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.