* Sediver – 03270 SAINT-YORRE (FRANCE)

RECOMMENDATIONS TO SOLVE CORROSION PROBLEM

ON HV INSULATOR STRINGS IN TROPICAL ENVIRONMENT

by O. L.S. PAIVA D. DUMORA L.F. FERREIRA R. SUASSUNA R. PARRAUD * M. NAMORA Chesf Sediver Electrovidro (Brazil) (France) (Brazil) ABSTRACT In tropical areas, insulators are subjected to specific environmental conditions : an almost permanent high humidity (RH > 70 %), a high mean temperature, associated with periods without rainfall. The consequence is an increasing of the conductivity of insulator surface when they are lightly polluted. The frequent leakage currents associated with arcs and discharges are the cause of the corrosion of insulator metal end fittings. The field experience of a new insulator design which is in operation since 5 years on 500 kV line in a severe typical tropical environment where the conventionnal insulators are submitted to corrosion problem is presented in this report. On the base of the field experience and the analysis of the corrosion, recommendations to improve the corrosion performance of insulators based on the analysis of the corrosion mecanism [5] on HV insulator strings are presented in this report. 1. INTRODUCTION : It is possible to distinguish two main corrosion mechanisms of insulator [2] : The first corresponds to the electrolytic corrosion due to the passage of a frequent leakage current on each insulator of a string [3]. The second corresponds to the ‘’arc corrosion’’ phenomenon [5] which concerns all the insulators of a string when the humidification of their surfaces is maximal, but also which increase the corrosion of the bottom units subjected to more arc activity duration than the other insulators during the pre-period of humidification and drying of the surface.

2. RECOMMENDED INSULATOR DESIGN FOR TROPICAL ENVIRONMENT 2.1 Insulator design The insulator design corresponds to improvements of the insulator for tropical environment subjected to a natural contamination (dusts, salts, ashes of sugar canes, …) to reinforce the insulator corrosion resistance (electrolytic corrosion and arc corrosion). 2.2 Protection of the metal parts of the insulators The cap and the pin of the insulator are protected by a continous zinc coating. A zinc protection with a very good bonding between the zinc layer and the pin forged steel or the cap cast iron, can be obtained by the hot dip galvanization process. A minimum zinc thickness of 110 µm can be obtained without risk of galvanization scaling due to a bad bonding. In some wet conditions, the zinc coating (galvanization) can be subjected to a formation of zinc hydroxide. Its corresponds to a white deposit. It is a non soluble and non conductive material which does not affect the electrical performance of the insulators. In tropical zones subjected to heavy contamination (areas close to the sea), the metal parts of the insulators must be protected against the effects of corrosion (atmospheric, electrical corrosion …), with reinforced galvanizing and sacrificial electrodes at the triple junction of the dielectric, metal parts and air. The pin must be protected with a sacrificial electrode in pure zinc. Even if the cap corrosion was never a serious problem from a mechanical point a view, in heavy polluted contamination, it is recommended, to avoid the formation of a rust deposit on the insulator dielectric surface.

21, rue d’Artois, F-75008 Parishttp://www.cigre.org

Symposium CAIRNS© CIGRÉ 2001300-05

2 300-05

The figure 1 represents typical toughened glass optimised insulators used in tropical areas subjected to heavy contamination : - to optimise the electrical stresses on the metal parts

(cap and pin), - to have a more uniform electrical field distribution

on the insulator, - to limit the effects of discharges around the metal

parts during ‘’wetted’’ periods.

Fig. 1 : Toughened glass optimised insulator design 3. FIELD EXPERIENCE OF TOUGHENED GLASS INSULATOR FOR 500 KV TRANSMISSION LINES IN TROPICAL ENVIRONMENT New optimised and conventional insulators were set up on a 500 kV transmission line built on the Nordeste area in Brazil. Insulators are tested as “ I ” and “ V ” strings composed of 26 units without protective devices on the same towers (Table 1). Various corrosion degrees from A to D were defined to quantify the performance of the insulator after a comparative exposure time in the same towers (from 1994 to 1999) in areas where conventional insulators were subjected to early corrosion. (see table 1) The various corrosion degrees are defined as indicated below : Corrosion degree A) The galvanization protects always the metal, but some little superficial traces of oxidation are visible (modification of the visual aspect) Corrosion degree B) Significant corrosion step with a rust formation ; Apparition of iron oxide which corresponds only to a partial attack of the galvanization Corrosion degree C) Complete attack of the galvanization which is destroyed totaly. A rust deposit covers the metal. Corrosion degree D) Heavy corrosion with a significant reduction of the metal thickness.

Table 1 : Comparative corrosion degree on ‘’I’’and

‘’V’’ strings after 50 months in severe tropical environment500 kV (tower 303)

line Sobradinho-Itaparica The table 1 shows: a) For the optimised insulator design (see fig 1) with a

cement joint between the cap and the dielectric and zinc retardation sleeve on the pin, no actual corrosion appears on the metal parts in comparison with standard insulator design No pin corrosion and no cap corrosion have started on ‘’I’’ and ‘’V’’ strings. Only some points of white zinc hydoxide from the galvanisation which does not affect the efficiency of the galvanisation coating are visible on caps and pins. The optimised insulators present an obvious corrosion improvement in comparison with the conventional insulators.

b) For the standard insulator, the pin corrosion is greater than the cap corrosion. The cap corrosion is starting on the live end units which are subjected to more arcs and discharges during the process of humidification and drying of the insulator surface.Corrosion is more important on ‘’I’’ strings than on ‘’V’’ strings. It can be explain by the fact that the pollution degree of ‘’I’’ strings is higher than the pollution degree on ‘’V’’ strings due to a better self cleaning of the insulator on inclined ‘’V’’ strings.

4. GENERAL RECOMMENDATIONS FOR THE CHOICE OF INSULATOR FOR TROPICAL ENVIRONMENT Tropical environment is characterised by an almost permanent high humidity (≥ 70 %), a mean high

Zinc corrosion retardation sleeve

Efficiency of the shell profile

Reinforced Galvanizing

Special shape to limit electrical field and arc corrosion

Cement joint between the cap and the shell

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temperature and a long period without rainfall corresponding to the accumulation of a natural contamination. We can distingued two main areas corresponding to different natural geographical zones : - The inland areas which can be subjected to a

natural contamination with dust from the ground

- The sea coast areas close to the sea which is subjected to a more conductive pollution (soluble salt), often associated with a solid contamination.

The table 2 below summarizes some recommendations for the choice of insulator to increase the life of the line with the prevention of corrosion damages, and to avoid other disturbances due to the flashover of the insulator strings

Contamination Level

Examples of Typical

Recommanded insulators used on AC transmission lines for tropical areas

Contamination In tropical

areas

Insulator profile

Leakage distance (*) in mm/kV (Phase to

phase)

Corrosion protection

Metal parts Cap Galvanizing Pin Cap Assembling

A – LIGHT :

ESDD

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5. CONCLUSION In tropical areas, the long term performance of insulators under specific environmental stresses corresponding to a high humidity and high mean temperature can be increases with the use of toughened glass optimised design. Field experience in Brazil on 500 kV lines confirms that toughened glass insulators equipped with zinc retardation sleeve on the pin, a cap with a cement joint between the cap and the dielectric shell, do have an excellent behaviour when used in tropical environment subject to contamination which causes not only a leakage current but also arcs and discharges during the cycle of humidification. 6. BIBLIOGRAPHY [1] ‘’Isoladores tipo disco de vidro temperado - Analise

de desempenho e vida util no sistema’’ CHESF - UFPB - XI SNPTEE 1991 - SILVA R/BASTOS O/XAVIER G

[2] ‘’Corrosao de isoladores de disco EM LT’s 500

kV da CHESF (Evolucao e solucao do problema) ROMERO O. SILVA/S.J. GUSMAO CAVALCANTI /R.PARRAUD/D. DE DECKER/F.KKEULER/RACHEL SUASSUNA/OSVALDO L.S. PAIVA

[3] CIGRE N° 22-203-Aging mechanisms of AC

energised insulators CROUCH A/SWIFT D/PARRAUD R/DE DECKER D

[4] ISH 87 N°51-14 The DC component of leakage current on AC energized outdoor insulation : the effect of dry band discharges

SWIFT D/FITTER C/LIS [5] Cigre Symposium Cairns 2001 – Corrosion

mecanism of insulators in tropical environment – R. PARRAUD – D. DUMORA KEY WORDS : INSULATOR - CORROSION - POLLUTION - TROPICAL RESUME L’humidité très élevée rencontrée dans les pays tropicaux associée à une contamination qui se produit sur la surface des isolateurs en dehors des périodes de pluie entrainent la formation fréquente de courant de fuite, associée à des arcs et à des décharges qui corrodent les ferrures des isolateurs. A la lumière de l’étude des mécanismes de la corrosion des isolateurs, un nouveau design d’isolateur en verre trempé à été developpé pour résoudre des problèmes de corrosion observés dans des zones tropicales sur des lignes de transmission 500 kV. A la lumière d’essais comparatifs réalisés en vraie grandeur sur des chaines 500 kV, des recommandations sont faites pour les isolateurs afin de résoudre les problèmes de corrosion rencontrés précocément dans les zones tropicales.

--> List of Papers / Liste des rapports=================================================SUJET/SUBJECT 1 : STRESSES CHARACTERISTIC OF TROPICAL ENVIRONMENT / CONTRAINTES CARACTERISTIQUES DES MILIEUX TROPICAUX100-01 : Estimation of lightning performance of a 500kV transmission line in Thailand, by C. Banmongkol - E. Pongpeerapat (Thailand) - K. Naito - Y. Mizuno - T. Hayashi - K. Tsuge (Japan)100-02 : Lightning performance of transmission lines in Malaysia, by A.B. Abdul Halim - Ahmad Darus (Malaysia)100-03 : A case study on the lightning performance of a North Queensland transmission line, by J.A. Gillespie (Australia)100-04 : Powerlines in Midwestern Grassland plains of tropical North Queensland, the old and the new, by J. Bulman - T. Sanders (Australia)100-05 : Fires in mediterranean area and high voltage lines, by A. Sekso-Telento - S. Bojic - Z. Bertalanic (Croatia)100-06 : Corrosion mapping of transmission lines in Queensland. (Australia), by B. McMahon - D.J. Tutticci- (Australia) R.A. Walker (New Zealand)100-07 : Lightning performance of HV transmission lines with insulated shield wire(s) energized at MV analysis and field experience, by F.M. Gatta - F. Iliceto - S. Lauria (Italy)SPECIAL REPORT by C. Fitzgerald (Australia)RAPPORT SPÉCIAL par C. Fitzgerald (Australie)

SUJET/SUBJECT 2 : INSULATION ASPECTS / ISOLEMENT200-01 : Chaotic surface discharge on porcelain housing of ZnO surge arrester under pollution, by Y. Kawaguchi , T. Shimizu, S. Nishiwaki, M. Kan (Japan)200-02 : Effect of the discontinuous pollution layer conductivity on the flashover and leakage current characteristics, by M. Teguar, A. Abimouloud, A. Mekhaldi, A. Boubakeur (Algeria)200-03 : Investigation of influence of the dust grains on electric strength of HV insulating air gaps, by A.J. Kaluzny - M. Kaluzny. - M. Kasprzyk - F. Siewiec (Poland)200-04 : Selection of suspension strings use of "V" string discarded in favour of "I" string (a case study conducted in India), by Pankaj Kumar - Hemlata Vyas (India)200-05 : Field experience of the pollution related factors responsible for porcelain insulator failures as well as tripping and adoption of remedial measures in EHV transmission system in India., by H. Agrawal - L. N. Agrawal (India) 200-06 : Temperature effects on sparkover and the density correction factor, by N.L. Allen (United Kingdom)200-07 : Service experience with composite insulators under tropical climatic conditions in Malaysia, by J.M. Seifert - P. Besold (Germany)200-08 : Surface discharges on composite polymer insulators, by I.A.D. Giriantari - T.R. Blackburn (Australia)200-09 : Recovery of hydrophobicity on silicone rubber surface for polymer insulator housing deposited with artificial pollutant, by Y. Koshino - T. Imakoma - I. Umeda - M. Nishitoba - K. Kondo (Japan)200-10 : Transmission line route assessment for prospective bird damage to insulators, by N. Pearce - J. Thornton (Australia)200-11 : Effects of deposited charge on impulse test techniques for polymer insulators, by M. Darveniza - T.K. Saha - S. Wright (Australia) - M.A. Leijon (Sweden)200-12 : MO-surge arresters in tropical environments - Natural stresses and artificial testing, by B. Richter - W. Schmidt (Switzerland)200-13 : Performance review of a 300 km, 132kV guyed monopole transmission line in the northern territory tropical zone, by C. J. Freeman - T. Horman (Australia)200-14 : Experience with composite insulators under stringent atmospheric conditions, by J.L. Bessede - D. Dufournet (France)200-15 : The requirements being applied in Russia, to dielectric strength of electrical equipment for tropical environment, by V.M. Pogostin - L.M. Pestryaeva (Russia)200-16 : Enhancement of the surface insulation performances by means of treatments and coatings, by M. De Nigris - A. Tomba (Italy)200-17 : Correlation between natural and artificial pollution in marine and desert environment in Tunisia, by R. Znaidi (Tunisia)200-18 : Performance of different insulator designs in tropical environments and diagnostic possibilities, by S. Gubanski - A.D. Dernfalk (Sweden) - M.A.R.M. Fernando (Sri Lanka)SPECIAL REPORT by A. Pigini (Italia)RAPPORT SPÉCIAL par A. Pigini (Italie)

SUJET/SUBJECT 3 : THERMAL, MECHANICAL AND STRUCTURAL ASPECTS / ASPECTS THERMIQUES, MECANIQUES ET STRUCTURELS300-01 : Instrument design for extreme environmental conditions. The SIEMENS's PD monitor for transformers, by J. Unsworth - D. Tallis - N. Booth - V. Kurusingal (Australia)300-02 : Standard HV circuit-breakers - even in tropical environments, by H.H. Schramm (Germany)300-03 : Numerical simulation of stresses distribution in the transformer corrugated wall structure, by M. Ciach - T. Nowak - K. Kaczmarek - W. Zielinski - A. Zbudniewek (Poland)300-04 : Corrosion mechanism of insulators in tropical environment, by R. Parraud - D. Dumora (France)300-05 : Recommendations to solve corrosion problem on HV insulator strings in tropical environment, by O.L.S. Paiva - R. Suassuna - L.F. Ferreira, M. Namora (Brazil) - D. Dumora - R. Parraud (France)300-06 : High voltage outdoor SF6 circuit-breakers in tropical environments - A Manufacturer's perspective, by A. Bosma - R. Thomas (Sweden)300-07 : Introduction of various type joints for EHV cable, by N. Yoshikawa - K. Ishibashi - S. Narisada - T. Nakanishi - K. Fudamoto (Japan)300-08 : High voltage switchgear and tropical environments, by S. Jones (Australia)300-09 : Structural design of subtransmission lines for a cyclonic environment, by C.A. Pemberton (Australia)300-10 : Benefits of non conventional instrument transformers in tropical environments, by A. Jaussi - H.-D. Schlemper (Switzerland)300-11 : Lightning current test of the overhead ground wire with embeded optical fibers, by R. Malewski - J. Dabrowski (Poland)SPECIAL REPORT by S.L. Jones (Australia)RAPPORT SPÉCIAL par S.L. Jones (Australie)

SUJET/SUBJECT 4 : OPERATION AND MAINTENANCE / EXPLOITATION ET ENTRETIEN400-01 : Diagnostic of the performance of transmission line in design phase, under the action of atmospheric discharges with installation of ZnO surge arresters, by A.C. Guarà Bezerra (Brazil)400-02 : Apport des travaux sous tension dans l'amélioration de la qualité de service, par Y. K. Lataille (Côte d'Ivoire)400-03 : Importance de la thermographie dans la surveillance et le diagnostic des réseaux, par Y. K. Lataille (Côte d'Ivoire)400-04 : The preventive diagnostic system for 765 kV substation in Korea, by D-J. Kweon - I-H. Choi - G-J. Jung - J.B. Kim - Y-J. Jung - H-J. Lee - H-C. Shin - H.R. Kwak S.B. Jin - D.J. Lee (Korea)400-05 : Behaviour of overhead-line conductors in tropical environments, by V. T. Morgan (Australia)400-06 : Operational readiness strategy for transmission lines within cyclonic zone of North Queensland, by A.S. Bonanno - (Australia)400-07 : Service ageing and monitoring of transformer insulation in tropical areas, by D. Allan (Australia)400-08 : Dynamic thermal rating system of overhead transmission lines using internet weather data, by A.K. Deb (United States)400-09 : Design and maintenance of insulation for a North Queensland transmission line, by J.A.T. Gillespie (Australia)400-10 : Towards the in service monitoring on silicone housed surge arresters, by W. Schmidt - B. Richter (Switzerland) - T. Klein (Germany)400-11 : Effect of load current on frequency response signatures of large power transformers, by S. Islam - N. Pinhas - J. Hullett (Australia)400-12 : Life management of transformers - a Northern territory historical perspective, by Yogendra Dev Vashishtha - P. Ascione (Australia)400-13 : Improved asset life management and utilisation through thermal monitoring, by G. R. Schmidt.- A. D. Stokes (Australia)400-14 : Condition assessment of power transformers : recent experiences in CESI, by M. De Nigris - R. Passaglia - P. Mazza (Italy)400-15 : Wooden pole lines in extreme climatic conditions in Dalmatia, by A. Sekso-Telento (Croatia) - M. Darveniza (Australia)400-16 : Les techniques de travaux sous tension HT et THT à Sonelgaz, par S.M. Sekhri (Algérie)400-17 : Incidents d'exploitation et avaries matériels, par H. Kadri (Algérie)400-18 : Maintenance scheduling of power transformer using Markov model, by Umar Khayam - Ngurah Satriyadi - Ngapuli I. Sinisuka (Indonesia)400-19 : Availability and reliability performance of power transformer operation in tropical area using Markov model, by Ir Ngapuli I. Sinisuka - Ig Ngurah Satriyadi H - Umar Khayam St-Mt (Indonesia)SPECIAL REPORT by P.L. Austin (Australia)RAPPORT SPÉCIAL par P.L. Austin (Australie)