Software Development for Direct Lightning Stroke Shielding of Substations

Transcription

1 Software Development for Direct Lightning Stroke Shielding of Substations P. N. Mikropoulos *, Th. E. Tsovilis, P. Chatzidimitriou and P. Vasilaras Aristotle University of Thessaloniki, High Voltage Laboratory, Thessaloniki, 54124, GREECE *Tel/Fax: , ABSTRACT: A user-friendly Windows application software has been developed for shielding design of high voltage substations against direct lightning strokes; shielding design can be achieved in a few minutes on the basis of a 3-dimensional analysis. With the aid of the software, an installed shielding system can be validated and/or a new system can be designed according to IEEE Standard 998:1996. The performance of different shielding design methods can be easily evaluated for various operating system voltages and equipment dimensions. The developed software has been applied to the shielding design of typical substations of the Hellenic Transmission System, 150 kv and 400 kv substations and a comparison of the design methods has been made. The application software is a useful tool for electrical engineers and can also be used for educational purposes in high voltage engineering courses. Keywords: Direct stroke shielding, lightning, substations. I. INTRODUCTION The design of the shielding system of a substation against direct strokes can be achieved by implementing geometrical methods and electrogeometric models. The geometrical methods, namely fixed angle and Wagner s method [1], have historically been employed in shielding design providing acceptable protection and they are still widely used [2]. These empirical methods assume that the protection offered by an air terminal is related to geometrical factors such as the heights of air terminal and protected equipment and their separation distance. On the other hand electrogeometric models based on more physical ground, take into account that lighting attractiveness is related to lighting peak current [3]-[11]. Thus, for a given shielding geometry, some of the less intense strokes may not be intercepted by the air terminal and strike to the protected equipment. An effective shielding design is achieved by limiting shielding failures to those lightning strokes with peak current less than the maximum current, which would not cause flashover of substation equipment insulation. An effective shielding design of a substation requires the positioning of the air terminals offering protection on the basis of a 3-dimensional analysis. This is a formidable task, thus software has been developed, which allows shielding design to be achieved in a few minutes. With the aid of a user-friendly graphics interface, an installed shielding system can be validated and/or a new system can be designed. Air terminals, namely masts or shield wires, can be easily placed at appropriate positions with respect to the protected equipment provided that the minimum air clearances ac- cording to [12] are met. The software incorporates the design methods adopted by IEEE standard [13] that is the fixed angle method, the Wagner s method [1] and the revised electrogeometric model introduced by Mousa et al. [10], [14]. Hence, the performance of the different shielding design methods can be easily evaluated for various operating system voltages and equipment dimensions. The developed software has been applied to the shielding System, 150 kv and 400 kv substations, and the design methods are compared with respect to the required number and height of air terminals and their positioning. II. SHIELDING DESIGN METHODS INCORPORATED IN THE DEVELOPED SOFTWARE The developed software incorporates three design methods for shielding of substations against direct lightning strokes, which are adopted by the IEEE Standard [13]. A. Fixed angle Shielding angle defines a protection volume provided by an air terminal with reference to the protected equipment height (Fig. 1). The fixed angle approach is used as a convenient approximation of the boundaries of the protection zone of an air terminal within which the equipment can be protected against lightning direct stroke. The fixed angles α and β shown in Fig. 1, are commonly used in shielding of substations as equal to 45 o. In case of overlapping between the protection zones of the air terminals (Fig. 1) and at relatively low substation system voltages a value of 60 o can be used for α [13]. This method is very simple in its application; however, it requires an extensive effort where an effective 3-dimensional shielding design is concerned [15]. Fig. 1: Schematic diagram of the application of the fixed angle method for two masts; in case of overlapping between the protection zones of the air terminals α > β. The developed software uses a fixed angle of 45 o, which is a conservative approach in shielding design in case of overlapping between the protection zones of air terminals. B. Wagner s method The shielding design according to Wagner s method is based on empirical curves, derived from scale model experiments [1], [16], which relate the separation distance between protected object and air terminal, mast or shield wire, with the ratio of their heights at various failures rates.

2 Typical such curves for the case of an object protected by a single mast are shown in Fig. 2; Wagner proposed similar curves referring up to 15% shielding failure rate. Also, instead of using a fixed shielding angle, Wagner s method implies that for a fixed failure rate the shielding angle varies with the ratio of air terminal to protected equipment heights (Fig. 3). hp/hm h p α h m 0.1% 0.1 ΔR ΔR/h m Fig. 2: Failure rate of an object protected by a single mast [1]. ΔR separation distance between protected object and mast; α shielding angle; h p, h m protected object and mast heights, respectively. Shielding angle (deg) % h p /h m Fig. 3: Shielding angles, derived from Wagner s empirical curves, at different failure rates; case of an object protected by a single mast. The developed software incorporates the empirical curves of Wagner referring to 0.1% and 1% failure rate, which are commonly used in practice. It also takes into account the type of air terminal i.e. mast or shield wire. C. Revised electrogeometric model Electrogeometric models were first developed for transmission line shielding [17]-[20] and their application has been extended to shielding of substations [3]-[11]. Typical of these models is the rolling sphere method; the rolling sphere radius representing the striking distance is correlated with the lightning peak current. According to this method, the protection zone of a system of air terminals is represented by circular boundaries defining a volume within which a fully situated structure is protected (Fig 4). Based on this method, a revised electrogeometric model was introduced [10], [14]; the striking distance is reduced by 10% to take into account its statistical nature and increased by 20% for strokes to mast. 1% 1% Fig. 4: Protection zone (shaded area) offered by two masts according to the rolling sphere method; S striking distance, h m height of masts. A shielding system, designed according to electrogeometric models for a striking distance corresponding to lightning peak current I s, is assumed to intercept all lightning strokes with peak current values higher than I s but it may be penetrated by lightning strokes of lower current. Thus, an effective shielding design may be accomplished for I s equal to the maximum current which would not cause flashover of substation equipment insulation. This critical current, I c, may be expressed as [20]: I c 2.2BIL Z = (1) where BIL (kv) is the basic insulation level and Z s (Ω) is the surge impedance of the conductor through which the surge is passing. A method for calculating Z s is given in [13], which considers geometrical parameters and the corona effect. The developed software incorporates the revised electrogeometric model [10], [14] and calculates the striking distance S (m) as a function of I c (ka) with the aid of the following expression: S s 0.65 = ki (1) 7.2 c where k takes values 1.2 for strokes to masts and 1 for strokes to wires and ground. III. DEVELOPED SOFTWARE The software for direct stroke shielding of substations has been developed in Microsoft Visual Basic 6 and it runs as a Microsoft Windows application. With the aid of a user-friendly graphics interface, an installed shielding system can be validated and/or a new system can be designed. Air terminals, specifically masts or shield wires, can be easily placed at appropriate positions with respect to the protected equipment provided that the minimum air clearances according to [12] are met. The software incorporates the shielding design methods adopted by IEEE [13] that is the fixed angle method, the Wagner s method [1] and the revised electrogeometric model [10], [14]. The basic input data required for the design, that is the system voltage, BIL and the dimensions of protected equipment, is entered in the first window (Fig.5). In the same window the user selects the design method, the number and the type of the air terminals and their symmetrical positioning with respect to the protected equipment (Fig.5).

3 In the second window (Fig. 6), depending on the selected design method, the height of the air terminals is plotted as a function of its separation distance from the protected equipment. Then the program calculates the required height of the air terminals for the entered separation distance from the protected equipment or visa versa. The user may accept the calculated values or select another air terminal arrangement. In case of an ineffective shielding design the unprotected equipment area will be shown in the results window displaying also a warning message. The results window displays a schematic diagram of the air terminals arrangement together with information related to input data and shielding design effectiveness (Fig. 7). IV. SOFTWARE APPLICATION AND DISCUSSION ON THE SHIELDING DESIGN METHODS Fig. 5: Input data window. Fig. 6: Calculations window. An application of the developed software to the shielding System, 150 kv and 400 kv substations, has been made. In these calculations the switchyard area (26 m x 30 m) is the same in both substations, however, the height of the protected equipment is set to 7.5 m and 14 m for the 150 kv and 400 kv substations, respectively. From the results of the program a comparison among the different design methods can be made with respect to the required number and height of air terminals and their positioning. Table 1 shows the results of the application of the developed software on shielding design of the substations by considering as air terminals 4 shield wires separated by each other by 6.5 m. It is evident that the revised electrogeometric model results in the most conservative shielding design, more conservative the lower the system voltage. Between the geometric methods Wagner s method yields a more conservative design (Table 1), however, this behaviour would depend on the ratio h p /h m as can be deduced from Fig. 3. Also, Wagner s method may result in a different shielding design depending on shielding failure rate; this is shown in Table 1 where the height of shield wires refers to 0.1% failure rate while that in parenthesis to 1%. Table 1: Input data and results of shielding design of 150 kv and 400 kv substations; 4 shield wires separated by each other by 6.5 m. 150 kv substation 400 kv substation BIL = 750 kv, Z s = 400 Ω BIL = 1425 kv, Z s = 350 Ω Design Method Height of shield wires (m) Height of shield wires (m) Fixed angle Wagner s 11.5 (11.0) 18.5 (17.8) Revised EGM Table 2 shows the results of the application of the developed software on shielding design of the substations by using fixed heights for the air terminals, 20 m and 30 m for the 150 kv and 400 kv substations, respectively. The minimum number of masts required for an effective shielding Fig. 7: Results window. Table 2: Minimum number of air terminals required for shielding design of 150 kv and 400 kv substations. 150 kv substation 400 kv substation BIL = 750 kv, Z s = 400 Ω BIL = 1425 kv, Z s = 350 Ω Air terminal height: 20 m Air terminal height: 30 m Design Method shield wires masts shield wires masts Fixed angle 2 >4 1 4 Wagner s Revised EGM 4 >4 3 4

4 design is greater than that of shield wires, especially for the geometric design methods. Shield wires are reasonably more commonly used in shielding of substations [2]. Concluding, both geometric design methods are simple in application. However, Wagner s method may provide a more accurate protection zone offered by the air terminals since in this method the shielding angle varies with the ratio h p /h m (Fig. 3) and depends on failure rate and type of air terminal. Also, both geometric methods do not take into account the prospective lightning peak current; the latter is considered by the electrogeometric models, which however are more time consuming in their application. Wagner s method is the only one of those methods a- dopted by IEEE [13] by which a shielding design can be achieved at a given failure rate. The latter approach is more realistic when considering that lightning interception phenomenon is statistical in nature, hence also its related design parameters namely striking distance and interception radius are statistically distributed. Actually, an effective shielding design of a substation should consider, besides lightning peak current distribution, interception probability. The necessity for such an approach in shielding design has been discussed in detail before [21] [25], where a new statistical shielding design method was introduced; an extension of this work to substations will be given elsewhere. V. LIMITATIONS OF THE DEVELOPED SOFTWARE The application of the developed software results in a conservative shielding design of a substation since the latter is considered as a rectangular structure. Also, only symmetrical positioning of the air terminals with respect to the protected equipment is considered; this is not always the case in practice. Finally, unequal air terminal heights and combination of shield wires and masts for a shielding design are not incorporated in the developed software. VI. CONCLUSIONS A user-friendly Windows application software has been developed for shielding design of high voltage substations against direct lightning strokes; shielding design can be achieved in a few minutes on the basis of a 3-dimensional analysis. With the aid of the software, an installed shielding system can be validated and/or a new system can be designed according to IEEE Standard 998:1996. The performance of different shielding design methods can be easily evaluated for various operating system voltages and equipment dimensions. An application of the developed software to the shielding System, 150 kv and 400 kv substations has been made, and a comparison of the design methods has shown that the electrogeometric model results in more conservative shielding design than the geometric methods, more conservative the lower the system voltage. The shielding design results obtained from the application of the developed software should be treated cautiously since the importance and the value of the equipment being protected must always be taken into account in shielding design. However, the developed software is a useful tool for electrical engineers and can also be used for educational purposes in high voltage engineering courses. VII. ACKNOWLEDGMENT Th. E. Tsovilis wishes to thank the Research Committee of Aristotle University of Thessaloniki for the support provided by a merit scholarship. VIII. REFERENCES [1] Shielding of Substations, C. F. Wagner, G. D. McCann and C.M. Lear, AIEE Transactions, Vol. 61, 1942, pp [2] A Survey of Industry Practices Regarding Shielding of Substations Against Direct Lightning Strokes, A. M. Mousa and R. J. Wehling, IEEE Transactions on Power Delivery, Vol. 8, No. 1, 1993, pp [3] Monte Carlo Simulation of the Lighting Performance of Overhead Shielding Networks of High Voltage Substations, M. A. Sargent, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-91, No. 4, 1972, pp [4] W. H. Dainwood, Lightning Protection of Substations and Switchyards Based on Streamer Flow Theory, M.Sc. Thesis, University of Tennessee, Knoxville, Tennessee, [5] Shielding of Modern Substations Against Direct Lighting Strokes, H. Linck, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-90, No. 5, 1975, pp [6] Shielding of High-Voltage and Extra-High-Voltage Substations, A. M. Mousa, IEEE Transactions on Power Apparatus and Systems, Vol. PAS-95, No. 4, 1976, pp [7] Protect your Plant Against Lightning, R. H. Lee, Instruments and Control Systems, Vol. 55, No. 2, 1982, pp [8] J. T. Orrel, Direct Stroke Lightning Protection, paper presented at IEE Electrical System and Equipment Committee Meeting, Washington, D.C., [9] The Protection of High Voltage Substations Against Lightning, F. Hofbauer, Proc. CIGRE, Paper No , [10] A Computer Program for Designing the Lightning Shielding Systems of Substations, A. M. Mousa, IEEE Transactions on Power Delivery, Vol. 6, No. 1, 1991, pp [11] Shielding of Substations Against Direct Lightning Strokes by Shield Wires, P. Chowdhuri, IEEE Transactions on Power Delivery, Vol. 9, No. 1, 1994, pp [12] IEC Standard , Insulation co-ordination, Part 2: Application guide, [13] IEEE, Guide for Direct Lightning Stroke Shielding of Substations, IEEE Standard 998, [14] A. M. Mousa and K. D. Srivastava, A Revised Electrogeometric Model for the Termination of Lightning Stroke on Ground Objects, Proc. of the International Aerospace and Ground Conference on Lightning and Static Electricity, Oklahoma City, USA, April 1988, pp [15] T. Horvath, Problems with Application of the Protection Angle Method at three-dimensional Structures, 29th ICLP, Uppsala, Sweden, June 2008, paper 4-5. [16] Shielding of Transmission Lines, C. F. Wagner, G. D. McCann and G. L. MacLane, AIEE Transactions, Vol. 60, 1941, pp [17] Monte Carlo Computer Calculation of Transmission Line Lightning Performance, J. G. Anderson, AIEE Transactions, Vol. 80, 1961, pp [18] Shielding of Transmission Lines, E. S. Young, J. M. Clayton and A. R. Hileman, IEEE Trans. Power Apparatus and Systems, Vol. S82, No. 4, 1963, pp [19] E. R. Whitehead, Mechanism of Lightning Flashover, EEI Research Project RP 50, Illinois Institute of Technology, Pub , February [20] The Mechanism of Lightning Flashover on High-Voltage and Extra-High-Voltage Transmission Lines, D. W. Gilman and E. R. Whitehead, Electra, 27, 1973, pp [21] P. N. Mikropoulos and Th. E. Tsovilis, Experimental Investigation of the Franklin Rod Protection Zone, 15th International Symposium on High Voltage Engineering, Ljubljana, Slovenia, August 2007, paper 461. [22] Striking Distance and Interception Probability, P. N. Mikropoulos and Th. E. Tsovilis, IEEE Transactions on Power Delivery, Vol. 23, No. 3, 2008, pp [23] P. N. Mikropoulos and Th. E. Tsovilis, Interception Radius and Shielding Against Lightning, 29th ICLP, Uppsala, Sweden, June 2008, paper 4-10.

5 [24] Interception Probability and Shielding Against Lightning, P. N. Mikropoulos and Th. E. Tsovilis, IEEE Transactions on Power Delivery, accepted. [25] P. N. Mikropoulos, Th. E. Tsovilis and T. Ananiadis, The Effect of an Earthed Object on the Interception Radius of the Franklin Rod: An Experimental Investigation, Med Power 08, Thessaloniki, Greece, November 2008, submitted. IX. BIOGRAPHIES Pantelis N. Mikropoulos was born in Kavala, Greece in He received the M.Eng. and Ph.D. degrees in electrical and computer engineering from Aristotle University of Thessaloniki (AUTh), Thessaloniki, Greece, in 1991 and 1995, respectively. He held postdoctoral positions at AUTh and the University of Manchester, Manchester, UK. He was Senior Engineer with Public Power Corporation SA, Greece. In 2003, he was elected Assistant Professor in High Voltage Engineering at AUTh, and since 2005, he has been the Director of the High Voltage Laboratory at AUTh. His research interests include the broad area of high-voltage engineering with an emphasis given on air and surface discharges, electric breakdown in general, and lightning protection. Thomas E. Tsovilis was born in Piraeus, Greece in He received the M.Eng. degree in electrical and computer engineering from Aristotle University of Thessaloniki, Thessaloniki, Greece, in 2005, where he is currently pursuing the Ph.D. degree in the High Voltage Laboratory. His research is dedicated to lightning protection, including theoretical analysis and scale model experiments. Panagiotis Chatzidimitriou was born in Thessaloniki, Greece in He received the M.Eng. degree in electrical and computer engineering from Aristotle University of Thessaloniki, Thessaloniki, Greece, in Panagiotis Vasilaras was born in Ioannina, Greece in He received the M.Eng. degree in electrical and computer engineering from Aristotle University of Thessaloniki, Thessaloniki, Greece, in 2007.