HTV SILICONE COMPOSITE INSULATORS FOR HVDC APPLICATIONS - LONG- TERM EXPERIENCES WITH MATERIAL AND DESIGN FOR 500KV AND ABOVE

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1 HTV SILICONE COMPOSITE INSULATORS FOR HVDC APPLICATIONS - LONG- TERM EXPERIENCES WITH MATERIAL AND DESIGN FOR 500KV AND ABOVE J. M. Seifert 1*, D. Stefanini and H. Janssen 2 1 Lapp Insulators GmbH, Bahnhofstrasse 5, D Wunsiedel, Germany 2 Dow Corning GmbH, Rheingaustrasse 34, D Wiesbaden, Germany * Jseifert@lappinsulators.de Abstract: Generation III silicone rubber composite insulators have been successfully applied in HVDC transmission lines for more than 3 decades [1]. The most popular installations are realised with the lines Pacific Intertie (±500kV) in Western USA, Cabora Bassa - Appollo (±533kV) in Mozambique/South Africa and in New Zealand (±270kV). The pollution severity for the HVDC line insulators at these installations can be evaluated in the range of light to medium, partly (locally) heavy in accordance with IEC The service experience with these line insulators has been very good regarding insulation performance and ageing resistance and forms an excellent base for further investigations and design reviews especially on material characteristics and creepage selection for future installations. Presently, several new HVDC lines are under construction or in the planning process in China, India, Brasil and South Africa. In contrast to the above mentioned existing lines, the pollution severity for the new projects is assumed to be much higher in the range of heavy to very heavy. This paper will give a design review of existing installations with silicone rubber composite insulators and will summarize the positive experience made with such insulators on HVDC lines during the last 30 years. During the design review the insulator characteristics will be investigated regarding creepage selection and housing design reflected by statistical methods applied to available pollution data. In addition to the electrical performance, the material ageing characteristic will be reviewed by laboratory experiments. Results of tracking and erosion material tests performed in similarity to IEC at DC voltage stress and those of a 5000 h multi-ageing test at DC voltage will be evaluated and discussed. The results will be compared to the behaviour of the insulators after 25 years of service in DC lines. 1. CASE STUDIES AND DESIGN REVIEW 1.1. Pacific Intertie ±500 kv Table 1: Line Data Parameter Line Voltage ±500 kv (before 1984: ±400 kv) Country USA, West Coast Year of installation 1984 (re-insulation of the former glass and porcelain discs with silicone rubber composite longrods) Service duration up to 24 years with silicone rubber composite Operated by LADWP, BPA Converter Stations Sylmar (Los Angeles, CA) Celeilo (Oregon) Pollution High in the Los Angeles area (industrial, pollution from traffic, natural) Medium in the desert/semi-desert area on the major line distance, dry arid, natural pollution Light, almost no pollution in the Oregon area ESDD [mg/cm²] and mg/cm² (in the LADWP district) [2], no available data from BPA Insulation HTV silicone rubber composite longrod, Generation III [1] Material HTV silicone compound with 48% ATH by weight Type 30/73 (168) mm, 30 mm FRP rod diameter, 73 sheds, uniform smooth shed profile, shed diameter 168 mm, connection length h1=4520 mm, creepage distance mm, 25.7 mm/kv Shed Profile Uniform, smooth sheds (open aerodynamic profile) Figure 1: ±500kV HVDC Pacific Intertie, CA, USA Figure 2: ±500kV HVDC Pacific Intertie Reinsulation, glass discs beeing substituted by silicone rubber longrod insulators, this procedure has been succesfully applied since 1984 [2]. Pg. 1 Paper C-12

2 Based on test station data (Koeberg Insulator Test Station, South Africa) the STRI Insulator Selection Tool (IST) was adapted to the type of silicone rubber composite insulator used in the Pacific Intertie line. The following calculations were performed using the STRI IST Ed. 2.0 for verification purpose: The service performance with silicone rubber composite insulators in this line is very good. Neither pollution flashovers nor tracking and erosion phenomena have been experienced during almost 30 years of application. Three different generations of composite insulators have been applied in this line which has globally the longest service records from all existing HVDC lines equipped with composite insulators. Although pollution severity is presently unknown and expected to be light-medium, the insulation described in this report is able to withstand pollution severities of the heavy-very heavy category. 2. LABORATORY TESTS Figure 3: IST Statistical Insulation Performance Calculation for the housing profile used in the Pacific Intertie Insulation, insulator type 30/73 (168) mm. number of insulators N=10000, Average ESDD value, NSDD/ESDD=5, based on test station data (KIPTS). The IST data shows that the insulation (red line) is well selected for the pollution conditions present. The choice of 25.7 mm/kv for silicone rubber composite insulators has been proven by 24 years of positive service experience. The theoretical approach for CD design can be confirmed by this case study also by applying statistical methods as indicated in IEC Cabora Bassa - Appollo ±533 kv Presently there are no IEC material and design test standards for HVDC composite insulators available, therefore established AC tests were modified for this work. Laboratory tests in accordance with modified tests similar to IEC [3] and IEC [4] were performed at DC voltage in order to compare excellent long-term in-service experience to the results obtained from artificial composite insulator material and design tests. 2.1 DC 5000 h Multi-Ageing (Design) Test A 5000 h DC test [4] was performed at STRI (Sweden) for the housing profile applied in the Cabora Bassa Appollo Line. The profile is an alterating one using shallow underibbed sheds (Type 1 acc. to Table 2). Table 2: Line Data Parameter Line Voltage ±533 kv Country Mocambique, South Africa Year of installation 1981/1996/1997/2002/2004 (various reinsulations / exchange of glass discs with silicone rubber composite longrods) Service duration up to 27 years with silicone rubber composite [2] Operated by ESKOM, RSA Converter Stations Cabora Bassa Dam (Mocambique), Appollo (Johannesburg, RSA) Pollution High in the Johannesburg area (industrial, pollution from traffic, natural) Light-Medium for the major line distance (natural pollution) ESDD [mg/cm²] Unknown, not determined systemically Insulation HTV silicone rubber composite longrod, Generation I-III [1] Material HTV silicone compound with 48% ATH by weight Type Type 1: 30/82 (178/138) mm (major type), 30 mm FRP rod diameter, 82 sheds, alternating shallow underrib shed profile, shed diameter 178 & 138 mm, connection length h1=4160 mm, creepage distance mm, 30.2 mm/kv Shed Profile Type 2: 30/127 (168/134) mm, 30 mm FRP rod diameter, 127 sheds, alternating smooth shed profile, shed diameter 168 & 134 mm, connection length h1=5100 mm, creepage distance mm, 33.4 mm/kv Alternating, shallow underrib sheds (open profile) and Alternating, smooth sheds (open aerodynamic profile) Figure 4: Leakage current measurement during DC 5000 h test, 14 kv DC test voltage. The highest peak value of 9 ma was recorded during 3340 and 3350 h in the salt fog phase. Summary of results: The maximum leakage current during the test was very low (9 ma) Pg. 2 Paper C-12

3 No flashover occurred, no tracking and erosion, no sheds were punctured, no core rod became visible There was only a slight reduction in hydrophobicity class (HC), HC=1 (before test), HC=2-4 (after the test) The hydrophobicity has not been lost during the test and the slight reduction after the test has fully been recovered to HC=1 some days after the end of the test. Within the tested parameter spectrum the erosion depth and length reach a maximum at DC+5,5 kv. Both DC polarities lead to significant higher erosions than obtained at AC voltage. For DC+ voltage stress the erosion severity increases steadily with the test voltage. For DC- voltage stress a clear tendency can not be recognized, but the severity is at high level independent of the test voltage level. For AC voltage stress the already known erosion maximum in the range of 3,5 4,5 kv [5] has been confirmed. 2.2 Inclined Plane Test at DC Voltage In order to obtain an orientation regarding severity of DC voltage stress compared to AC stress (reference), the inclined plane test in accordance with IEC was applied. Presently, this test procedure is standardised only for AC voltage stress. Three test series were performed where both polarities of the DC voltage (DC+/DC-) and AC (r.m.s) voltage were applied to the HV electrode of the test setup [3]. The test voltage was varied at constant conductivity of the electrolytic test liquid. Tab. 3 summarizes the main test parameters. In deviation to the line insulator material, these test samples are made with 58 % (by weight) of ATH content as it is standard today. The 10 % point higher level typically gives a slightly improved T+E performance in the inclined plane test. Table 3: Test setup and test samples [3] Test procedure: IEC modified, DC+, DC-, AC (r.m.s.) Test samples: HTV silicone rubber, 58% ATH filled (by weight) Voltage [kv]: Flow rate [ml/min]: 0,15 0,30 0,60 0,90 R [k ]: Conductivity [ms/cm] 2,5 Test duration [h] 6 The individual tests were evaluated regarding erosion depth and erosion length. In order to obtain a comprehensive survey, the test results were normalised to the erosion maximum found at DC+ (5,5 kv) and expressed in per unit (p.u.). Fig. 5 shows the average erosion depth and Fig. 6 the respective average erosion length obtained from each test Erosion Depth [p.u.] 1,0 0,8 0,6 0,4 0,2 0,0 Test Voltage [kv] DC+ AC Figure 5: Erosion depth at DC+, DC- and AC (r.m.s.) depending on the test voltage. Erosion Length [p.u.] 1,0 0,8 0,6 0,4 0,2 0,0 Test Voltage [kv] DC+ DC- DC- AC Figure 6: Erosion length at DC+, DC- and AC (r.m.s.) depending on the test voltage. It can be concluded that the erosion severity in the inclined plane test at DC voltage stress is generally much higher than for AC (r.m.s.) stress. Even with 10 % points more ATH filled material much higher erosion depth values occur compared to AC tests and compared to DC service experience. In consequence, while the inclined plane test according to IEC is a helpful and commonly accepted tool in the evaluation and selection process for AC outdoor housing materials, it seems not to reflect the same correlation for DC stress. The experience from other ageing tests (5000 h DC test) and those from service performance show no correlation to the results of the DC modified inclined plane test. The applicability and representativity of the test at DC has therefore to be questioned critically and especially can not be used for any standardization today. 2.3 DC Performance and Hydrophobicity Fig. 7 was derived from former investigations regarding hydrophobicity, hydrophobicity transfer mechanism (HTM) and tracking and erosion resistance for HTV silicone rubber material compounds that are filled with ATH (alumina tri-hydrate, Al(OH) 3 ) [5]. Both, HTM and tracking and erosion resistance are indispensable features that influence the long-term performance of housing materials. The outdoor performance, respectively general dependence (black curve with a maximum between 55 and 60% b.w. ATH content) is the same for AC and DC stress, but the individual features (red and blue) differ of course depending on the relevant stress type (AC or DC). Pg. 3 Paper C-12

4 Performance 1 p.u. 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 HTM Tracking & Erosion Resistance Outdoor performance % ATH content (by weight) Figure 7: Outdoor performance of HTV silicone rubber compounds [5]. We assume that the hydrophobicity and the HTM behave completely differently under DC than under AC stress. One indication is the leakage current development in the 5000 h DC test and the observed hydrophobicity after the test as described in Chapter 2.1. A loss of hydrophobicity under DC stress here seems to be more unlikely than under AC stress, referring to available results from 5000 h AC tests. For a better comprehension the detailed coherences have to be studied in future scientific work. The data from service experience as well as laboratory tests clearly show that suitable HTV silicone rubber housing materials filled with suitable ATH can be used for AC as well as DC applications. The tracking and erosion resistance can be adapted by adding ATH filler while good hydrophobicity properties are in a well balanced setup. Furtheron, the ATH mineral filler is antistatic and avoids the formation and storage of critical space charges inside the material or on the housing surface. No additional antistatica are therefore necessary. 3. DISCUSSION 3.1 Simplified DC partial arc model Housing erosion is regarded as the most severe and critical ageing mechanism that could occur on composite insulators under DC and AC stress. Erosion processes and mechanisms are initiated by surface pollution and driven by effects such as pre-arcing and there mainly by stable low current partial arcs [5] also known as dry band arcing (DBA). To understand the differences in dry band arcing at AC and DC stress the simplified model according to Obenaus shown in Fig. 8 can be applied. Pollution driven DBA takes place with discharges to earth ground. R P is representing the ohmic resistance which is given majorily by the surface pollution of the insulator part that is free of discharges and supplying ohmic leakage current to the plasma discharges in the DBA zone(s). C D is the operating capacity of the discharge free parts of the insulator, e.g. between end fittings and DBA discharge zone(s). At AC stress a significant amount of capacitive leakage current can be supplied to the discharges in the DBA zone(s). Since =o and the impedance over Z D = for DC voltage, all the feeding current for potential DBA discharges has to be supplied via R P. R P on the other hand strongly depends on two parameters: the conductivity of surface pollution (pollution severity expressed in ESDD or surface conductivity) and on the hydrophobicity. Thus, critical feeding currents for DC DBA in present installations (Pacific Intertie, Cabora Bassa) seem to be avoided by both, non-critical surface pollution and/or intact surface hydrophobicity. Therefore no critical damages can be observed. Figure 8: Simplified partial arc model. The modified inclined plane test results at DC show severe damages at both polarities, but no correlation to real HVDC line installations can be derived as they do not show erosion. This can be explained also with the Obenaus model in Fig. 8: the inclined plane test suppresses intentionally all hydrophobicity effects and is performed at relative high surface conductivity, also the local stability of pre-arcs and partial discharges that cause the damages are very high since there is no current zero at DC stress. R P is low and sufficient leakage current is supplied for critical stable discharges with sufficient energy that will lead to severe erosions. Therefore one interesting question has to be placed: what is the critical value of R P, Crit for real HVDC line insulators? Here, it should be kept in mind that R P is mainly dependent on surface pollution severity and hydrophobicity effects. 3.2 Critical pollution stress situations The HVDC results from service experience and laboratory tests are obviously contradictionary. The following questions / concerns are therefore raised for further research and development: 1. is the modified inclined plane test at DC voltage stress with the present test setup and parameters representative for the material selection process for DC materials? 2. if yes, then a critical pollution condition is mainly dominated by R p, Crit. If R P < R p, Crit then severe damages have to be expected for HVDC line insulators at high/very high pollution severities. 3. since existing installations are presently operating at light/medium pollution severity and show no traces of ageing but excellent insulation characteristics, it has to be assumed that in these cases they operate at R p > R p, Crit? 4. is it possible to develop a future artificial pollution / ageing test that is evaluating these effects and finally allow proper material and insulator design selection? Pg. 4 Paper C-12

5 3.3 DC Ageing and Pollution Test Presently, CIGRE WG C is developing a pollution / ageing test [6]. The experiences, findings and ideas of this paper should be considered for the definition of test parameters and conditions. 4. CONCLUSION Long-term service experience with existing silicone rubber composite suspension/tension insulators in HVDC overhead transmission lines is excellent. The direct evidence is the superior service performance of the overhead transmission lines Cabora Bassa Apollo and Pacific Intertie. Both lines have successfully been in service for more than 25 years insulated with silicone rubber composite insulators [2]. A review on the insulator designs showed that 25 years ago the selection of creepage distance and shed profile has been done adequately and even in accordance with today's knowledge and new draft IEC standards for HVDC insulators. This is as well confirmed by application of statistical design methods by means of IST software [6]. Both lines are operating under light/medium (partly/locally high) pollution conditions. The hydrophobicity of the silicone rubber housing and the hydrophobicity transfer effect was maintained over the whole service period at both lines up to now. A 5000 h multi-ageing (design) test was performed at DC test voltage. The leakage current during the test was much lower than in comparable AC tests in accordance with IEC [4]. The hydrophobicity of the silicone rubber housing was maintained during the whole test period and resulted in HC=2-4 obtained after the test in the final evaluation. The low leakage current during the test was a result of the maintained surface hydrophobicity. No damages caused by tracking and erosion were detected. The characteristics of these test results indicate that the hydrophobicity loss mechanism is apparently completely different at DC than at AC voltage stress. The test results of the 5000 h test are fully reflecting the long-term service experience of the HVDC lines presented above and in [2]. Material tests were performed using the inclined plane test in accordance with IEC which was modified for DC test voltage. A variation of voltage stress was done for both polarities of the DC voltage from 2,5 to 5,5 kv. For reference purpose, also a series of AC test voltage was tested with the same voltage interval and steps. The material tested was a typical high quality HTV silicone rubber compound filled with 58 % (by weight) with ATH filler for extrusion processing. Such materials are applied for more than 25 years and are also used in today s Generation III silicone rubber composite longrod insulators for HVDC and HVAC applications [1]. The obtained inclined plane test results which show severe erosion at DC are in complete contradiction to the excellent service performance and the results of the 5000 h DC test. The test erosion severities at both DC polarities are signficantly higher those at AC. The dependence on voltage level is also different for DC and AC. The surface hydrophobicity is intentionally fully suppressed during the inclined plane test. Since the hydrophobicity seems to be the protecting feature in real service and also during the 5000 h test, the results of the inclined plane DC test likely will mislead if applied for the material selection process for DC materials. Nevertheless, the inclined plane test has been accepted and shown representativity for the material selection process for AC materials. With the results obtained in this study it has to be regarded as not representative for DC applications. This must be considered if a DC material test for tracking and erosion is discussed for future standardisation process. A simple model for leakage current supply of partial arcs has been introduced and discussed. Since at DC voltage stress there is no capacitive part of the leakage current, the feeding current for partial arc is only supplied via the ohmic current (represented by R P in the equivalent circuit scheme). R P is mainly dependent on the surface pollution of the silicone rubber housing and its hydrophobic effects (intrinsic hydrophobicity and hydrophobicity transfer mechanism). As long as these hydrophobic effects are able to suppress the ohmic leakage currents, no critical leakage currents, dry band and/or partial arcs come into effectiveness and in such a state no critical tracking and erosion effects are expected. Theoretically, the existence of a critical value for R P can be derived from this simplified model. This fundamental theoretical insight is very important and has to be considered when line insulation design is made for heavy and very heavy pollution severities or if critical hydrophobicity states have to be expected from environmental or climatic impacts. This should be considered in future standardisation work within CIGRE and IEC, namely when IEC will be discussed. For the time being, worldwide there is no report about negative performance of silicone rubber longrod insulators at HVDC. On the other hand, there are very positive long-term service experiences with Generation III silicone rubber longrod insulators over more than 25 years at HVDC. Since the dominating problems of conventional insulators (hardware corrosion, cement growth, electrolytic elements and accumulation effects) are per se avoided with composite insulators, these products offer the best alternative for such line insulations and are superior to glass and porcelain disk insulators. HTV silicone rubber with ATH filler are suitable for both AC and DC applications. The results of this study are very encouraging to apply these products for future HVDC installations for 500 kv and above. Very special care has to be taken if for material testing existing AC tests are applied similarly with DC voltage. The results may be misleading and not transferable to real insulator applications. 5. REFERENCES [1] J. M. Seifert, Innovations in the Field of Composite Insulators Review on 40 Years of Pg. 5 Paper C-12

6 Experience, LAPP RODURFLEX 40 Years Convention, th February 2008, Tröstau, Germany. [2] CIGRE WG B2.03, Service Performance of Composite Insulators used on HVDC lines, ELECTRA No. 161, August [3] IEC 60587, Electrical insulating materials used under severe ambient conditions - Test methods for evaluating resistance to tracking and erosion (IEC 60587:2007). [4] IEC 62217, Polymeric insulators for indoor and outdoor use with a nominal voltage > 1000 V - General definitions, test methods and acceptance criteria (IEC 62217:2005). [5] J. M. Seifert, D. Stefanini, High Pollution Resistant Composite Insulators, ICHVE 2008, Nov th, 2008, Chongqing, China. [6] CIGRE WG C , A laboratory test method for polymeric insulators, IWD draft [7] W.L. Vosloo, R. Stephen, P. Naidoo, I. Gutman, D. Muftic, N. Ijumba, External Insulation Functional Specification for the Upgrade of a 400 kv AC Transmission Line to 500 kv, DC, SAUPEC 2007, South Africa. Pg. 6 Paper C-12