Manufacturing and Application of Cage Design High Voltage Metal Oxide Surge Arresters

Transcription

1 Manufacturing and Application of Cage Design High Voltage Metal Oxide Surge Arresters K. Steinfeld, B. Kruska and W. Welsch Siemens Surge Arresters and Limiters, Berlin, Germany Abstract: Metal oxide surge arresters protect electric power systems from overvoltages. The conventional design of high voltage metal oxide surge arresters includes a stack of metal oxide non-linear resistors and a housing, either a porcelain hollow core insulator or a polymer hollow compound insulator (tube design). However, the technological progress in the polymer industry makes designs possible which are simpler and more cost effective. As an alternative to the above mentioned tube design, the active part of the arrester may be directly covered by silicone rubber. The cage design introduced here consists of a stack of metal oxide elements which are kept in between the aluminium end fittings under high mechanical pressure by pre-stressed fibre reinforced plastic rods. The silicone rubber sheds are then directly moulded onto the construction. Due to their cost effectiveness, cage design surge arresters are advantageous over tube design arresters for standard mechanical requirements. 1. Introduction Electric power systems are exposed to overvoltages of different origin which might endanger the equipment such as transformers, instrument transformers or circuit breakers, only to name a few. Fig. 1 summarises the different kinds of overvoltages as a function of their duration in comparison to the highest voltage for equipment. Lightning strokes into the electric power system or its vicinity lead to lightning overvoltages in the range of microseconds, switching action within the system cause switching overvoltages in the range of milliseconds and certain operating conditions due to load flow control cause temporary overvoltages which can last for several seconds. As can be seen from Figure 1, the equipment is designed to withstand the highest voltage for equipment as well as temporary overvoltages. However, the insulation of the equipment is not capable of withstanding lightning and switching overvoltages. Without countermeasures, occurrences of these overvoltages in the system can lead to breakdown of the equipment insulation and its failure. In order to protect electric power system equipment from lightning and switching overvoltages, surge arresters are used within the system as a tool for insulation co-ordination. Figure 1: Overvoltages in high voltage electric power systems compared to the highest voltage for equipment The purpose of using a surge arrester is to always limit the voltage across the terminals of the equipment to be protected below its insulation withstand voltage. This is achieved by connecting elements with an extremely non linear voltage current characteristic (varistor) in parallel to the terminals of the equipment. So called metal oxide (MO) surge arresters containing ceramic MO elements mainly made from zinc oxide (ZnO) and bismuth oxide are used nowadays [1]. Due to the high non linearity of the material there is no need for series spark gaps any more as they were used in silicon carbide (SiC) surge arresters. 2. Design of High Voltage Surge Arresters In the past twenty years there were two major changes in the technology of surge arresters. Firstly, the gapped SiC arrester technology was replaced by the gapless metal oxide (MO) arrester technology in the late seventies to early eighties. As a consequence, the protection characteristic was improved, the reliability was dramatically increased to failure rates close to zero and the design became much simpler. In the late eighties to early nineties, polymeric housings were introduced using fibre reinforced plastic (FRP) tubes with sheds made from silicone rubber or ethylene propylene diene copolymer (EPDM) [2]. Since then, porcelain housings have almost been fully replaced by polymer housings in the medium voltage distribution systems for new installations and polymer technology is increasingly used in high voltage power systems even up to 800 kv. The success of polymer housings lies in the 1

2 versatility of the possible designs and properties which allow a wide range of arresters with respect to mechanical properties, short circuit behaviour and costs. An MO high voltage surge arrester basically consists of a stack of cylindrical MO elements kept together by a supporting structure and a housing. The general purpose of the housing is to protect the MO elements from environmental impacts such as humidity and pollution as well as damages due to transport, carry external forces, e.g. by conductor wires, wind or earthquake, control the pressure relief behaviour in case of electrically overloading the arrester, provide a dielectric strength (withstand voltage) above the protection level of the arrester and to keep the stack of MO elements together by maintaining a certain pressure within the stack. There are different approaches to categorise the various designs of MO surge arresters. Basically, designs with and without internal gas volume can be distinguished. The former includes both porcelain and polymer tube design and the latter includes cage and wrapped design. In the following, the different above mentioned designs of MO surge arresters are explained and compared to each other Tube Design MO High Voltage Surge Arresters As indicated in Figure 2 and Figure 3, a tube design MO surge arrester includes a hollow core housing either of porcelain or polymer. A stack of MO elements forms the electrically active part and flanges including sealing and pressure relief system form the top and bottom of the arrester. The housing is the mechanically supporting part which carries the mechanical forces. The design of porcelain and polymer tube design surge arresters is almost identical, the only difference is the housing which in case of the polymer housing consists of an FRP tube with polymer sheds directly moulded onto it. Usually, silicone rubber is used as shed material rather than EPDM due to its excellent chemical resistance and hydrophobicity [3]. The mechanical resistance of the FRP tube material is significantly better than of porcelain resulting in higher cantilever strength and headload of the housing with a much lower weight at the same time. Furthermore, the mechanical properties may be adjusted according to the customer s needs by varying the parameters of the FRP tube such as wall thickness or fibre angle. Thus, polymer tube design arresters are used where there are highest mechanical requirements, particularly for stations in areas with seismic activity. Figure 2: Tube design MO surge arrester with porcelain housing (flange section only) Figure 3: Tube design MO surge arrester with polymer housing (flange section only) A very important part of tube design MO surge arresters with respect to safety and reliability is the sealing and pressure relief system. The sealing system must be designed to prevent ingress of moisture for the whole lifetime. This can only be achieved by an appropriate but simple construction and a very careful choice and combination of the materials used. In case of an arrester failure and internal short circuit, the pressure relief system must release the pressure inside the housing which is caused by the arc heat before the housing is violently destroyed by the pressure shock wave. Furthermore, the pressure relief forces the arc out of the housing to prevent from further pressure built-up and burning of the internal parts and housing. In this context, it must be pointed out that failure of an arrester is a very rare event. Gapped SiC surge arresters failed quite frequently and it is generally recommended to replace them by modern MO surge arresters [4], the failure rates of which are by far lower and appear to be comparable with other equipment such as transformers. In most cases failures were caused by deficiency of the sealing system due to transportation damage or stressing the arrester above its specification, e.g. by direct lightning strokes with extremely high currents or by voltage transfer. 2 Steinfeld, K., Kruska, B. & Welsch, W.

3 A porcelain housing is designed to withstand the internal pressure and arc heat and remain mechanically undamaged during flow of short circuit current and pressure relief. However, occasionally the arrester housing will collapse due to thermal mechanical stresses within the housing which are caused by the arc heat on the surface of the porcelain. As shown in Figure 4, the effect is that the arrester falls into pieces non-violently neither endangering the surrounding substation equipment nor personnel in its vicinity. Although such a result is considered successfully passed in a short circuit test according to IEC [5], it implies that porcelain arresters must not be used as supporting insulator but a post insulator requiring additional space must be installed. As opposed to porcelain, the FRP tube material does not show a secondary thermal break due to its thermal-mechanical properties. Even after short circuit and pressure relief the remaining mechanical strength of the FRP tube is at least 75% of the initial value. Figure 5 shows a tube design polymer surge arrester after a short circuit test. Only thermal decomposition of the silicone rubber sheds occurred leaving silicone dioxide (sand) on the surface, but there was no burning and the arrester housing has kept its full mechanical strength. Thus, tube design polymer housing surge arresters may be used in a double function as arrester and post insulator saving space in the substation [6] Cage Design MO High Voltage Surge Arresters Instead of using a mechanically supporting porcelain or FRP tube to accommodate the stack of MO elements, the MO elements themselves can be used as mechanically supporting part. As shown in Figure 6, this can be achieved by clamping them in between the end fittings using a cage of FRP rods, which contributes to the name of this design. The silicone rubber insulation is then moulded directly onto the MO elements without any internal gas volume left. Figure 6: Cage design surge arrester with polymer sheds directly moulded onto MO elements and FRP rods Figure 4: Secondary thermal break of a porcelain arrester after short circuit test Figure 5: Tube design polymer surge arrester before (left) and after (right) short circuit test Due to the high compressive strength of the MO elements (about 500 MPa) and the high tensile strength of the FRP rods (700 to 1000 MPa) arresters with high mechanical resistance can be manufactured with this design principle. To obtain a sufficient mechanical resistance in terms of cantilever strength or headload a pre-stress of about 100 kn typically is applied which is however far from utilising the above mentioned strength of either MO elements or FRP rods. There are some specific advantages of cage design surge arresters over arresters with other design principles: Cage design surge arresters can be manufactured more cost effective as compared to tube design surge arresters due to the simple construction and the use of a comparatively little amount of material. Manufacturing and application of cage design high voltage metal oxide surge arresters 3

4 As opposed to tube design surge arresters there is no pressure relief and thus no sealing system needed. The MO elements are safely kept in place by the cage together with the high compressive force and they are embedded into the silicone rubber. Thus, the active part is almost perfectly protected from mechanical impact resulting in a high transport safety. Furthermore, cage design surge arresters are of comparatively low weight and thus easy to transport, handle and install. Since there is a direct contact of the MO elements with the polymer material, heat produced by the MO elements is more easily dissipated through the housing into the environment as compared to tube design surge arrester. This increases thermal stability and allows to utilise more the MO elements with respect to electrical stress. If silicone rubber is used as the housing material, the only combustible material is the 30% epoxi resin fraction of the FRP rods, which results in a high safety considering the burning behaviour. When there is light, there must be shadow. Of course, cage design surge arresters have their limits particularly if compared to tube design surge arresters: Among arresters without internal gas volume, cage design surge arresters provide the highest mechanical resistance, in terms of static cantilever strength 2,8 knm is a typical value. In comparison, tube design surge arresters with a static cantilever strength of as much as 50 knm are available. Thus, cage design surge arresters can not be used if standard mechanical requirements are exceeded. As a consequence of the mechanical properties, cage design surge arresters do have a comparatively large, clearly visible deflection when stressed with the specified headloads. The mechanical performance of cage design surge arresters relies on the internal pre-stress and thus on an intact cage: Since rods may be broken in case of failure after overloading the arrester, there is a certain risk of a collapse of the arrester. Consequently, as opposed to tube design surge arresters, cage design surge arresters can not be used as post insulators Wrapped Design MO Surge Arresters Another possible surge arrester design which is shown in Figure 7 is the so-called wrapped design. Hereby, a fibre glass cloth impregnated with epoxi resin is wrapped around the stack of MO elements including the end fittings. After polymerisation of the epoxi resin, the housing material which forms sheds and insulation is moulded directly onto the wrap. Alternatively, pre-manufactured sheds may be slipped over the wrap. This comparatively simple design allows to manufacture cheap arresters particularly for medium voltage systems where mainly costs are decisive and reliability plays a less important role as compared to high voltage systems. Figure 7: Wrapped design MO surge arrester with polymer sheds directly moulded onto FRP layer Consequently, tube design porcelain arresters have almost totally disappeared from the medium voltage distribution systems for new installations and were replaced by polymer surge arrester either of cage or wrapped design. The FRP cloth forms a mechanically stable enclosure around the MO elements. In case of an arrester failure, the pressure built-up by the arc burning inside this enclosure is limited by its mechanical resistance. If the wrap thickness is chosen too large, a dangerous pressure built-up may occur, violently shattering the housing, whereas a thin wrap results in low mechanical resistance towards external forces. Thus, the thickness has to be balanced between safe pressure relief behaviour and mechanical resistance. As indicated in Table 1, this compromise usually results in a lower mechanical strength as compared to the tube or cage design. Other measures against violent shattering such as providing pressure relief slots along the FRP wrap weaken the mechanical resistance, too. Occasionally, medium voltage surge arresters are arranged in series and parallel connection to form a high voltage surge arrester. By connecting in series, the required rated voltage is achieved and connecting in parallel at the same time results in the necessary energy absorption capability and mechanical stability. Since low cost medium voltage surge arresters can be used for this arrangement, cost effective high voltage surge arresters can be produced this way. However, this design bears certain limitations and risks: The mechanical resistance is very low resulting in only little cantilever strength and headload. Without careful adjustment of the residual voltages of the arresters in parallel, the current distribution will be non-uniform. This may lead to electrical and thermal overloading of the arrester with the lowest residual voltage even if the total current is within the specifications. The series connection of medium voltage surge arresters includes conductive end fittings or flanges in 4 Steinfeld, K., Kruska, B. & Welsch, W.

5 between the insulating housing material. In case of pollution layers on single arrester units which might occur e.g. on the lower units in the morning hours due to dew close to the ground, surface currents occur flowing over the housing via the metallic parts. This results in a significant field distortion which might cause thermal runaway and failure. This phenomenon generally is a concern for multi unit surge arrester and is the reason why the unit length should be as long as possible. Figure 8 shows an example for a wrapped design medium voltage surge arrester after failure and short circuit. As can be seen, a collapse of the housing must be taken into consideration when applying this type of surge arresters to high voltage power systems. Figure 8: Medium voltage wrapped design MO surge arrester after short circuit 3. Manufacturing of Cage Design MO Surge Arresters As explained in chapter 2.2, the mechanical resistance of cage design MO surge arresters results from prestressing both the FRP rods and the MO elements with considerable force. Two techniques to achieve a sufficient pressure on the MO elements are possible depending on the mechanical requirements: For lower mechanical resistance of the arrester a lower pre-stress is required. In this case a spring can be used on top or bottom of the stack of MO elements. For higher mechanical resistance of the arrester a higher pre-stress is required. Here, the FRP rods are stretched for some millimetres e.g. using a hydraulic machine. The gap occurring within the stack of MO elements is then filled e.g. by spacers or a threaded bolt. Thus, the cage of FRP rods itself acts as a spring keeping the mechanical force. The self-supporting, pre-stressed arrangement of MO elements, end fittings and cage of FRP rods, the socalled active part, is then carefully cleaned by using a solvent. To ensure a sufficient bonding of the different materials of the active part (glass surface of the MO elements, aluminium and FRP) to the silicon rubber, it is treated with a primer. The active part is then prewarmed in an oven in order to achieve the necessary thermal conditions for the silicone rubber moulding and polymerisation process as well as a short process time. After inserting the primered, pre-warmed active part into the mould, the housing is produced by silicone rubber injection moulding. The final step of the manufacturing process is to clean the arrester from flash. 4. Application of Cage Design MO Surge Arresters There are basically two typical applications for cage design surge arresters, the use as a station surge arrester in regular upright installation and the suspended installation. The mechanical limits of cage design surge arresters mentioned in chapter 2.2 do restrict their use as station surge arresters to a system voltage (highest voltage for equipment, U m ) of about 300 kv. Above this voltage, due to the required length of the arrester there is only very little permissible headload left which will not be sufficient in many cases. In addition, there will be a significant deflection at specified headload which might lead to insufficient clearance between phases. Above 300 kv, only the tube design seems to safely provide the necessary mechanical resistance. These restrictions do not apply to suspended installation of a cage design surge arrester either in substations or in the power line where the arrester can be used as a so-called transmission line surge arrester (TLSA) [7]. In this case there is little or no cantilever force acting on the arrester and the cage design can be used up to the highest system voltage levels. The application of a cage design surge arrester as TLSA can be seen in Figure 9. Figure 9: Cage design surge arrester used as transmission line surge arrester (TLSA) in a high voltage overhead power line Manufacturing and application of cage design high voltage metal oxide surge arresters 5

6 5. Conclusion There are different design alternatives for high voltage MO surge arresters with polymer housing which include tube design (either porcelain or polymer housing) and cage design. The wrapped design is restricted to medium voltage applications due to its limited mechanical resistance. Arrangements of cage or wrapped design medium voltage surge arresters in parallel and series connection to form a high voltage surge arrester are the exception rather than the rule, since this solution provides only limited mechanical, electrical and pollution performance. Both tube design and cage design MO surge arresters provide safe pressure relief behaviour, but there is a significant difference in mechanical resistance in terms of cantilever strength, headload and deflection. Typical values for the different designs regarding an arrester for 170 kv system voltage as an example are summarised in Table 1. Table 1: Typical mechanical properties of different high voltage polymer surge arrester designs Design Property System voltage in kv Static cantilever strength in knm Length in m Headload in kn Deflection in mm Tube Cage Wrap d design design design ,7 2,8 0,8 1,76 1,77 1,56 8,4 1,6 0, n.a. Cage design high voltage polymer surge arresters are appropriate for system voltages below 300 kv and standard mechanical requirements if regularly installed in upright position. However, if installed suspended, e.g. as TLSA, cage design surge arresters may be used up to the highest system voltages. Cage design surge arresters should not be applied when a mechanical resistance is essential even after short circuit, e.g. if the surge arresters serves a dual function as arrester and post insulator to carry a busbar. In this case, tube design surge arresters are the only alternative. Above 300 kv system voltage or in case of highest mechanical stresses, cage design surge arresters may not provide the necessary mechanical resistance any more. Thus, for system voltages above 300 kv, tube design surge arresters appear to be more appropriate. 6. References [1] Einzinger R., Metal Oxide Varistors, Annual Review Materials Science, , pp [2] V. Hinrichsen, Fien H., Solbach H.B., Priebe J., Metal Oxide Surge Arresters with Composite Hollow Insulators for High-Voltage Systems, CIGRÉ 1994 Session, 28 August - 3 September, Paris, France, paper [3] Gubanski M., Wettability of Naturally Aged Silicone and EPDM Composite Insulators, IEEE Transactions on Power Delivery, Vol. 5, No. 3, July 1990, pp [4] M. Darveniza, D.R. Mercer, R.M. Watson, An Assessment of the Reliability of In-Service Gapped Silicon-Carbide Distribution Surge Arresters, IEEE Transactions on Power Delivery, Vol. 11, No. 4, October 1996, pp [5] IEC , Edition 1.2, , Surge Arresters, Part 4: MO Surge Arresters Without Gaps for AC Systems [6] K. Steinfeld, R. Göhler, Rating and Design of Metal-Oxide Surge Arresters for High Voltage AC Systems, PowerCon 2002, Kunming, China October 13-17, 2002 [7] E. Tarasiewicz, F. Rimmer, A. Morched, Transmission Line Arrester Energy, Cost and Risk of Failure Analysis for Partially Shielded Transmission Lines, IEEE Transactions on Power Delivery, Vol. 15, No. 3, July 2000, pp Author adress: Dr. Kai Steinfeld, Director R&D Siemens PTD H 42, Nonnendammallee 104, D Berlin kai.steinfeld@siemens.com Bernd Kruska R&D engineer Siemens PTD H 42, Nonnendammallee 104, D Berlin bernd.kruska@siemens.com Wolfgang Welsch Director Sales Siemens PTD H 43, Nonnendammallee 104, D Berlin wolfgang.welsch@siemens.com 6 Steinfeld, K., Kruska, B. & Welsch, W.