An optimal design method for improving the lightning performance of overhead high voltage transmission lines

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Abstract

This paper presents a method for the optimal design of high voltage transmission lines taking into consideration shielding and backflashover failure rates. The minimization of suitably defined performance indices, which relate the failures caused by lightning in a transmission line to both line insulation level and tower footing resistance, is aimed. Optimum values for both line insulation level and tower footing resistance are calculated. The method is applied on several operating Hellenic transmission lines of 150 and 400 kV, respectively, carefully selected among others, due to their high failure rates during lightning thunderstorms. Special attention has been paid on open loop lines, where a possible failure could bring the system out of service causing significant problems. The obtained design parameters, which reduce the failure rates caused by lightning, are compared with the existing design parameters of the transmission lines leading up to useful conclusions. The proposed optimization method can be proved a valuable tool to the studies of electric power systems designers, intending to reduce the failure rates caused by lightning.

Introduction

Lightning strikes to overhead high voltage transmission lines are a usual reason for unscheduled supply interruptions in the modern power systems. In an effort to maintain failure rates in a low level, providing high power quality and avoiding damages and disturbances, plenty of lightning performance estimation studies have been conducted [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13] and several design methodologies have been proposed [14], [15], [16], [17], [18], [19]. Designing appears to be the most important issue in the lightning performance of a transmission line, not only because differences in the design parameters values affect significantly the lightning performance but also because is practically impossible to make improvements and modifications in an existing transmission line.

Chang and Zinn [14] determined a minimum cost design of transmission lines. They demonstrated a methodology for the design of an electric transmission line system constructing a mathematical model, which represented the total cost of the system as a function of the system design variables. Grant and Clayton [15] developed a methodology, which used to explore the sensitivity of the required present worth of revenue, to several design variables in order to achieve design performance at minimum cost. Significant was also the study of Kennon and Douglass [16], who conducted an interesting study presenting a range of line optimization techniques which can be applied to decide whether standard or optimized line designs are appropriate, concluded that even simple methods of optimization can help the designer keep his costs to a minimum. Saied et al. [17] presented a method for the optimal design of overhead high voltage transmission lines with main objective the minimization of the line total annual cost, considering the relevant technical constraints and both fixed and running cost items. Katic and Savic [18] analysed the economical aspects of the overhead distribution line lightning performance taking into account customer and utility costs of line outages. An alternative design procedure for uncompensated overhead transmission lines introduced from Saied [19]. It was based on the derivation of two closed-form analytical expressions for both the line power and current ratings, in terms of the geometrical data of the line tower and its bundled conductors.

Generally, two different methods exist in the design of a transmission line. The first method uses a good tower footing resistance and relatively low line insulation while the second one uses an average tower footing resistance but relatively high line insulation [8]. The current work presents in detail a design method for the optimum selection of the transmission line insulation level and the tower footing resistance. Suitable performance indices are defined in order to relate the line insulation level and the tower footing resistance cost values to the lightning failure costs. Using an iterative optimization algorithm, optimal values of these two design parameters are calculated in order to minimize the defined performance indices.

The developed method is applied on several operating Hellenic transmission lines, (including open loop lines), of 150 and 400 kV, with high lightning failure rates in order to validate its effectiveness. New values for the transmission line design parameters are proposed, which have as a result the reduction of the lightning failures. Useful conclusions are extracted from the comparison between the proposed values and the actual line design data.

Section snippets

Flashover rate of a transmission line

An approximation to the number of flashes to earth that are intercepted by a transmission line is calculated using the equation [8]:NL=0.004T1.35(B+4h1.09)where NL is the number of lightning flashes to a line per 100 km/year, T the lightning level in the vicinity of the line, h the average height (in m) of the shielding wires and B is the horizontal spacing (in m) between the shielding wires.

The lightning parameters, i.e., the peak value and the lightning current derivative, are randomly

Formulation of the optimization problem

The examined transmission line is divided into N regions, due to the different meteorological conditions and the different average values of tower footing resistance, which exist in each one region of the line. For each region, an analysis is conducted and suitable values for the insulation level and the tower footing resistance are computed. The total flashover failure rate is also computed for each region, using (7).

A performance index is defined for each region of the examined transmission

Transmission lines parameters

The method presented in this paper has been applied and tested on 150 and 400 kV operating transmission lines of the Hellenic interconnected system [32]. These lines, which are presented in Table 2 (Fig. 1), were carefully selected among others, due to: (a) their high failure rates during lightning thunderstorms [33], (b) their consistent construction for at least 90 present of their length and (c) their sufficient length and their sufficient time in service in order to present a reasonable

Conclusions

The paper describes in detail an optimal design method for improving the lightning performance of overhead high voltage transmission lines taking into consideration both shielding and backflashover failure rates. The method calculates and proposes the most suitable line insulation level and tower footing resistance values, for each one region of the examined lines, in an effort to minimize the total failures caused by lightning. Suitable performance indices are defined in order the line

Acknowledgements

The authors want to express their sincere gratitude to the electrical engineers Mr. A. Vlachos and Mr. N. Spiliotopoulos of the Hellenic Public Power Corporation S.A. for their kind supply of various technical data and the National Meteorological Authority of Hellas for the supply of meteorological data.

Lambros Ekonomou was born on January 9, 1976 in Athens, Greece. He received his bachelor of engineering (Hons.) in electrical engineering and electronics in 1997 and his master of science in advanced control in 1998 from University of Manchester Institute of Science and Technology (U.M.I.S.T.). Since 1999, he is a Ph.D. student and a research associate, at the High Voltage Laboratory of National Technical University of Athens. His research interests concern high voltage transmission lines,

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    Lambros Ekonomou was born on January 9, 1976 in Athens, Greece. He received his bachelor of engineering (Hons.) in electrical engineering and electronics in 1997 and his master of science in advanced control in 1998 from University of Manchester Institute of Science and Technology (U.M.I.S.T.). Since 1999, he is a Ph.D. student and a research associate, at the High Voltage Laboratory of National Technical University of Athens. His research interests concern high voltage transmission lines, lightning and artificial neural networks.

    Dimitris P. Iracleous was born on July 23, 1969 in Athens, Greece. He received his diploma in electrical engineering and Ph.D. degree from University of Patras in 1993 and 1999, respectively. He is a teaching assistant at the Hellenic Naval Academy. His research interests concern optimization control algorithms and techniques in static and dynamic systems.

    Ioannis F. Gonos was born on May 8, 1970 in Artemisio, Arcadia, Greece. He received his diploma in electrical engineering and his Ph.D. from the National Technical University of Athens in 1993 and 2002, respectively. He was a teaching assistant at the Hellenic Naval Academy and the Technological Education Institute of Athens (1996–2001). He is working at the High Voltage Laboratory of NTUA (since 2001). His research interests concern grounding systems, insulators, high voltages, measurements and genetic algorithms. He is the author of more than 60 papers in scientific journals and conferences proceedings.

    Ioannis A. Stathopulos was born in Volos, Greece in 1951. He studied in the Faculty of Electrical and Mechanical Engineering of the National Technical University of Athens (1969–1974). He carried out his doctor thesis at the Technical University of Munich (1974–1978). He become teaching assistant at the Technical University of Munich (1974–1978), production engineer in the company “Vianox–Franke” (1979–1980), teaching assistant at the National Technical University of Athens (1979–1983) and thereafter lecturer (1983–1987), assistant professor (1987–1991), associate professor (1991–1995) and professor (since 1995) in the High Voltage Laboratory of the NTUA. He is the author of 8 books and more than 90 papers in scientific journals and conferences proceedings. He is lead assessor of the Hellenic Accreditation Council.

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