APPLICATION OF KNOWLEDGE-BASED FUZZY INFERENCE SYSTEM ON HIGH VOLTAGE TRANSMISSION LINE MAINTENANCE A Thesis Submitted for the Degree of Master of Engineering By Mohd Junaizee Mohd Noor, B.Sc Elect Eng (Missouri) School of Electrical & Electronic Systems Engineering Queensland University of Technology 2004 KEYWORDS High voltage transmission lines; insulators; tower structures; foundations; conductors; maintenance management; visual inspection; artificial intelligence; fuzzy logic; fuzzy inference systems; knowledge-based systems ABSTRACT A majority of utilities conduct maintenance of transmission line components based on the results of routine visual inspection The inspection is normally done by inspectors who detect defects by visually checking transmission line components either from the air (in helicopters), from the ground (by using high-powered binoculars) or from the top of the structure (by climbing the structure) The main problems with visual inspection of transmission lines are that the determination of the defects varies depending on the inspectors’ knowledge and experience and that the defects are often reported qualitatively using vague and linguistic terms such as “medium crack”, “heavy rust”, “small deflection” As a result of these drawbacks, there is a large variance and inconsistency in defect reporting (which, in time, makes it difficult for the utility to monitor the condition of the components) leading to ineffective or wrong maintenance decisions The use of inspection guides has not been able to fully address these uncertainties This thesis reports on the application of a visual inspection methodology that is aimed at addressing the above-mentioned problems A knowledge-based Fuzzy Inference System (FIS) is designed using Matlab’s Fuzzy Logic Toolbox as part of the methodology and its application is demonstrated on utility visual inspection practice of porcelain cap and pin insulators The FIS consists of expert-specified input membership functions (representing various insulator defect levels), output membership functions (indicating the overall conditions of the insulator) and IF-THEN rules Consistency in the inspection results is achieved because the condition of the insulator is inferred using the same knowledge-base in the FIS rather than by individual inspectors The output of the FIS is also used in a mathematical model that is developed to suggest appropriate component replacement date It is hoped that the methodology that is introduced in this research will help utilities achieve better maintenance management of transmission line assets CONTENTS Keywords i Abstract ii Contents iii List of Figures vi List of Tables ix List of Abbreviations x Statement of Original Authorship xii Acknowledgments xiii Chapter 1: Introduction 1.1 Justification for and Introduction to the Research 1.2 Aims and Objectives of the Research 1.2 Organization of the Thesis Chapter 2: Components of Transmission Lines and their Failure Modes 2.1 Chapter Overview 2.2 Towers and Structures 2.2.1 Functions of Transmission Towers 2.2.2 Failure Modes of Steel Transmission Towers 12 2.3 Foundations 13 2.3.1 Functions of Foundations 13 2.3.2 Failure Modes of Foundations 16 2.4 Conductors and Earth Wires 18 2.4.1 Function of Conductors and Earth Wires 18 2.4.2 Failure Modes of Conductors 21 2.4.2.1 Conductor Corrosion 22 2.4.2.2 Conductor Vibration 24 2.4.2.3 Conductor Annealing 27 2.5 Insulators 27 2.5.1 Functions of Insulators 27 2.5.2 Failure Modes of Insulators 31 2.5.2.1 Mechanical Failures 31 2.5.2.2 Electrical Failures 34 2.5.2.3 Audible Noise and Radio Interference 38 2.6 Chapter Summary .39 Chapter 3: Inspection, Diagnosis and Maintenance of Transmission Line Components 41 3.1 Chapter Overview .41 3.2 Review of Component Diagnosis Methods 41 3.2.1 Test for Tower Structural Strength 42 3.2.2 Diagnostic Tests on Tower Foundations 43 3.2.3 Diagnostic Tests on Conductors 45 3.2.4 Insulator Diagnostic Tests 46 3.3 Review of Inspection and Maintenance Methods 53 3.3.1 McMahon Survey of Inspection Practice of Australian and New Zealand Utilities 54 3.3.2 CIGRE Survey of Utility Assessment of Existing Transmission Lines 55 3.4 Visual Inspection 56 3.4.1 Ground-level and Climbing Inspection 56 3.4.2 Aerial Inspection 58 3.4.3 Drawbacks of Visual Inspection 60 3.5 Chapter Summary .61 Chapter 4: Fuzzy Logic and Fuzzy Inference System .63 4.1 Chapter Overview .63 4.2 Fuzzy Logic 63 4.2.1 Membership Functions 64 4.2.2 Fuzzy IF-THEN Rules 67 4.3 Fuzzy Inference System .68 4.3.1 Inference and Defuzzification Techniques 69 4.3.2 Developing Fuzzy Inference Systems 72 4.4 Applications of Fuzzy Logic in Utility Environment 73 4.5 Chapter Summary .75 Chapter 5: Using Fuzzy Logic for Transmission Line Defect Assessment 77 5.1 Chapter Overview .77 5.2 Application of the FIS on Porcelain Cap and Pin Insulators .78 5.2.1 Factors Affecting Pin Corrosion 79 5.2.2 Factors Affecting Chipped/Broken Porcelain Insulator Disc 83 5.2.3 The Insulator Visual Inspection Process 89 5.2.4 The Fuzzy Inference System for Visual Inspection of Insulators 91 5.2.4.1 Assessment of Single Insulators 91 5.2.4.2 Assessment of Multiple Insulators in a String 112 5.2.4.3 Assessment of Multiple Insulator Strings in a Transmission Line 114 5.3 Application of the FIS on Other Transmission Line Components 118 5.4 Potential Savings due to Introduction of FIS in Tenaga Nasional Berhad’s Transmission Line Inspection and Maintenance Practice 120 5.5 Chapter Summary 122 Chapter 6: Conclusions and Future Work 124 6.1 Summary of the Research 124 6.2 Major Research Contributions 129 6.3 Future Work 130 References 131 Appendix 138 LIST OF FIGURES Figure 2-1: Example of a typical overhead transmission line Figure 2-2: Monopole and steel lattice tower designs for 220 kV transmission lines 11 Figure 2-3: Structural failures of a transmission line due to an ice storm in Canada 12 Figure 2-4: Uplift and compression forces on tower foundations 14 Figure 2-5: Typical construction of transmission tower steel grillage foundation 15 Figure 2-6: Typical construction of transmission tower rock anchor foundation 16 Figure 2-7: Typical ACSR conductor stranding arrangements 18 Figure 2-8: Arrow showing evidence of rust on steel core of ACSR conductor 22 Figure 2-9: Sectional view of ACSR conductor illustrating galvanic corrosion mechanism .23 Figure 2-10: Arrow showing signs of corrosion at a conductor mid span joint 24 Figure 2-11: Multiple fatigue cracks of outermost aluminum strands of an ACSR conductor 25 Figure 2-12: Cross-sectional view of a cap and pin insulator 29 Figure 2-13: Porcelain cap and pin insulator string 29 Figure 2-14: Typical polymeric insulator construction .30 Figure 2-15: Pin corrosion is most severe at cement interface 32 Figure 2-16: Brittle fractures on composite insulators .33 Figure 2-17: ‘Doughnut’-like separation of the porcelain shell from its cap and pin 36 Figure 3-1: Sequence photograph of a transmission tower collapse during a structural insitu test .43 Figure 3-2: Diagram of the half-cell measurement method 44 Figure 3-3: Electric field distribution across an insulator string with a punctured unit in the middle 49 Figure 3-4: Variation of electric field along an 18-unit insulator string which shows defective insulator units at insulator number 7, 11, 14 and 15 50 Figure 3-5: Aerial visual inspection of transmission line components using the helicopter .59 Figure 3-6: Broken conductor strands due to gunshot 59 Figure 4-1: Triangular membership function (10, 20, 30) 65 Figure 4-2: Intersection of membership functions (A AND B) 66 Figure 4-3: Union of membership functions (A OR B) 67 Figure 4-4: Components of a fuzzy inference system 68 Figure 4-5: Mamdani fuzzy inference method .70 Figure 4-6: Defuzzification schemes to derive a crisp output 72 Figure 5-1: Voltage profile across a 132 kV insulator string .80 Figure 5-2: No rust insulator pin condition 81 Figure 5-3: Light rust insulator pin condition 82 Figure 5-4: Medium rust insulator pin condition 82 Figure 5-5: Heavy rust insulator pin condition 82 Figure 5-6: P-F curve for rust condition of insulator pin 83 Figure 5-7: Cutaway drawing of a normal type cap and pin insulator .84 Figure 5-8: Cutaway drawing of an anti-fog type cap and pin insulator 85 Figure 5-9: Chipped porcelain disc 86 Figure 5-10: Small breakage of porcelain disc 86 Figure 5-11: Major radial breakage of porcelain disc 87 Figure 5-12: Total porcelain disc breakage 87 Figure 5-13: Arrows show partially broken porcelain discs in an insulator string 87 Figure 5-14: Arrows show totally broken porcelain discs in an insulator string 88 Figure 5-15: Structure of insulator inspection FIS 92 Figure 5-16: Input membership functions for pin rust conditions 93 Figure 5-17: Input membership functions for porcelain shell conditions 94 Figure 5-18: Output membership functions for insulator condition .94 Figure 5-19: A 3-dimensional plot of insulator inspection FIS showing the relationship between the two inputs and output .98 Figure 5-20: Insulator Sample 99 Figure 5-21: Pin rust condition for insulator Sample 100 Figure 5-22: Porcelain shell condition for insulator Sample 100 Figure 5-23: Resultant of invoking Rule 101 Figure 5-24: Resultant of invoking Rule 11 101 Figure 5-25: Aggregation of Rules and 11 resultants 102 Figure 5-26: Insulator Sample 103 Figure 5-27: Porcelain shell condition for insulator Sample 104 Figure 5-28: Pin rust condition for insulator Sample 104 Figure 5-29: Resultant of firing Rule 105 Figure 5-30: Resultant of invoking Rule 106 Figure 5-31: Aggregation of Rules and resultants 106 Figure 5-32: Insulator Sample 107 Figure 5-33: Porcelain shell condition for insulator sample 107 Figure 5-34: Pin rust condition for insulator sample 108 Figure 5-35: Resultant of invoking Rule 12 109 Figure 5-36: Resultant of invoking Rule 17 109 Figure 5-37: Aggregation of Rule 12 and Rule 17 resultants 110 Figure 5-38: Proposed structure of tower inspection FIS 119 Figure 5-39: TNB transmission line forced outages 1997-2003 121 LIST OF TABLES Table 2-1: Basic transmission structure types Table 2-2: Requirement for zinc coating thickness as per BS EN ISO 1461 11 Table 2-3: AN and RI limits for insulator at typical system voltages 39 Table 3-1: ASTM C867:1999 criteria for corrosion of steel in concrete .44 Table 3-2: Emerging and available aerial inspection technologies 60 Table 4-1: Determination of membership function from -cut sets .66 Table 5-1: Category of in-service porcelain insulator disc defects 88 Table 5-2: Pin corrosion description used by Powerlink during visual inspection .90 Table 5-3: IF-THEN rules used in the insulator inspection FIS 97 Table 5-4: Suggested insulator maintenance decision 98 Table 5-5: Introduction of environmental coding for areas based on IEC’s pollution severity classification 115 Table 5-6: Environmental coding for zinc loss 116 Table 6-1: Summary of transmission line components, their functions and failure modes 125 Table 6-2: Summary of TNB and Powerlink porcelain cap and pin insulator inspection practice 127 10 outage rate Figure A-2 provides a snapshot of TNB’s transmission line forced outage rate for the periods between 1998/1999 and 2002/2003 [88] Figure 5-39: TNB transmission line forced outages 1997-2003 [88] It can be seen from Figure A-2 that the annualized 275 kV and 132 kV transmission line performance in 2002/2003 is 1.09 per 100 km-circuit and 1.03 per 100 km-circuit respectively Most forced outages experienced by TNB have been due to bad weather or lighting strikes This is not surprising because Malaysia is located in the tropics where the isokeraunic level is about 180 thunder days per year [89] Each year TNB spends between RM 32 million and RM 37 million on inspection and maintenance of its transmission line [90] This expenditure is due to current inspection and maintenance activities which include:  Monthly routine ground patrol inspection for transmission lines located in accessible regions  3-yearly routine climbing inspection for all transmission lines  Yearly routine climbing inspection for lines that cross roads, highways, railways and rivers  Yearly helicopter inspection for transmission lines located in inaccessible regions  Bulk replacement of wooden cross arms and cap and pin insulators that have been installed for 25 years 135 As with many utilities worldwide, TNB has been working towards reducing its maintenance and inspection expenditure whilst maintaining or further reducing its outage rate One way of achieving this objective is to reduce the frequency of inspection and maintenance based on the condition of the component The improved inspection process using the FIS as proposed in this thesis provides a means for TNB to use a numerical rating scheme to quantify the condition of the component Specific maintenance actions can therefore be made on those components that are bad and/or achieving its end of service life Expenditures can therefore be channelled to components with such conditions rather than on the whole component population The net effect is savings in inspection and maintenance expenditures At the same time, information regarding the condition of the components that are collected using the FIS can be stored in a computer As more data are collected, an analysis of the component deterioration trend can be made The trends would enable TNB to make projections of future component replacement or repair activities thus taking better control of its maintenance budget It is rather difficult to estimate the exact savings that can be achieved by introducing the new inspection process, but based on the current annual transmission line inspection and maintenance expenditure, a savings of 10% would translate to between RM million to RM million a year With time, the savings could potentially be more significant as more data about component condition is available for future maintenance projections 5.5 Chapter Summary In this chapter, we have primarily presented the development of a novel methodology that utilizes principles of fuzzy logic to handle uncertainties associated with visual inspection of porcelain cap and pin insulators Current practices of two utilities namely TNB (Malaysia) and Powerlink Queensland (Australia) were discussed which indicated the need for such a methodology to overcome the problem of uncertainty Together with an inspection guide, it has been shown that using the insulator inspection fuzzy inference system significantly reduces the subjectivity of assessing insulator pin rust 136 and porcelain shell conditions The development of the fuzzy inference system, whereby the design of input/output membership functions and linguistic variables were utilized in the rule base, has also been explained It has also been elucidated how the fuzzy inference system was used to determine the condition of three sample insulators taken from the field It has been further shown that qualitative analysis based on linguistic indicators used during assessment of the insulators is converted to crisp numbers by the fuzzy inference system The results of the output of the fuzzy inference system and how they can be used as a decision support for maintenance have also been discussed The numerical rating of each insulator unit has made it possible to define the rating of an insulator string It has been shown that the numerical rating of multiple insulator strings in a transmission line can be used in a mathematical model to estimate the appropriate replacement date for bulk replacements due to pin corrosion deterioration Further in this chapter, we have also discussed the application of the FIS on visual assessment exercise of other components of the transmission line Finally, we have discussed the possible savings that might be achievable if the new inspection methodology is implemented in TNB 137 CHAPTER 6: CONCLUSIONS AND FUTURE WORK 6.1 Summary of the Research The main goal of the research presented in this thesis was to develop a novel methodology that would improve utility practice of visual defect assessment of transmission line components In reaching this objective, this thesis seeks to answer the research question, “How can we reduce or remove the uncertainty associated with inspector visual assessment during field inspection so that the information gathered from the field inspection can be used effectively to manage maintenance actions?” The answer to this question is, as has been proposed in this thesis, a knowledge-based fuzzy inference system This chapter presents an overall conclusion of the research and reflects on the objectives listed in Section 1.2 The first stage of the study was aimed at identifying the functions and understanding the failure modes of the major transmission line components This was an essential part of the study because the reason utilities carry out visual inspection was to identify defects that could lead to imminent failure In Chapter 2, it was highlighted that environmental factors such as wind, weather and atmospheric pollution have a major influence on failures of transmission line components Except for very severe cases such as major storms with high incidental winds or earthquakes which could result in immediate transmission line failure, failures of transmission line components generally start with small defects followed by gradual deterioration Failure, as defined in this thesis, is loss of function In this regard, Chapter presented the main functions of the major components and their associated failure modes These are summarized in Table 6-1 138 Component MAIN FunctionS Failure Modes Steel Withstand static and Buckling or collapse due to structural Tower/Structure dynamic forces failure made worse by effects of corrosion Foundations Transfer mechanical loads Foundation failure due to corrosion to ground for stability of steel reinforcement bar Excessive soil movement Conductors Insulators Carry rated current while Conductor snap due to corrosion, maintaining even sag vibration fatigue or annealing Provide mechanical Broken pin due to corrosion or metal support fatigue Withstand electrical Damaged insulator shells stresses Table 6-1: Summary of transmission line components, their functions and failure modes As shown in Table 6-1, a majority of the defects is due to deterioration of steel components as a result of corrosion Having understood the functions of transmission line components and their failure modes, the second phase of the study was to review the available methods for detecting these defects from the various sources of literature and the third chapter includes information about various types of component monitoring systems It was found that, depending on the component and its functions, a majority of these methods utilized equipments that were able to detect either electrical or physical defects on transmission lines It was also found that, among the major components, the insulator has the greatest number of different equipments available determination of its condition However, these equipments were either found to be expensive (i.e the Daylight Corona camera), still in the development stages (i.e analysis of sample polymeric insulator sheds using Fourier transform infrared spectroscope) or highly affected by the environment (i.e the thermovision camera or corona phone) Chapter three also provided a review of inspection and maintenance practices of utilities worldwide, which was the third objective of the research For this purpose, the results of 139 two industry surveys were analysed: one was conducted by CIGRE [1] on 90 utilities worldwide and the other by McMahon [2] on utilities in Australia and New Zealand The CIGRE survey indicated that utilities regard the most typical form of defect is corrosion attack on steel components However, the significance of both the survey results to this research is that, despite the advent of new monitoring equipments, both the survey results indicated that the most widely used utility practice for location of defects on transmission lines is by visual inspection The method requires inspectors to visually inspect the component either from the air (in helicopters), on the ground (using visual aids such as binoculars or zoom cameras) or at the top of the tower (by climbing) The main problem with visual inspection, as discussed towards the end of the chapter three, is that there exists a high level of subjectivity and uncertainty when the inspectors are making the defect evaluation Another problem is that because the defects are recognized by their perceived physical conditions, defect indicators are normally reported qualitatively using vague and linguistic terms such as “medium crack”, “heavy rust”, “small deflection” and “large breakage” Unfortunately, inspectors are human beings and not machines or robots that can be programmed to produce accurate results all the time As a consequence, there is a large variance in defect reporting (which, in time, makes it difficult for utilities to monitor the condition of the component) that can lead to wrong or ineffective maintenance decisions This thesis recommends solving this problem by using the artificial intelligence technique of fuzzy logic, a technique which was first introduced in the 1960’s to deal with the fuzziness of human judgment The aim of chapter four in this thesis was to provide an understanding of the principles of fuzzy logic for it to be applied in this research In this chapter, the main building blocks of fuzzy logic, namely the construction of membership functions and IF-THEN rules, were discussed It was shown in the chapter how the combination of membership functions that represent linguistic variables and IF-THEN rules were implemented in a knowledgebased Fuzzy Inference System (FIS), one of the ways by which fuzzy logic is used in realworld applications To assist in understanding further the subject matter, several applications of fuzzy logic in the utility environment were also discussed in this chapter The fifth and main objective of this research was to design, apply and test a knowledgebased FIS on the utility visual inspection of one of the most important components of the 140 transmission line – the porcelain cap and pin insulators For this purpose, chapter five examined two of the most common failure mechanisms of porcelain cap and pin insulators: corrosion of the steel pin and breakage of the porcelain shells Pin rust increases gradually with time and the rate of deterioration is affected by the environment wherein the insulator is located Breakage of porcelain shells is mostly due to vandalism, does not increase with time and normally occurs on lines that are located in areas accessible to the public Both types of defects are normally detected visually by utilities during routine inspections For the purpose of this research, current insulator inspection practice of two utilities, Powerlink Queensland (Australia) and Tenaga Nasional Berhad Malaysia (TNB), was studied Table 6-2 provides a summary comparison of the inspection methods of both utilities TNB powerlink  Visual inspection is done by  Visual inspection is done by linesmen during routine line linesmen during routine live line outage for inspection inspection  No inspection guides are used  For rust conditions, a 4-category  Inspection results are recorded rust in inspection guide is used (scale: 1-no rust to 4-heavy rust) in inspection form: (√) means good – no action, (x) means bad  Maintenance decision is based – replace immediately; anything on the reported condition of the in between is recorded in insulator referred to one of four separate forms defect categories  Maintenance is done based on  Bulk replacement is taken if the reported condition of the necessary insulator  Bulk replacement is sometimes taken if the same defects are detected on many insulators Table 6-2: Summary of TNB and Powerlink porcelain cap and pin insulator inspection practice It was deduced that the TNB practice results in high level of variance in defect reporting due to the linesmen not using any inspection guides when conducting the inspection 141 Powerlink practice narrows down the variance level but there is still uncertainty in the defect evaluation because the actual perceived condition of the insulator may not be exactly the same as described in the inspection guide Chapter five then proceeded with the design of a knowledge-based FIS for use during insulator visual inspection The first step shown was to develop defect rating tables based on the linguistic description of the severity of porcelain shell breakage (see Table 5-1) and pin rust (see Table 5-2) Using Matlab’s proprietary Fuzzy Logic Toolbox program, these defect ratings were then converted to triangular membership functions The overall condition ratings of the insulator were also represented by membership functions In the FIS, the relationship between the inputs (pin rust condition and porcelain shell breakage condition membership functions) and the output (insulator overall condition membership function) was represented by IF-THEN rules For this purpose, a table of twenty IFTHEN rules (see Table 5-3) was then created to infer the overall condition of the insulator based on its pin rust and porcelain shell conditions respectively Mamdani’s Center-of–Area (COA) defuzzification method was used in the FIS, where the overall condition of the insulator was inferred by calculating the point which is central to the area under the aggregated output membership function Finally, the output of the FIS was cross-referenced to a table of suggested maintenance action (see Table 5-4) to guide the user to either “do nothing”, “flag for next maintenance cycle”, “increase monitoring frequency” or “replace immediately” The insulator inspection FIS was then tested on three sample insulators taken from the field Experience from the three sample applications showed that:  together with an insulator inspection guide for rusty pins and broken porcelain shells, the insulator inspection FIS assisted visual evaluation of defective insulator units on the string by inferring the defect level of each insulator with respect to its physical appearance and condition  the inspector was only required to indicate the appearance of rust at the insulator pin and chipping/breakage of the porcelain shell by comparing these conditions with the condition ratings used in Tables 5-1 and 5-2 respectively  inference was undertaken by the Inference Engine of the FIS and not by the inspector thereby relieving the inspector from making any judgment with regards 142 to the overall condition of the insulator This greatly reduced subjectivity and uncertainty when evaluating insulator defects  because the output of the FIS was referred to standardized maintenance actions (see Table 5-4), consistency in decision-making for maintenance actions was therefore possible Chapter five also showed how the output of the insulator inspection FIS was used in a mathematical model that was developed to assist maintenance managers in making bulk replacement of rusty porcelain cap and pin insulators The assumptions used in the model were that the rate of pin rust deterioration over time follows a linear relationship and that the rate of pin rust deterioration is higher in areas of aggressive environments For this intention, an environmental rating condition based on the International Electrotechnical Commission’s (IEC) definition of pollution severity was created Application of the model on several combinations of insulator and environmental conditions showed the model’s usability as a tool for managing insulator assets, thus achieving the sixth and final objective of this thesis As finally discussed in chapter five, the ideas used during the design of the insulator inspection FIS can be applied towards the design and implementation of a transmission line inspection FIS which takes into account the visual assessment of other transmission line components namely the tower structure, the conductors and the foundations True to this thesis’s title and research objectives, thus it has been shown “The Application of Knowledge-based Fuzzy Inference System on High Voltage Transmission Line Maintenance” 6.2 Major Research Contributions Several benefits of the FIS include:  Together with visual inspection guide, FIS can effectively reduce the level of uncertainty when assessing transmission line defects This results in a more objective and consistent evaluation of defects as well as provides support to making maintenance decisions 143  FIS works by transforming fuzzy qualitative indicators that are prevalent when assessing defects visually to crisp numerical values that can be used as defect rating  Numerical values make it possible to represent transmission line component defect condition in computer database for trending and future maintenance strategies  Numerical values can be used in a replacement model to plan for bulk replacement of transmission line components  Programmed into mobile computing devices, FIS makes available expert knowledge at site  FIS can be fine-tuned by adjusting membership functions and IF-THEN rules based on available expert knowledge 6.3 Future Work The following are recommendations for further work on this research topic, which in the author’s opinion, will prove to be particularly fruitful:  Examining the effect of using different membership function shapes such as trapezoidal, Gaussian or sigmoid on the output of the FIS  Further reducing the level of uncertainty associated with visual inspection of defects The use of a pictorial representation of defect levels in the inspection guide (rather than descriptions of defect as used in this research) may help achieve this 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Assessment of Multiple Insulator Strings in a Transmission Line 114 5.3 Application of the FIS on Other Transmission Line Components 118 5.4 Potential Savings due to Introduction of FIS... compression forces on tower foundations 14 Figure 2-5: Typical construction of transmission tower steel grillage foundation 15 Figure 2-6: Typical construction of transmission tower