Measurement of Leakage Current for Monitoring the Performance of Outdoor Insulators in Polluted Environments

Transcription

1 F E A T U R E A R T I C L E Measurement of Leakage Current for Monitoring the Performance of Outdoor Insulators in Polluted Environments Key words: insulators, pollution, leakage current, diagnostic techniques, flashover risk Introduction The measurement of leakage current is used for monitoring the performance of insulators to minimize system outages attributable to pollution []. The technique is applied to transmission lines exposed to severe and varying conditions of pollution [2]. According to this technique, preventative maintenance is initiated when a critical level of current is reached on polluted insulators. However, the technique requires continuous supervision to analyze the condition of the monitored insulators. Important differences in the severity of pollution between sites can be detected by measuring the leakage current. In some sites, the severity of pollution can cause high leakage current activity in a short time, whereas in others, a longer time is needed and monitoring at different sites is necessary to determine the best insulator profile for each site. With the selection of the best insulator profile for each site, maintenance can be scheduled accordingly, thereby minimizing pollution flashover outages. Since 995, the Mexican Electrical Research Institute (Instituto de Investigaciones Eléctricas), together with the Mexican electric utility (Comision Federal de Electricidad), has had several projects for online measurement of leakage current that have been applied to various designs of insulators in an attempt to minimize pollution flashovers on transmission lines [3]. To date, the Comision Federal de Electricidad has had 44 leakage current monitoring systems installed on several transmission lines [4]. These projects have provided a much broader perspective on the performance of outdoor insulators for various pollution zones, and based on laboratory tests, critical leakage current magnitudes have been determined for various insulator profiles [5]. Although the maximum permissible levels of leakage current are considerably different for each profile, this does not necessarily indicate that insulators that permit a lower leakage current will have the best operating performance because the Isaias Ramirez, Ramiro Hernández, and Gerardo Montoya Instituto de Investigaciones Eléctricas, Cuernavaca, Morelos, México Measurement of leakage current can be an effective method for minimizing the pollution flashover of insulators on transmission lines, but to be useful, the technique requires additional information from measurements and analysis of the pollution. profiles themselves will permit the accumulation of pollutants, which defines their pollution performance. Furthermore, a leakage current may be sporadic, continuous, periodic, or increasing with time, all of which are factors in the preventive maintenance of insulator outages resulting from pollution. This article presents an analysis of the monitoring of various types of insulators, namely, porcelain, room temperature vulcanizing silicone (RTV)-coated porcelain, toughened glass, and nonceramic insulators installed on transmission lines close to the Villa de Garcia Substation, in the metropolitan area of Monterrey, Mexico. On one occasion during heavy fog, pollution caused outages on 400- and 230-kV transmission lines, resulting July/August Vol. 28, No /2/$3/ 202/IEEE 29

2 Table. Leakage Current Classifications. Range Leakage current magnitude (ma) in a very serious event in which 6.8 MW of load was dropped for 5 minutes and 5 MW of load was dropped for minute. Leakage Current Monitoring The monitoring system classifies leakage current peaks in five ranges, as shown in Table, and the microprocessor monitors all leakage current pulses in each half cycle (8.33 milliseconds) for all three phases and provides a data summary every 9 hours. The data contain the maximum leakage current pulses, have the accumulated peaks classified into each of the five ranges, and have autonomy for 6 months, after which the data need to be downloaded and the system reset to begin a new monitoring period. Table 2 outlines the basic parameters of the insulators monitored. The insulators are porcelain, glass, and silicone rubber nonceramic insulators (NCI). The critical leakage current for flashover as determined in the laboratory is also shown in the table. Table 3 shows the transmission line voltage and the string configurations monitored. Example of Leakage Current Measurements During a period of severe fog lasting 2 days, leakage current activity was monitored on insulator strings; the results for insulators on tower 59 are shown as examples (Table 4). The leakage current on nonceramic insulator NCI-2 on phase A, shown in Figure (a), reached peaks of 38 and 44 ma during the event. During the same event, RTV-coated superfog insulators on phase C showed higher leakage current activity, reaching levels up to 25 ma. The Vee-string configuration on phase B with type NCI-2 insulators showed leakage currents of 35 and 60 ma on each leg of the Vee string, respectively. The NCI on phases A and B showed lower leakage currents compared with the RTVcoated superfog insulators on phase C on the same tower. Table 4 summarizes the highest magnitude of leakage current registered by the monitoring systems. No leakage current levels above the critical ones, as determined in the laboratory (Table 2), were recorded for the operating conditions of this site. Therefore, no pollution flashover should have taken place. To understand the reason for flashovers, several of the flashed insulators were removed from the site for laboratory flashover tests and chemical analysis of the pollutant. Laboratory Tests Insulator strings on line A3450, tower 59, and A3720, tower 58, which showed the highest leakage currents during the 2-day fog event (Table 4), were removed for flashover tests in a clean fog chamber. These tests were done following the rapid flashover method in IEC [6]. The removed strings were made into shorter strings of four units and tested, obtaining an average flashover per unit of 0.9 kv rms. Because this flashover is close to the nominal voltage per insulator (0.5 kv rms ) on the transmis- Table 2. Basic Parameters of the Insulators Monitored. Insulator designation Material Insulator type Rated strength (kn) Units spacing (mm) Shed diameter (mm) Leakage distance per unit (mm) Critical leakage current for flashover (ma) 27SVC Glass Standard SVC Glass Fog SPC Porcelain Fog SPC Porcelain Superfog SVC Glass Superfog SV60C Glass Fog NCI- Silicone rubber NCI 30 3,550 NCI-2 Silicone rubber NCI 30,928 NCI-3 Silicone rubber NCI 33 3,475 NCI = silicone rubber nonceramic insulator. 35 major 00 minor 4 major 02 minor 72 major 42 minor 0,420 6,2 4, IEEE Electrical Insulation Magazine

3 Table 3. Transmission Line Designation, Tower Number, Line Voltage, and String Configurations Monitored. Transmission line, tower number and line voltage Phase A Phase B Phase C A3450, tower 67, 400 kv 32SPC I string 32SPC I string A3450, tower 98, 400 kv 28SVC I string 28SVC I string A3D60, tower 5, 400 kv 32SVC I string 32SVC I string A3450, tower 7, 400 kv 32SV60C Vee string 32SV60C Vee string RTV-coated 29SPC I string NCI- I string 9330, tower 59, 230 kv NCI-2 I string NCI-2 Vee string RTV-coated 32SPCS I string A3G20, tower 55, 400 kv NCI-3 Vee string NCI-3 Vee string A3720, tower 58, 400 kv 28SVC I string 29SPC Vee string RTV-coated 29SPC I string RTV = room temperature vulcanizing silicone; NCI = silicone rubber nonceramic insulator. sion lines, it was demonstrated that the pollutant on the insulators was indeed the cause of the flashovers. The pollution level in terms of equivalent salt deposit density according to IEC 6085 [7] and chemical analysis of the pollutant were determined. These results are shown in Table 5. The pollutant level corresponds to pollution class III (high pollution) in IEC The pollutant was analyzed by X-ray diffraction using a Siemens D500 diffractometer with filtered Cu radiation. To identify the pollutant crystalline components, a diffraction pattern file was used [Mineral Powder Diffraction File, Joint Committee on Powder Diffraction Standards (JCPDS), Newtown Square, PA, 980]. The crystalline compounds identified for this pollutant type and the equivalent salt deposit density values are shown in Table 5. As indicated in Table 5, gibbsite was found on insulators at tower 65 and at tower 4, but gibbsite was not found in an earlier analysis of the pollutant in this area [8]. It was concluded that the source of the pollutant must be very near the affected insulators. Because gibbsite contains aluminum, a review of the manufacturing facilities in the area identified an aluminum automobile engine block manufacturing facility as the probable emitter of aluminum, and gibbsite is formed from aluminum under specific environmental conditions. Under certain conditions of moisture, gibbsite crystals on insulators can result in sufficient leakage current for flashover, and this was thought to be the reason for Table 4. Leakage Current During the Period Analyzed. Maximum leakage current during the period analyzed (ma) Tower Channel Insulator Channel 2 Insulator Channel 3 Insulator Channel 4 Insulator SPC 4 32SPC SVC.38 28SVC SVC 69 32SVC SV60C 24 32SV60C 38 RTV-coated 29SPC 5 NCI NCI-2 25 RTV-coated 32SPC 35 NCI-2 60 NCI NCI-3 2 NCI-3 2 NCI-3 0 NCI SVC 22 29SPC 3 29SPC 93 RTV-coated 29SPC A dash ( ) indicates channel not installed. RTV = room temperature vulcanizing silicone; NCI = silicone rubber nonceramic insulator. July/August Vol. 28, No. 4 3

4 Figure. Leakage currents recorded on insulators on tower 59 (Table 3) during a period of severe fog: (a) nonceramic insulator (NCI)-2 on phase A; (b) room temperature vulcanizing silicone (RTV)-coated superfog insulators on phase C; (c) one leg of the NCI-2 Vee-string insulator on phase B; (d) other leg of the NCI-2 Vee-string insulator. the high leakage current [up to 25 ma in Figure (b)] at tower 59 of the 230-kV 9330 transmission line. Table 6 shows the elemental analysis by atomic absorption spectrometry of the solid constituents of the pollutant layer; it showed a large weight percentage of aluminum, with the top surface showing 3.5 to 6.4 times higher level than the bottom surface. This implies that the mechanism of deposition of aluminum on the insulator was likely precipitation. The higher levels of calcium and sulfates on the bottom of the insulator were associated with the formation of gypsum [9]. Because of its chemical properties and solubility, as well as the shape and design of the insulator, this pollution tends to accumulate on the bottom surface of the insulators. Gypsum is a product of the reaction of calcite-dolomite in the soil and SO 2 from the atmosphere, the latter generated mainly by the combustion of hydrocarbon fuels. [9]. Dihydrate CaSO 4 is a compound with low solubility that presents a high water absorption capacity, which aids in the flashover process. RTV-Coated Insulators To simulate the field pollutant for laboratory tests, a mixture of 600 g of kaolin, 200 g of CaSO 4, and 5 g of NaCl (common salt) per liter of water was made and applied to various types of insulators for clean-fog flashover tests. For these tests, the contaminant layer was applied to the top, bottom, or entire surface of the insulators [0]. These results showed that when the entire surface of the insulator was coated, the flashover voltage was the highest, whereas coating only the bottom surface resulted in only a slight reduction in the flashover, as illustrated in Figure 2. Su et al. [] indicated that RTV-coated insulators could operate longer than 6 years without preventive maintenance in subtropical climates. However, in another study reported by Su et al. Table 5. Pollutant Equivalent Salt Deposit Density (ESDD) and Crystalline Components. ESDD (mg/cm 2 ) Crystalline components Insulator Top surface Bottom surface Top surface Bottom surface String, line A3450, tower Gibbsite [Al(OH) 3 ], quartz (SiO 2 ) Calcite (CaCO 3 ), quartz (SiO 2 ), gypsum (CaSO 4 2H 2 O) String 2, line A3720, tower Calcite (CaCO 3 ), quartz (SiO 2 ), gibbsite [Al(OH) 3 ] Calcite (CaCO 3 ), quartz (SiO 2 ) 32 IEEE Electrical Insulation Magazine

5 Table 6. Elemental Analysis of the Solid Part of the Contaminant Layer from the Affected Insulators. Weight % Item Top of insulator C Top of insulator C2 Bottom of insulator C Bottom of insulator C2 Element Magnesium (Mg) Calcium (Ca) Sodium (Na) Potassium (K) Aluminum (Al) Silicon (Si) Iron (Fe) Sulfate (SO 4= ) Chlorine (mg/l) ND ND Weight of the solid (g) The content of metals was analyzed by atomic absorption spectrometry following standard ASTM E Calibration of equipment to determine ph was performed according to standard ASTM D using at least two reference buffer solutions traceable to the National Institute of Standards and Technology. The determination of total chlorine was carried out according to standard ASTM C Sulfates were obtained following standard NMX-AA Acidity and alkalinity were determined as specified in standard NMX-AA-036-SCFI-200. ND = not determined. 2 Please indicate what the boldface line in the table represents. [2] on insulator performance on a 220-kV transmission line in a coastal environment, in which heavy contamination accumulated when rain was sparse, high leakage current and continuous partial discharge activity contributed to the drop in hydrophobicity, increasing the likelihood of flashover. However, fully coated insulators were installed in the Monterrey area, even though the cost of this preventive solution was considered too high. According to the leakage current measurements on the 230- kv circuit 9330 at tower 59 and flashovers on the 400-kV circuit A3450 at tower 4, RTV-coated fog-type insulators could operate reliably for 9 years in this environment without flashovers (insulators installed in 995). As a comparison, nonceramic insulators that were installed at the same time and in the same location as the RTV-coated fog-type insulators on tower 7 could operate for at least 5 more years, which was estimated by taking into consideration the very low leakage current and good condition of the insulators. Conclusions Monitoring of leakage currents on various types of insulators, in the field and in laboratory tests, has provided a much broader perspective on the performance of outdoor insulators in various pollution zones. The technique can be effective for corrective maintenance of transmission lines, provided the type of pollutant and the critical leakage currents are analyzed periodically. A leakage current that is continuous, periodic, or increasing with time is one consideration in the performance of insulators in pollution zones, whereas the maximum permissible levels of leakage current at flashover, which is considerably different for each Figure 2. Clean-fog flashover tests of room temperature vulcanizing silicone (RTV)-coated insulators for no coating, coated on the upper side, coated on the lower side, and fully coated. July/August Vol. 28, No. 4 33

6 insulator profile, is another consideration. What this means is that insulators showing lower leakage currents will not necessarily have the best operating performance. The insulator profile affects the accumulation of pollutants, which defines their pollution performance, and control of the leakage current by the insulator design depends largely on the type of pollutant. As a result, there is no unique solution, and leakage current values with flashover risk from laboratory tests for various types of insulators must be determined by using the type of pollutant in the field. This combination of strategies will provide an effective means of ensuring reliable transmission line operation in pollution zones. Acknowledgment The authors thank M. C. Georgina Blass Amador, and María Guadalupe Cruz González, personnel from Comision Federal de Electricidad-Coordinacion de Transmision y Transformacion, Mexico, for their invaluable help with this project. References [] J. Y. Li, C. X., Sun, W. X. Sima, and Q. J. Yang, Stage pre-warning based on leakage current characteristics before contamination flashover of porcelain and glass insulators, IET Gener., Transm., Distrib., vol. 3, no. 3, pp , [2] A. J. Carreira, Condition detection of non-ceramic insulators for live line work at electric utilities, INMR, vol. 7, no. 84, pp , [3] G. Montoya-Tena, R. Hernández-Corona, and I. Ramírez Vázquez, Experiences on pollution level measurement in Mexico, Electr. Power Syst. Res., vol. 76, pp , [4] Montoya, G., Ramirez, I., and Hernandez, R., The leakage current as a diagnostic tool for outdoor insulation, in Proc. IEEE/PES Transmission and Distribution Conference and Exposition: Latin America, Bogota, Colombia, Aug. 2008, pp. 4. [5] Isaías Ramírez-Vázquez, José Luis Fierro-Chávez, Criteria for the diagnostic of polluted ceramic insulators based on the leakage current monitoring technique, in 999 IEEE Conf. Electrical Insulation and Dielectric Phenomena, Austin, TX, 999, pp [6] Artificial Pollution Tests on High-Voltage Insulators to Be Used on A.C. Systems, IEC 60507, 2nd ed., Apr. 99. [7] Guide for the Selection of Insulators in Respect of Polluted Conditions, IEC 6085, Ed..0, [8] G. N. Ramos, M. T. Campillo, and K. Naito, A study on the characteristics of various conductive contaminants accumulated on high-voltage insulators, IEEE Trans. Power Del., vol. 8, no. 4, pp , 993. [9] R. T. Campillo, Characterization of pollutant in outdoor insulators of the industrial area of Monterrey, N.L., Classification IIE/30/3/RP- 00/526/993, Instituto de Investigaciones Eléctricas, Cuernavaca, Morelos, Mexico, Mar. 3, 993. [0] B. Segura Bahena, Evaluation of electrical Insulators for transmission lines with pollution problems, B.S. Thesis, Faculty of Chemical Sciences and Engineering, Cuernavaca, Morelos, Mexico, Mar [] H. Su, Z. Jia, Zhicheng Guan, Durability of RTV-coated Insulators Used in Subtropical Areas, IEEE Trans. Dielectr. Electr. Insul., vol. 8, no. 3, pp , June 20. [2] Huafeng Su, Zhidong Jia, Zhicheng Guan, and Licheng Li, Mechanism of Contaminant Accumulation and Flashover of Insulator in Heavily Polluted Coastal Area, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 7, No. 5; pp , October 200. Isaias Ramirez-Vazquez (S 05) received the BS degree from the Facultad de Ingeniería Mecánica Eléctrica y Electrónica (FI- MEE), Salamanca, Guanajuato, México, in 990, the MS degree from FIMEE in 999, and the PhD in electrical and computer engineering from the University of Waterloo, Ontario, Canada. He is a researcher at the Instituto de Investigaciones Eléctricas (Electrical Research Institute) in Cuernavaca, Morelos, México. His current research interests include nano materials for outdoor insulation, insulation coordination, and electromagnetic transients in power systems. Ramiro Hernández-Corona was born in Cuernavaca, Morelos, México, on September 7, 966. He received the BS degree in electrical engineering from the University of Morelos in 99. In the same year, he joined the Instituto de Investigaciones Eléctricas (Electrical Research Institute) located in Cuernavaca, Morelos, México. In 995, he received an MS degree in electrical engineering from the University of Salford, UK. He is currently a researcher of the Transmission and Distribution Department. He has been working in activities of pollution flashover on external insulation and of lightning protection in transmission and distribution lines. Gerardo Montoya-Tena was born in México City, México, on April 27, 962. He received the BS degree in electronic and communications engineering from the Instituto Politécnico Nacional (National Polytechnic Institute). In 990, he received a MS degree with honors in electrical engineering from the Universidad Nacional Autónoma de México (Autonomy National University of Mexico). In 986, he joined the Instituto de InvestigacionesEléctricas (Electrical Research Institute) located in Cuernavaca, Morelos, México. He is currently a researcher in the Transmission and Distribution Department. He is author of several papers about external insulation and overhead transmission lines. He has been working on activities around pollution flashover on external insulation in transmission and distribution lines. He is member of specialists committee of the Mexican utility. 34 IEEE Electrical Insulation Magazine