         The uncovered width

·         The maximum strike distance

·         The maximum current

·         The probability of minimum and maximum current occurrences

Figure 1-27: critical values

Lightning Flash Density Specification

You can specify the Keraunic level or the Ground Flash Density to characterize the lightning activity level corresponding to your analysis. For simplicity, it has usually been assumed that the number of flashes to earth or transmission line in a specific locality is proportional to the keraunic level in that locality.

Figure 1-28: Ground Flash Density or Keraunic level specification

Report Analysis

The report is an important element in the transmission line analysis because it regroups in the same document shown in Figure 1-29 the:

·   Characteristics of the transmission line and shield wires as input data

·   Intermediate values as output results

·   The effective shielding status of the analyzed transmission line

Figure 1-29: Edition of one analysis report in order to visualize computations.

Content of the Report File

The report generated in SES-Shield is composed of five parts:

·         Headline

·         Characteristics of phase conductors and shield wires

·         Intermediate computed values

·         Analysis of the required position of shield wires and effective angles

·         Shielding failure rate results

Headline’s report: It contains information about the software version used, the name of the analysis and the date of the study.

Characteristics of phase conductors and shield wires: reflects all the values which were specified by the user. Thus, you can find the number of phase conductors and shield wires, various height and also position of conductors and shield wires.

Intermediate computed values: all values which were used in the computation are displayed. These values provide important information, such as the average height of shield wires, the uncovered width and the number of flash to the line (Ground Flash Density).

Analysis of position of shield wires and effective angles: The optimal position of the shield wires as well as the shielding angles is displayed in the Analysis section. This part also displays intermediate computed values such as withstand voltage, surge impedance, nominal stroke current and striking distance, for each conductor being analyzed.

Shielding failure rate: Only flashes having a stroke current between I_min and I_max can cause a shielding failure according to the electrogeometric theory. The maximum value of the strike distance and stroke current are displayed in the Shielding Failure Rate Computation section. The minimum current, the maximum current and the uncovered width determine the number of flashes per 100 km per year. The number of flashes is a function of the probabilities for the minimum and maximum current flashes to occur. These values are displayed in this section.