Transcript
Experience at Venezuelan Utility withCeramic and Non-Ceramic Insulators INTRODUCTION C.V.G. ElectrificacióndelCaroní (EDELCA) is an electric power generation andtransmission company created by the Venezuelan Government in 1963. Itstransmission system comprises 2100 km of 765 kV lines, 2400 km of 400 kV linesand 500 km of 115 kV to 230 kV lines. This system is connected to the Guri Dam(10,000 MW) as well as to the Macagua Complex (2950 MW), which in 1998together generated 56,190 GWh - equivalent to approximately 70% of the country¶stotal electricity consumption. 765 kV TRANSMISSION SYSTEM EDELCA first began construction of its 765 kV transmission system in the early1980¶s and, by 1986, the first two lines began operating covering a length of 1250km. During 1991, a third transmission line was completed adding 850 km to thissystem. Together, these 2100 km of 765 kV lines have now been in service for more than 10 years.In terms of insulators, almost 700,000 ceramicinsulator units have been utilized on this system, of which approximately 60 per cent are porcelain and40 per cent glass. These lines are 4-bundle ACAR1300 MCM per phase with two overhead groundwires of alumoweld 7#8 AWG. The configuration of the insulator string is I V I (i.e. I string at both outsidephases and V string in the center).The 765 kV lines from the hydro-electric stations in the southern Guayana Regionto the urban centers of the northern and western parts of Venezuela, pass througha region known as Mesa de Guanipa which is noted for its high levels of agricultural pollution and adverse climatic conditions. Weather studies indicate thatthe dry season in this area lasts from 4 to 6 months beginning in November. Thisresults in an accumulation of pollution and during the evenings the relative humiditylevels approach 100%. These heavy contamination conditions have affected thedesign of the insulation level of the lines more than any considerations related toswitching surges. At first, EDELCA analyzed the performance of two 400 kV lines as well as a 230 kVline located in this area and which had been in service since 1971 and 1968 The performance of insulators affected bypollution is not only aresult of creepagedistance but is alsorelated to the shape of theinsulators. respectively. During a period of eight years from 1971-1978 there were 17 faults onthe 400 kV lines during the January-March period - all attributed to insulator contamination. Although there were no outages due to contamination on the 230kV line, a high level of corrosion was observed on the pins of insulators.Since 1978, five porcelain and glass test-insulator strings have been installed tomonitor ESDD levels. Also, international consultants have been called in toconduct additional studies in this area. They came to the conclusion that theperformance of insulators affected by pollution is not only a result of creepagedistance but is also related to the shape of the insulators being used. This analysiswas taken into account in defining the type of insulators to be specified byEDELCA for each pollution level, as follows:Low Contamination Areas (ESDD of 0.05 mg/cm2): Thirty-seven 160 kN and 210 kN insulators with a creepage distance of 370 mm for suspension strings and thirty-five 300 kN insulators with a creepage distance from425 to 465 mm for tension strings. Both porcelain and glass insulators have beenused.High Contamination Areas (ESDD of 0.24 mg/cm2): Thirty-seven 160 kN and 210 kN insulators with a creepage distance of 550 mm(anti-fog type) in suspension strings and forty-five 300 kN insulators with acreepage distance of 460 mm for tension strings. Only porcelain insulators havebeen used.On the other hand, a conservative attitude was adopted when considering insulator performance to be equivalent for both I-strings and V-strings. The increase of stringlength was estimated at 5 per cent to take into account the non-linearity effect of the discharge voltage.In summary, the first 765 kV lines have been operational for about 14 years nowwithout presenting any major problems. During this time, only seven pollution-related failures have occurred of which five were within a high contamination zone. Another issue related to the insulators on these 765kV lines has been the problem of vandalism and arecord has been kept of the numbers of insulatorsbroken due to this reason. From the time these linesbecame operational up until 1998, 890 brokeninsulators have been detected, representing close to0.1 per cent of the total number of insulators on thelines. It is interesting to note that the majority of broken insulators were glass. This has resulted in The continuousinterruption of both 400kV lines was due to aninadequate insulationlevel given the degree of pollution whichunfortunately wasunknown at the time thelines were designed. EDELCA¶s maintenance units adopting a policy of replacing glass insulators withporcelain in those areas where vandalism occurs. 400 kV TRANSMISSION SYSTEM EDELCA¶s 400 kV transmission system is approximately 2400 km long andconsists of lines of two and four conductors per phase, mostly ACAR 1024.5 MCM,with two alumoweld overhead ground wires. The configuration of the strings is I II(i.e. three I strings). From the insulation point of view, the three most critical lineshave been the Guri-El Tigre-San Gerónimo lines in the sections crossing theMeseta de Guanipa, which is a region characterized by heavy agricultural activity.This has caused a high level of dust to accumulate on insulators.The first line was commissioned in 1970. At the time, it was not known that theregion presented heavy levels of contamination and the srcinal design of thesuspension strings used 19 glass insulators of 280 mm x 146 mm type with acreepage distance of 370 mm and a mechanical load of 160 kN. The totalcreepage distance of each string was 7030 mm.The second line, Guri-El Tigre-San Gerónimo-Sta. Teresa, entered service in 1975.Based on the limited experience gained regarding pollution-related failures on thefirst line, a more conservative insulation design was adopted here. This designused 160 kN porcelain insulators of 292 mm x 159 mm type with a creepagedistance of 470 mm. Each string¶s total creepage distance was now 9400 mm,which represented an increase of 34 per cent compared to the first 400 kV line.Due to the fact that pollution-related outages continued, the contamination levelswere measured on test strings installed on lines 1 and 2 for a period of two yearsstarting in January 1978. These test insulators were then sent for chemicalanalyses to Japan and revealed an ESDD level of 0.2 mg/cm2. Given these resultsand the knowledge that dust accumulates here over a long dry period, EDELCAconservatively established a value of 0.24 mg/cm2 as the contamination level for the design of lines which pass through the Mesa de Guanipa. TABLE 1: Comparison of Design of Three 400 kV TransmissionLines Passing Through Heavy Contamination Areas LINE YEAR ORIGINAL DESIGN DESIGN (INSERVICE) INSULATOR SIZEAND NUMBERINSULATORCREEPAGEDISTANCECREEPAGEPATH LENGTH Guri-El Tigre- SanGerónimo N° 11969 (1970)19 of 280 mm x146 mm370 mm 7030 mmGuri-El Tigre- SanGerónimo N° 21972 (1975)20 of 292 mm x159 mm470 mm 9400 mm Guri-El Tigre N° 3 1986 (1989)21 of 320 mm x170 mm550 mm 11,550 mm Considering this value and taking into account the types of insulators installed, Line1 would now need more than thirty normal-type 284 mm x 146 mm units while Line2 would now need twenty-three anti-fog 292 mm x 159 mm insulators. Thisconfirmed that the continuous interruption of both these lines was due to aninadequate insulation level for the degree of pollution which was unknown at thetime the lines were designed. In order to reduce outages due to contamination, apermanent hot line wash for both lines was implemented in 1978.In addition, the results of studies on the performance of different types of insulatorsunder different contamination levels revealed that the anti-fog 292 mm x 159 mminsulator and the conventional 280 mm x 146 mm insulator behave similarly, whilethe anti-fog 320 mm x 170 mm insulator has a far superior performance. Based onthese findings, in 1986 EDELCA designed the Guri-El Tigre Line 3 using twenty-one anti-fog 320 mm x 170 mm insulators, with a total string creepage distance of 11,550 mm, which is 64 per cent more than Line 1 and 23 per cent more than Line2.Table 1 shows a comparison of the three projects indicating the type and quantityof insulators used and the total creepage distance for each string. The threetransmission lines have parallel trajectories and the distance between them doesnot exceed 100 m making the comparison between their design and performancepertinent. As can be seen from Table 1, Lines 1 and 2 were designed with acreepage distance lower than the insulation requirements for the degree of pollution in the area.In 1988, after periodic washing of the strings and with a better awareness of theperformance of the anti-fog 320 mm x 170 mm insulator, sixteen insulators of thistype were installed on Line 1 and eighteen units on Line 2, in order to improve theinsulation level of both. It was impossible to install twenty-one insulators asdesigned for Line 3, because of the conductor to tower clearance of the existingstructures.The performance of these lines after these changes had been made was asfollows: y Although Line 1 has been hot line washed and the 19 conventionalinsulators replaced by 16 anti-fog insulators, this line had 25 outages dueto pollution-related faults during the 1988-1998 period. y Line 2 has towers with a longer conductor to tower clearance, permittingthe installation of a string of 18 anti-fog units, and has had a betterhistorical performance than Line 1 with 30 per cent fewer failures.Duringthe 1988-1998 period, 17 pollution-relatedfaultshaveoccurred.
