A3-110 LONG TERM PERFORMANCE OF POLYMER HOUSED MO-SURGE ARRESTERS

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1 21, rue d'artois, F Paris A3-110 Session 2004 CIGRÉ LONG TERM PERFORMANCE OF POLYMER HOUSED MO-SURGE ARRESTERS B. RICHTER*, W. SCHMIDT K. KANNUS, K. LAHTI ABB SWITZERLAND LTD. TAMPERE UNIVERSITY OF TECHNOLOGY (Switzerland) (Finland) V. HINRICHSEN C. NEUMANN W. PETRUSCH K. STEINFELD DARMSTADT UNIVERSITY RWE NET AG FGH MANNHEIM SIEMENS AG OF TECHNOLOGY (Germany) Years after the market introduction of metal oxide (MO) surge arresters with polymeric housings in medium voltage (MV), high voltage (HV) and extra high voltage (EHV) systems an IEC-standard [1] was published recently, dealing with test procedures for this type of surge arresters. Besides mechanical tests, climatic tests, and the short circuit test, which is still under consideration, the longterm tests have led to controversial discussions. One reason is that it is virtually almost impossible to define a test procedure, which considers the features of hydrophobic (e.g. silicone) and nonhydrophobic (e.g. EPDM) materials at the same time. Further on, HV MO-surge arresters can generally not be tested completely in long-term cyclic tests, while MO-surge arresters for MV systems can be tested completely in the today existing test labs. This is important, because not only the insulating material of the housing has to be tested. New test procedures and long-term tests have been developed in close cooperation between manufacturers and users under guidance of research institutes. The aim of the long-term tests is to prove the validity of existing test methods, to develop new tests, and to compare artificial lab tests with real existing stresses in the system. The test results show that the design of the MO-surge arresters (internal structure), the material of the housing, and the quality of the production process have significant influence on the performance of the arresters. The overall experience with polymer housed MO-surge arresters in MV, HV, and EHV systems is very good. Long-term tests (testing time longer than two years) under very high humidity proved, that directly moulded (e.g. by silicone) designs show the best performance. Keywords: MO-surge arresters polymeric housing silicone long-term tests humidity 1. INTRODUCTION Surge arresters are indispensable devices within electrical power supply systems to ensure appropriate insulation coordination and to protect valuable equipment like power transformers, GIS, cables etc. from lightning and switching overvoltages. They have undergone considerable technical changes during the last two decades. A today s modern surge arrester has only few things in common with a surge arrester for the same application but 25 years ago. The first change was the transition from gapped silicone-carbide (SiC) to gapless MO-surge arresters. This development started at the end of the 1970s, and latest by end of the 1980s MO-surge arresters had definitely been established as state of the art, since their technical and commercial benefits are quite evident. There is in general no indication for any severe degradation of the MO material even after more than 20 years of service. The failure rates of MO-surge arresters are in the range of 0.1%/a * bernhard.richter@ch.abb.com

2 to 1%/a (with geographic variations) in MV, and virtually zero in HV systems. The very simple and robust structure of an MO-surge arrester the active part basically consists of a stack of cylindrical MO resistor elements also pushed another development only a few years later: the increasing application of polymer instead of porcelain housings. Polymer housed arresters were introduced for the first time in the mid 1980s in MV systems. A main driving force might have been the partly poor performance of several porcelain housed MV arrester designs, which often suffered from sealing deficiencies, extreme sensitivity to pollution and an unsatisfying overload performance. While sealing problems are not automatically solved when using a polymer housing, it turned out that this technology offered some more advantages with respect to automation of production, lead time, transportation, handling, and so on. Today the market share of polymer housed MV arresters is close to 100 %. The situation is somewhat different for HV arresters. First polymer housed HV arresters appeared on the market around 1990, being niche products in this time. The actual share of polymer housed HV arresters is estimated to be 25% to 30%, with increasing tendency. It has to be noted that MO-surge arresters with polymeric housings are used more and more in railway facilities (a.c. and d.c.), generator protection, cable and cable sheath protection and other special applications. 2. DESIGN OF MO-SURGE ARRESTERS After about 15 years of development some few basic design principles of polymer housed MO-surge arresters can be distinguished with a variety of individual sub-solutions. In principle MO-surge arresters consist of two main parts: the active part and the housing. The active part consists of a stack of MO resistors with a highly non-linear voltage-current characteristic, kept together by a mechanical structure. An insulating housing gives protection against the environment. The arrester designs can be grouped according to the manufacturing technique and internal structure. 2.1 Design principles of polymer housed HV arresters It is suggested to make a differentiation between designs using a hollow core insulator with an intentionally enclosed gas volume and such designs, where the housing is put onto the MO column without any intentional gas volume [2]. The mechanical designs can be characterized as follows: Type A: This design the tube design is a more conventional approach, looking quite similar to that of porcelain housed arresters. The stack of MO resistors is mechanically supported by an internal cage structure, for example made from FRP (fibre reinforced polymers) rods or loops. This insert is clamped between the end flanges. What is important (and sometimes criticised) is the fact that this arrester due to its enclosed gas volume needs sealing and pressure relief systems. This has in fact to be designed and manufactured with the same high care as it is the case with porcelain housed arresters. But just for this reason this does not constitute a real problem for an experienced manufacturer of HV arresters. Some of the potential benefits of a Type A design are high mechanical strength, safe short circuit performance, and the possibility of producing tall units, which can serve as single-unit arresters up to 300 kv systems. Type B: Here we have to differentiate between the so called wrapped design (B1) and the cage design (B2) [2]. Both designs were first introduced for MV arresters, and then adopted to HV arresters [3]. Fig. 1 shows photographs of real arrester modules, produced by two different manufacturers. Main technical advantages of the cage designs are that they offer comparatively high mechanical strength combined with an inherent good short circuit performance. As for the mechanical design, there are numerous possibilities to realize the outer housing and sheds. With regard to material, however, with only few exceptions, there has been a clear tendency towards SR (silicone rubber). All the other materials are usually not excepted in HV systems. The reason is quite obvious: only SR offers hydrophobicity (i.e. the ability to repel water from its surface) which lasts for decades, and from its chemical structure it is inherently less sensitive to solar radiation.

3 FRP loop FRP tension rod MOV column End fitting Fig. 1: 1: Internal Internal mechanical mechanical structure structure of Type B2 of arresters, type B2 cage arresters, design. cage Left: Loop design design, Left: right: loop rod design. Fig. 2: Principle designs of MV arresters. Left: Group I, right: Group II (III). Fig. 3: Internal structure of a MV arrester, Group I. 2.2 Design principles of polymer housed MV arresters MV arresters designs can be grouped according to the manufacturing technique and the internal structure, similar to the differentiation of the design principles of HV arresters. In the MV field only very few designs have a Type A design, and if so, for special applications only. In [4] the designs of MV arresters are differentiated as follows: Group I: The polymeric material (e.g. silicone) is directly moulded onto the internal parts, the MO resistors and the mechanical structure. A primer is used to ensure chemical bonding of the different materials with the silicone. End caps are not needed. Group II: The insulating housing is pressed or slipped over the separately manufactured active part. The materials are only attached mechanically one to the other. A sealing system with end caps is needed. Group III: As Group II, but with considerable internal gas space. This gas space may be intended, due to the design, or not intended, due to an uncontrolled manufacturing process. The differences in the design differentiation of HV and MV arresters are due to the fact that in HV the mechanical strength is more important, while in the MV field the production process is of higher importance. The design principle B2 for HV arresters is almost identical to Group I for MV arresters. 2.3 Pollution performance and radial electric field strength The internal axial voltage distribution of an MO-surge arrester is nearly even, due to its high self capacitance, and, in the case of HV arresters, additionally controlled by grading rings. The external distribution shows statistical behaviour in case of surface conductivity due to pollution. The electric potential of an intermediate fitting or flange may be transferred along the insulator, and thus a high electric field stress may occur between the outer surface of the housing and the MO column, see Fig. 4. In case of a Type A arrester, which has a gas-filled gap between the housing wall and the MO column, the highest fraction of the radial voltage drops across the gap due to the geometry and the ratios of different dielectric constants of the materials. The electric field stress at the MO column can therefore reach values above the initiation level of partial discharges. There are two means to make a Type A arrester resistant against this effect: one is to apply a sufficiently large gap. Investigations under salt fog stress have shown that a gap distance of 10 mm to 20 mm for polymer housings with SR

4 MO column Gas Solid U axial, int Conductive layer Type X, boiling Type Y, boiling Type X, + 70 C Type Y, + 70 C U radial Fig. 4: Possible voltage distributions of an arrester unit under polluted condition. Fig. 5: Water immersion test with two different water temperatures. Test duration up to 800 h. sheds is sufficient to effectively avoid internal partial discharges. Additionally, exclusively materials and MO resistors should be used, which are insensitive to possible partial discharges. Radial field stress is also a concern for Type B arresters. Here the radial voltage drops across the small distance of only few millimetres between the outer surface and the MO column, and a week design may be punctured by this stress. For this reason the actual arrester standard [1] requires a weather aging test (which includes salt fog stress under applied voltage) at the longest unit for all polymer housed arrester types. 3. TESTS Recently an IEC-standard (IEC , Ed. 1.2, ) [1] was published, including test procedures for polymeric housed MO-surge arresters. Besides mechanical, climatic, and the short circuit test (which is still under consideration), the long-term tests have led to controversial discussions. One reason is, that it is virtually almost impossible to define a test procedure, which considers the features of hydrophobic (e.g. silicone) and non-hydrophobic (e.g. EPDM) materials at the same time. Additional it is not possible to cover all climatic requirements world wide in one test. The idea to apply modified material or insulator tests (e.g. acc. IEC 61109) as test for electrical equipment leads to some problems. Equipment with active internal parts exhibit electrical field stresses very different from those of e.g. insulators and therefore require other test procedures and criteria than pure material tests. HV MO-surge arresters can generally not be tested completely in long-term cyclic tests. This is possible on sections or units only. For MV MO-surge arresters the situation is different, it is possible to test complete MV arresters in the today existing test labs. This is important and necessary, because not only the insulating material of the housing is of importance, but the complete design and the processing. This is especially true when long-term tests are performed. 3.1 Long-term performance of the outer insulation Several procedures have been developed in the past to check the performance of polymeric insulators under severe ambient conditions. Most of the tests are long-term tests, lasting for 1000 h or 5000 h. The tests are performed with medium voltage, this means, as already mentioned, that from HV arresters only pro rated sections can be tested. An overview about the most common procedures and information about the correlation between test procedure and stresses in the system is given in [5] and [6]. It is obvious, that for the time being only insufficient knowledge is available for the definition of cyclic tests. One important point is, that the test procedure should consider the polymeric material used. Decisive for the long-term performance of polymeric material is the dynamic behaviour of the

5 hydrophobicity. Depending of the material a loss of the hydrophobicity can be permanent (no regeneration, e.g. EPDM) or temporary (regeneration after a stress period, e.g. silicone). Based on the existing standardized test procedures (e.g. VDE 278 for cable terminations (1970s), VDE 441 for line insulators (1982), IEC for line insulators (1992), and IEC for hollow insulators (1998)) it was tried to establish a long-term test for HV arresters with polymeric housings, in which the longest unit had to be tested. This may give problems, because the existing test labs can perform salt fog tests with test durations of 1000 h and more only with test voltages in the range of 100 kv to 150 kv. Cyclic tests with 5000 h test duration can be performed practically only with MV arresters. 3.2 Standardized tests for arresters For MO-surge arresters with polymeric housings a weather ageing test is given in [1]. Two test series are specified: test series A is a 1000 h salt fog test, while test series B is a 5000 h cyclic test. It is mandatory to run at least one of them on the longest unit of a design. It has to be noted, that the 5000 h cyclic tests are given just as examples. Any other cycle fulfilling the requirements given in the standard may be chosen. As long as it is not sufficient to test only the outer insulation, but the complete design of an arrester, an additional test is given in [1]. This moisture ingress test is a sealing test. In a preconditioning sequence the arrester is stressed mechanically in different directions, simultaneous with thermal cycles. Afterwards the arrester is kept in boiling water for 42 h. After cooling down in the water to ambient temperature some electrical a.c. measurements are repeated, and the results are compared with previous measurements. There should be no relevant changes in the electrical characteristics, and no sign of water ingress into the design. 3.3 Non-standardized tests To get more information and better understanding about the different designs and materials used special tests have been developed in close cooperation between manufacturers and users, under guidance of research institutes. Tests, as for instance given in the which -reports by ea-technologies, UK, are comparing tests, checking the performance of different designs under very severe stresses. In such test procedures emphasis is given to the hydrophobicity, the long term behaviour under salt fog, and the performance under extreme temperature cycles in air, water (including icing) or in liquid (water with additives). Tests, performed at the Tampere University of Technology, Finland, had the purpose to evaluate different designs of MV arresters under severe long-term stresses, and to compare new test methods with the standardized procedures Water immersion tests Two different MO-surge arresters with silicone housing, both of design Group I (here named X and Y) have been immersed in de-ionised water with 1 kg/m 3 of NaCl. The water ingress was detected by accurate d.c. leakage current measurement. Two water temperatures (70 C and 100 C, boiling) have been chosen to study the influence of the water temperature. The resistance to moisture penetration was quite different in the performed test, see Fig. 5. Arrester Y withstood more than 100 h of boiling, before a significant change in the leakage current could be observed. Arrester X had a relatively low resistance to water penetration. It took only some hours to reach leakage currents of some µa. The temperature of the water has a significant influence on the leakage current growth rate. It has to be mentioned, that both the types X and Y withstood the moisture ingress test acc. to [1] without any problem Recovery tests The same procedure and the same measuring technique as for the water immersion test have been used. The recovery was measured after the water immersion for the same two MV arresters X and Y.

6 Type X Type Y Fig. 6: Change of leakage current due to diffusion of humidity into the design and recovery in dry periods. Fig. 7: Results of long duration moisture ingress tests. During the recovery period the test samples were stored under normal room conditions. Fig. 6 shows that the recovery from moisture ingress is very fast for the tested arresters. Both MO-surge arresters, belonging to design Group I, recovered almost completely. Not only the moisture ingress has to be considered, but the recovery behaviour of a design as well. As long as humidity cannot condense in the arrester and form water or water films along interfaces between different materials, the design is not endangered by humidity Long-duration moisture ingress tests This test gives emphasis to the diffusion of humidity into the arrester and the sealing system under very long test times. In test sequence a total of seven different designs of MV MO-surge arresters have been tested. The samples were placed in a chamber with high humid conditions (temperature 30 C to 35 C, relative humidity 95% to 100%). Additional rain was applied for 2,5 h each day. Alternating voltage of 12 kv was applied. After each 4 weeks electrical and mechanical tests were performed for 5 days. This cycle was repeated for a total testing time of 780 days. Fig. 7 shows, that all designs from Group II and III failed after a period of time. Especially endangered are designs of Group III, which have remarkable gas volumes in the inside of the housing. Only the designs of Group I, where the silicone is directly moulded onto the internal parts, and chemically bonded with a primer to the other materials, withstood this severe long duration test. 4. FIELD TESTS A total of 67 MV MO-surge arresters with polymeric housings of 8 different makes (age 4 to 15 years) were taken from various 24 kv networks in Finland. The following measurements were performed: 1) Visual inspection 2) Insulation resistance measurement with a d.c. voltage of 1000 V 3) Measurement of the d.c. voltage at 3 ma d.c. 4) Measurement of the peak value of the total a.c. current at U c 5) Measurement of the a.c. voltage and the power losses at a current of 3 ma (peak) 6) PD measurement at 1.05 U c 7) Residual voltage at current impulses (2.5/70 µs), 3 positive and 3 negative impulses with I = 1.6 ka to 2.5 ka, depending on type of arrester 8) Measurement of the d.c. voltage at 3 ma d.c. (repetition of 3)) All the studied MO-surge arresters with polymeric housings were in good conditions after being in service for 4 to 15 years. Comparing measurements on some unused MO-surge arresters showed that the protection levels remained stable, and the a.c. characteristics did not change remarkable.

7 5. OUTDOOR TEST STATIONS MO-surge arresters with polymeric housings are installed in trial lines and outdoor test stations. For example, MV arresters of type Y (design Group I, see 3.3.1) are installed in the ESKOM test station in South Africa, directly at the shore of the sea, Fig. 8, withstanding the very severe stresses of salt from the sea, sand from the desert, and the strong UV radiation for years without any failure. The National Grid Company (NGC) from UK requires the proof from the manufacturer that the arrester withstands the special ambient conditions close to the channel coast. For this the arrester has to withstand at least two winter periods in the outdoor test station at Dungeness. The arresters are stressed in this place almost constantly with a strong wind from the sea, and consequently have all the time a pollution layer on the surface of the housing. Fig. 9 shows a HV arrester (420 kv system, U r = 360 kv) in this station. After the two winter periods the arrester was brought back to the manufacturer for final investigation. There was no relevant change in the hydrophobicity compared to the arrester in new condition. Fig. 8: MO-surge arrester for MV system in an outdoor test station of ESKOM, South Africa. Left: View of the test station, located directly at the shore of the Atlantic Ocean. Right: MO-surge arrester with insulating bracket and disconnector under test. Fig. 9: 420 kv MO-surge arrester (U r = 360 kv) with polymeric housing (outer insulation: RTVsilicone) under test in Dungeness (NGC/UK). MO-surge arresters with silicone housings for MV systems are installed in trial lines, close to cement factories and in regions with extensive agriculture, in very different regions of the world, and show everywhere a very good performance. It has to be noted that even in regions with severe pollution standard creapage distances and single shed profiles are sufficient. The MO-surge arrester as shown in Fig. 8 has a specific creapage of 28 mm per kv test voltage. 6. CONCLUSION MO-surge arresters with polymeric housings can be grouped according to their internal structure and the production process. Silicone as housing material shows generally better performance than other polymeric materials. In the MV field MO-surge arresters with polymeric housing have a market share of almost 100%. They have now successfully been in service for nearly 20 years. Polymer housed HV arresters actually have a market share of about 30%, with increasing tendency. First installations have been made around 1990, and so far there is no indication that they will not show the same good performance as MO arresters in general. Different design principles have emerged in the meantime, which basically

8 can be divided in tube designs and wrapped or cage designs. These different designs usually serve different market segments. The technology can be considered mature. Thus it can be predicted that the market share of polymer housed HV arresters will increase, because of the economical and technical advantages against porcelain housed arresters. In 2001 an IEC test standard [1] for gapless MO-surge arresters was published, including long-term tests for polymer housed arresters. However, the discussion is still ongoing. Due to restrictions in test labs it is not possible to test complete HV arresters in long-term test, especially when cyclic tests are considered. The actually used material should be considered when long-term tests for the outer insulation have to be performed. Polymers, which do not recover after severe pollution stresses (complete loss of hydrophobicity), should preferably be tested by a 1000 h salt fog test. Polymers, which recover after severe pollution stresses (e.g. silicones), should better be tested by cyclic tests. Generally, the correlation of test results from lab tests and the performance of the different arresters in the field is still questioned. Long-term tests (testing time longer than two years) with MV arresters under high humidity proved, that directly moulded (e.g. by silicone) designs show the best performance. Additionally it is very important not only to consider the ingress of humidity into a design, but also the recovery under normal ambient condition after a period of high humidity. It has to be ensured that there is no accumulation of humidity in the design. Large field studies and the experience from outdoor test stations in South Africa (ESKOM) and UK (NGC) prove, that the overall performance of polymer housed MO-surge arrester in MV, HV and EHV systems is very good. The different designs, developed for different applications, allow special solutions for overvoltage protection and insulation coordination in general. 7. REFERENCES [1] IEC , Ed.1.2, ; Surge arresters Part 4: Metal-oxide surge arresters without gaps for a.c. systems [2] V. Hinrichsen, Latest Designs and Service Experience with Station-Class Polymer Housed Surge Arresters, [World Conference on Insulators, Arresters and Bushings, Marbella/Spain, November 16-19, 2003; proceedings] [3] W. Schmidt, New POLIM medium-voltage surge arresters with silicon insulation, [ABB Review 2-96] [4] K. Lahti, B. Richter, K. Kannus, K. Nousiainen, Internal Degradation of Polymer Housed Metal Oxide Surge Arresters in Very Humid Conditions [Proceedings ISH London, UK, 1999] [5] Oberflächenverhalten von Freiluftgeräten mit Kunststoffgehäusen. Technischer Bericht Nr. 291 der FGH, Januar 1999 (in German) [6] Natural and artificial ageing and pollution testing of polymeric insulators. CIGRE Brochure 142, June 1999