Introduction
Methods for estimating transmission line lightning performance are outlined in Chapter 6 of the
EPRI Transmission Line Reference Book: 200 kV and Above [Red 2006]. These calculations are
time-consuming to perform by hand, even with the applets provided, when there are several mitigation options to investigate. In general, it is impractical to undertake any such calculations without a computer and the proper software.
The same is true for evaluating possible strategies for improving lightning performance. The placement of arresters, the improvement of tower grounds, the placement or relocation of shield wire, and the replacement of shield wires with arresters should be evaluated with the proper computer modeling in order to compare the effectiveness of each and, ultimately, to select the most cost-effective lightning performance design.
Lightning Performance Design Workstation (LPDW)
Several software packages can be used to simulate the effects of lightning on transmission lines. Programs giving a highly detailed model of power system transients, with user-defined source current and attachment point, include EMTP, TLP, ATP, Matlab Power System Blockset and PSCAD. Programs giving a simplified model of the power system transient, but incorporating models for the lightning source current statistics, include the IEEE FLASH program provided with IEEE Standard 1243.
Two programs, SIGMA slp and TFLSAH, offer the specific features that allow tower-by-tower parameter variation along the statistical variations in lightning surge currents. In particular, the distribution of soil resistivity or footing resistance along the line can be entered if the tower-to- tower variation has been established. The balance of this section will discuss the EPRI TFLASH routine, Version (?4.1).
TFLASH Overview
This section is not intended to be a step by step guide through the TFLASH program or a detailed description of the algorithms used. These are covered in the online tutorials and help features of the program. The on-line information will be kept updated as new research data is incorporated into the program and algorithms are changed or updated. This section is merely a summary of TFLASH capabilities and features.
Application Software
TFLASH is a Windows TM based program that allows the user to model an entire transmission line or sections of a line quickly and easily on a PC. The line can have an arbitrary number of towers, and there are no restrictions on the number or order of unique towers that can be entered in the model other than the memory and computation capability of the computer used. Several other programs allow for a limited number of unique towers that can be repeated periodically. TFLASH provides libraries of towers and equipment to assist the user in building a model. Once the model is completed, the user can perform a number of different analyses.
Building a TFLASH Model
TFLASH Capabilities
Before describing the details of constructing a TFLASH model, the user should be aware of general capabilities and operating characteristics of the software, which are outlined below: • TFLASH can simulate a maximum of three shield wires and 12 phase wires, for a total of 15
conductors.
• TFLASH can simulate up to four circuits.
• The number of conductors must remain constant throughout the model. For instance, a single-circuit tower with a shield wire cannot be connected to a single circuit tower without a shield wire. The first tower has four conductors and the second has only three. (This scenario can be analyzed with two separate models, however .)
• TFLASH cannot perform analyses for corridors with circuits on different towers.
• TFLASH will calculate circuit response to a discrete, specified lightning event at a tower or series of towers. Output graphs will plot specified voltages and currents against time for specified locations on the span or tower and will also report arrester energy absorption. • TFLASH will estimate the average statistical performance of a transmission line or line
section for the full spectrum of lightning events that would be expected for a given location. This spectrum can be selected from IEEE or CIGRE Curves or from local data when the area is covered by the North American Lightning Detection Network (NALDN).
• TFLASH will analyze simultaneous multi-phase or multi-circuit flashover performance.
General Procedure for Constructing a Line Model
To construct a tower model, the user should be aware of the information that TFLASH requires and the procedure for supplying that information.
First, the user must select a tower from a library of tower structures, as shown in . Once a tower is selected, the user must enter information about the tower:
• General properties • Ground system 7-2
Application Software
• Equipment on the tower
Figure 7-1
Tower Modeling Screen from EPRI TFLASH (dummy)
The tower general properties are as follows:
• The length of the span that connects the tower to the next tower.
• The ground flash density (GFD), which is given in terms of the number of lightning strokes to ground per square kilometer or per square mile per year. This can be determined from the NLDN program data base for the U.S., or it can be specified by the user.
• The location of natural shielding, or lack thereof, from hilltops, valleys, trees, buildings, and other structures
The user has a number of options for the tower grounding. First, the user can select from three different ground types:
• Driven rods
• Continuous counterpoise • Radial counterpoise
The user can then enter the ground resistance directly. If the ground resistance is not known, or the user selects driven ground rods, TFLASH will calculate the resistance using the following data:
• Soil resistivity
• Ground rod diameter • Ground rod length
Application Software
• Number of ground rods
If the user selected a continuous counterpoise, the following must be specified: • Soil resistivity
• Wire diameter
• Depth to which the wire is buried
If the user selected a radial counterpoise, all of the data that would have to be entered for a continuous counterpoise must be entered, PLUS:
• The number of branches that make up the counterpoise • The length of each branch
The tower equipment includes conductors, insulators and arresters. As Figure 7-2 illustrates, for each conductor in the tower, the user must enter:
• The type and name of the conductor (i.e., ACSR, Pheasant). These values can be selected from a database of conductor types and names.
• The conductor use (i.e. Circuit 1, Phase A) • The height of the conductor above the ground
• The horizontal location of the conductor from the center of the tower • The sag of the conductor
Application Software
Figure 7-2
Conductor Information Screen from TFLASH (dummy)
For each insulator, the user must enter:
• The type and name of the insulator (Again, these are also in a database to assist the user.) The name field also contains the number of units in the string for ceramic suspension insulators.
• The geometry of the insulator (i.e., a V-string, an I-string, or a dead-end) For each arrester, the user must enter:
• The arrester name and rating (These are also in the database.) • The series gap length, if gapped arresters are used.
There are many more options within TFLASH. This section is intended to give the reader an idea of the minimum data required to get TFLASH results. When all of this information has been provided, the tower is complete. Then the user can copy the tower, create new towers, or copy repeating sequences of towers until the line is modeled.
Analyzing a TFLASH Model
Once a TFLASH model has been constructed, two different types of analyses can be done. The first calculates the statistical performance of the line, and the results it produces represent the "average" yearly performance of the line if the line were observed for a long time. The second type of analysis simulates the performance of the line during a single, user-specified lightning event. Both types of analyses are useful lightning performance calculations.
The Classical Solution - The Average Performance of the Line
The process of determining the average performance of the line is called the classical solution. The classical solution works by first using the electrogeometric model (EGM) to determine the distribution of strokes along the line. This is accomplished by applying the EGM at each unique tower at back midspan, quarter-span at the tower and the forward quarter span for strokes of incremental magnitude and perpendicular distance. A traveling wave model is then initiated that computes the propagation of currents through the towers, grounds, arresters and spans. Voltages are then computed to determine whether a stroke caused a flashover. The process is repeated for strokes of various peak currents until the critical current is found, below which there are no more flashovers.
Menu options include performing calculations only to the first insulator flashover which is a relatively quick task for the computer, or continuing calculations to also determine if multiple phase or multiple circuit flashovers occur. Other options include limiting calculations to any continuous sequence of towers or performing calculations on the entire line model. The effects of power frequency voltage can also be included if desired.
Application Software
The statistical results of the solution are presented in terms of the average number of flashovers per year that the line will experience if operated for a very long time. This data is arranged into a number of different reports:
• Phase Flashover Report • Tower Flashover Report
• Multiple-Phase, Single-Circuit Flashover Report • Multiple-Circuit Flashover Report
• Arrester-Failure Report
The Phase Flashover Report lists the average number of times a year a given phase would be expected to have a breaker operate. For example, phase A of circuit 1 may be expected to flashover 11.52 times per year. This is a convenient overview of the total number of breaker operations that could be expected in a year.
The Tower Flashover Report lists the average number of times a year that an insulator flashes over on a given tower. For example, tower #34 may have an insulator flashover 1.66 times per year. While the user will find this information particularly helpful in identifying problem
sections on the transmission line, caution must be exercised in using this information for arrester placement. Chapter 5 provides information on arrester placement strategies.
The Multiple-Circuit Flashover Report lists the average number of times a year that more than one circuit trips out. For example, the combination of circuit #1 and circuit #2 may flashover simultaneously 0.16 times per year. This is an important number in terms of power quality because multiple-circuit outages can be far more serious than single-circuit outages.
The Multiple-Phase, Single-Circuit Flashover Report lists the number of times two or more phases of the same circuit flash over. For example, an A to B phase-to-phase flashover of circuit #1 may occur 0.34 times per year. This is not as serious an occurrence as a multiple-circuit flashover, but it is of greater consequence than a single phase flashover.
The Arrester Failure Report gives two important pieces of information: the total number of arresters that fail (and therefore must be replaced), and the total number of additional breaker operations that arrester failures cause. For example, in one year 3.22 arresters on a given line may be expected to fail and be replaced. Of those failures, 2.80 per year would cause a breaker operation even though an insulator did not flashover. This is useful in terms of making arrester lifetime assessments and in determining the relative improvement in lightning performance from the application of arresters.
Oscillographs - Line Behavior for a User-Specified Stroke
The second type of analysis that the user can perform is to simulate the response of the line to a single lightning event. The user defines the stroke characteristics, and location on the line and T- flash computes the voltages and currents at various points on nearby towers. This type of
Application Software
analysis is helpful in determining how arresters improve performance. For example, if NLDN data indicate that a particular set of towers was hit with a 65 kA stroke (which subsequently caused a flashover), the user could simulate the single lightning event using the oscillograph. The user could then try different mitigation techniques to preventing the event from happening again.
This feature can also be used when the classical solution Tower Flashover Report lists a section of line as a problem area. The user could focus on this section of line using oscillographs to evaluate various mitigation options.
General Procedure / Sample Application
As an example, assume that the user has a transmission line ridge crossing with a high exposure factor and high footing resistance. A first step could be to build a line model without arresters and to run the classical solution. Using the Phase Flashover Report, the user could obtain an estimate of the total number of flashovers expected each year and compare it with past
performance. Next, the user could use the Tower Flashover Report to find the problem locations. If the user could also run an oscillograph solution for a sample stroke for an indication of which conductors would benefit the most from the application of TLSA. The user could then edit the model, adding arresters at various points in the section of line. Finally, the user could run the classical solution again, checking the Phase and Tower Flashover Reports so see if there has been an acceptable improvement in the line.
These techniques also could be applied to test the performance of different tower designs and different line routing tower placements, all with the goal of optimizing the design of a
transmission line.
Other mitigation techniques, such as the addition of counterpoise, can be applied. Then an economic analysis can be used to determine the most cost-efficient mitigation strategy. Similar analyses can be performed on long transmission line sections through open ground, trees, valleys, and ridges to determine mitigation strategies.