Arresters For Railways

Transcript

English translation of publication in „eb - Elektrische Bahnen“ 100 (2002), Heft 8-9, Seite 321-328 Metal-Oxide Surge Arresters for Electric Railways Kai Steinfeld, Reinhard Göhler, Berlin Power systems of electric railways may be protected against overvoltages by metal-oxide (MO) arresters. Basic designs of the housing are porcelain, polymer composite and directly moulded polymer. With respect to safety, their different short-circuit behaviour must be considered. The permissible bending moment of directly moulded arresters is lower as compared to arresters with porcelain or polymer composite housing. Furthermore, the MO varistors may be pre-damaged by improper handling. Metalloxidableiter für elektrische Bahnen Metalloxid-Ableiter können zum Überspannungsschutz in Anlagen elektrischer Bahnen verwendet werden. Die Gehäuse sind aus Porzellan, Kunststoff-Verbund oder direkt vergossenem Kunststoff mit unterschiedlichem Kurzschlussverhalten. Die direkt vergossenen Ableiter sind preiswerter, jedoch geringer auf Biegung belastbar und empfindlicher gegen unsachgemäße mechanische Handhabung. Parafoudres à oxyde métallique pour réseaux de traction Des déviateurs en oxyde métallique peuvent servir à la protection contre les surtensions dans les installation des chemins de fer électriques. Leurs carters consistent de porcelaine, de matières synthétiques assemblés ou des matières synthétiques coulées directement, présentant des comportements différentes en courtcircuit. Les déviateurs coulées en direct sont plus avantageux, mais moins résistant contre le maniement mécanique maladroit. 1. Introduction The power systems of electric railways, like all electrical energy systems, are at risk of damage from voltage surges. This is because the equipment insulation is invariably of only limited dielectric strength, which can be exceeded by the overvoltage, leading to a breakdown of the insulating materials. Breakdown of a solid or liquid insulation generally results in permanent damage to the equipment because it allows a short-circuit of varying degree of conductivity to develop. An event of this nature usually puts the affected item of equipment or the surrounding power system out of operation pending replacement or repair. Apart from the actual electrical systems, track-side electronic equipment is also increasingly at risk as it becomes more ubiquitous from overvoltages. Factors which cause voltage surges in the power systems of electric railways include atmospheric discharges, switching operations as well as certain circuit and/or loading conditions of the power system. Atmospheric discharges, i.e. lightning strikes actually on the power system or close by, produce surges lasting typically around 10 µs. Switching operations in the power system may produce overvoltages with a typical duration of several 100 µs, depending on the impedance switched. Certain circuit and loading conditions in a power system cause temporary overvoltages (TOV), whose duration is highly dependent upon the way the power system is managed. The figure specified in IEC 60099-4 is 100 s, for example [1]. Lightning strikes pose the greatest hazard for electric railway traction systems in terms of overvoltages. The incidence and current amplitude of such strikes are a function of the local topography and the structural design of the power system. With the resources currently available, full protection is very costly and is possible only in exceptional cases. A lightning protection concept is therefore often a compromise between operational requirements and economic criteria and is only able to cope with a certain proportion of possible strike incidents. Moreover, the overvoltage protection concepts needed in traction system applications must also protect against the indirect consequences of a lightning strike, for example induced voltages in electronic equipment. Metal-oxide (MO) surge arresters are an important and nowadays indispensable element of modern lightning protection concepts. These protective devices limit the level of overvoltages within a power system to values below the withstand voltage of the equipment used within that system [2]. 2. Metal-Oxide (MO) Varistors for Surge Arresters The operating principle of an MO surge arrester is essentially based on the strongly non-linear currentvoltage characteristic of specific ceramic elements, so-called MO varistors, as shown here in Fig. 1, taking the example of an arrester for a 2 kV DC traction system. These MO varistors limit the voltage acting on them, and fluctuating in a range equivalent to several powers of ten of the current, to a relatively constant level and in this way protect parallelconnected equipment against an impermissibly high voltage stress. The most important parameter is the surge arrester’s so-called protection level, i.e. the voltage received by the surge arrester under exposure to a lightning impulse with waveform 8/20 µs and a current amplitude of 10 kA. Fig. 1: Current-voltage characteristic of a surge arrester for 2 kV DC traction systems. The material used for MO varistors is approximately 90% zinc oxide (ZnO) with 5% bismuth oxide (Bi2 O3 ), doped with 5% other metal oxides. As can be seen in Fig. 2, the microstructure of an MO varistor consists of zinc oxide grains with a diameter of around 10 to 50 µm, which are separated by bismuthoxide-rich phases during production. During this process, every boundary between the essentially electrically conductive zinc oxide and bismuth oxide forms a potential barrier of approximately 2.9 V, resulting in an overall electrical performance that is similar to a statistical series-parallel circuit of many Zener diodes. For a given grain size of the MO ceramic material, the protection level provided by the surge arrester is determined by the height of the MO varistor or stack thereof. A typical, specific figure for the normalgradient varistors commonly used for rail traction systems is approximately 300 V/mm height, i.e. the varistor stack in the surge arrester described in Fig. 1 is about 16 mm high. MO surge arresters used in traction system applications absorb large amounts of energy when a surge occurs and the varistors can undergo a significant temperature rise of up to 50°K as a result. To prevent overheating of the surge arrester, it must contain sufficient material to provide an adequate heat storage capability. This is assured for a given protection level by providing elements of appropriate diameter. For example, the surge arrester illustrated Fig. 2: Microstructure of an MO varistor material. in Fig. 1 has a diameter of approximately 70 mm. Previously, surge arresters which contained silicon carbide (SiC) and featured a series gap were used. These are no longer manufactured, but are still found in some power systems. 3. Types of MO Surge Arresters for Electric Railway Systems 3.1. General The electrical dimensioning of surge arresters produces a stack of MO varistors with a specific height and a specific diameter. This stack is enclosed in a housing which, for the entire life of the arrester, must retain the required electrical and mechanical properties for the specific application as well as resistance to the specified ambient conditions. Previously, porcelain was the only suitable material for insulated housings, but polymer housings are now also available, allowing for certain design innovations. 3.2. MO Surge Arresters with a Porcelain Housing The classic insulation material for surge arrester housings in the power engineering sector and for railway traction systems is porcelain. Fig. 3 shows a typical arrester with a porcelain housing for a DC traction system. The stack of MO varistors is tensioned with a construction of fibreglass-reinforced plastic (FRP) Fig. 3: Surge arrester type 3EC3 with porcelain housing for DC traction systems (Table 1). Total height 233mm 1 case 3 MO-Varistors 5 seal 2 compression spring 4 flange 6 pressure relief membrane -2 - bolts, contact and pressure springs and retaining plates. It is contained inside a porcelain housing. The flange-like metal end fitting is fastened to the porcelain housing using sulphur cement. This has significant advantages in terms of production over Portland cement, which can also be used, particularly because it becomes stable under load soon after pouring. The end fitting is also equipped with a pressure relief and sealing system, which are vital for reliable surge arrester operation. Although modern MO surge arresters are exceptionally reliable items of equipment, adverse conditions, e.g. incorrect dimensioning, direct lightning strike or voltage overspill can nevertheless cause them to fail. In such cases, a short-circuit occurs inside the surge arrester and the resultant arc produces a significant pressure increase inside the housing. To prevent the housing from exploding, which would be hazardous for personnel and materials, the surge arrester flange is sealed with a fine metallic membrane only a few tenths of a millimetre thick, which quickly ruptures in the event of a short-circuit, so protecting the housing against internal overpressure. This process, known as blowout, also expels the arc from the housing where it can continue to burn until the power system has been shut down. While surge arresters with a porcelain housing must never be allowed to explode after shorting, a so-called thermal secondary breakdown as defined in EN 50123-5 or IEC 60099-4 [1] is permissible. In this situation, the thermal stresses produced by the action of the electric arc on the porcelain cause the housing to break down. This disintegration is a depressurised event, however; in other words, fragments merely drop off the housing. The pressure relief system must be reliably sealed for the entire service life of the surge arrester, a criterion which is met through meticulous yet relatively uncomplicated design principles and the selection of suitable and, above all, correctly coordinated materials. 3.3. MO Surge Arresters with a Polymer Composite Housing The availability of polymer materials with the right properties for power engineering applications has also led to more surge arresters with polymer housings being used in traction systems for electric railways. Initially, the previously described design features of surge arresters with porcelain housing were adopted and only the material or design of the housing was modified. A surge arrester with a polymer composite housing designed for DC or AC traction systems is shown in Fig. 4. As these diagrams show, the internal configuration of MO varistors and retaining device conforms with that of the surge arrester with porcelain housing. Where the design differs, however, is with regard to the insulating enclosure, which consists of an FRP tube with integral injection-moulded shielding of silicone elastomer or with individually bonded shields. In this design configuration, the FRP tube gives the housing its mechanical strength, while a coordinated system of FRP tube and silicone elastomer shielding safeguards the arrester’s electrical properties. The FRP tube is bonded to the flange, so permitting maximum bending moments. The polymer composite housing, like the porcelain housing before it, is a mechanically stable, hermetically sealed system and therefore requires a pressure relief system to prevent its explosive disintegration in the event of a short-circuit. However, it differs from the porcelain housing in that the material properties of the FRP tube do not allow for any thermal secondary breakdown and also because the housing retains a bending moment of at least 75% of the original figure following a shortcircuit. This type of housing is therefore the safest version from the point of view of personnel and plant protection, despite being the most complex and expensive of the various alternatives available. 3.4. MO Surge Arresters with a Directly Moulded Polymer Housing Commercial demand for costoptimised surge arresters for electric power systems has resulted in the development of directly moulded designs. With this design, a mechanically stable, sealed housing has been dispensed with in favour of a very much simpler structure and minimised use of materials. The disklike MO varistors and their respective end fittings are fastened together by means of FRP rods, screws or loops to form a self- Fig. 4: Surge arrester type 3EB1 for DC- or AC traction system with polymer composite housing (Table 1) mounted on ICE 2 power car. Total height 296 mm 1 compression spring 3 FRP tube 5 flange with seal and 2 MO-Varistors 4 HTV s ilicone sheds pressure relief membrane -3 - supporting configuration around which the silicone elastomer is then moulded. This design is similar to that of a reinforced concrete bridge in that its mechanical stability relies on a sufficiently high level of prestressing; in other words the MO varistors are mechanically prestressed. The term “direct moulded“ indicates that the silicone elastomer is applied directly to the surface of the MO varistors. Directly moulded surge arresters are now used almost exclusively in medium-voltage threephase systems, and they will become increasingly commonplace in highvoltage three-phase systems in future. Surge arresters with directly moulded polymer housings are also available for the power systems of electric railways, and appropriately modified directly moulded surge arresters are frequently used for medium-voltage three-phase systems. The design of such surge arresters is remarkably uncomplicated and essentially consists of metallic terminal fittings with the MO varistors fastened between them by means of FRP tensioning elements. The housing is then formed by moulding a layer of silicone elastomer directly onto these elements, as shown in Fig. 5; this design has no pressure relief or sealing system. The arc produced in the event of a short-circuit exits to atmosphere through the mechanically weak silicone elastomer layer, and continues to burn there without any explosive destruction of the housing. Even without a pressure relief system, therefore, there is no highspeed ejection of housing fragments. However, the arc accompanying a short-circuit generally damages the surge arrester’s mechanically selfsupporting internal structure seriously enough to destroy its mechanical stability, with the result that it is no longer able to absorb forces from bolted on busbars or connecting cables, for example. Another possible scenario, similar to that occurring with thermal secondary breakdown of a surge arrester with a porcelain housing, is that internal fragments of the arrester or of the MO varistors may be expelled from the housing. However, short-circuit tests on the surge arrester performed in the context of its type testing are required to demonstrate that any fragments ejected in this manner merely drop away loosely from the housing and that there is consequently no risk to personnel or adjacent plant components from high-speed splinters. The mechanical strength of directly moulded surge arresters, particularly their permitted bending moment, is generally significantly lower than that of alternative designs with a porcelain or composite polymer housing. Moreover, forces applied through the connections are added to the pre-tensioning force applied to the MO varistors, the elements responsible for the surge arrester’s electric response, potentially resulting in imperceptible damage to the MO varistors through improper handling. There is no chance of such damage with surge arresters with a porcelain or composite polymer housing because mechanical forces are absorbed by the housing and the only mechanical load on the MO varistors comes from their spring loading which is necessary to ensure perfect electrical contact. Table 1 compares the permitted bending moments of Siemens surge arresters of the various types described above. 4. Use of MO Surge Arresters in DC Traction Systems as per VDV 525 Publication No. 525 [3] of the Association of German Transport Undertakings (VDV) gives operators of DC traction systems recommendations on effective surge protection in the event of lightning strikes. It first of all distinguishes between external and internal lightning protection, whereby external lightning protection applies directly to the overhead traction line systems, and also the running rails in the case of traction systems with contact lines, while an overhead contact line acts as a lightning arrester device. Internal lightning protection refers to measures implemented in the substations, but also encompasses the use of surge arresters in low-voltage systems or signalling and communications equipment close to or in the tracks. 4.1. Earthing of Power Systems for Electric Railways The planning of lightning protection concepts pays particular attention to the earthing of power systems. If the rails of DC traction systems are insulated to earth to reduce stray Table 1:Permissible bending moments of Siemens surge arresters with porcelain housing, polymer composite housing and directly moulded polymer housing as used for the power systems of electric railways Type Housing 3EC3 Porcelain Max. system voltage [kV] DC AC Permissible bending Moment Fig. 5: Surge arrester type 3EB2 with directly moulded polymer housing (Table 1). Application kV kV 4 Nm 2000 stationary and on cars -4 - 3EB1 Polymer composite 4 37 3EB2 Directly moulded 2 2600 1250 mainly on cars mainly stationary current corrosion, as is required for new tracks, they are unsuitable as earth terminations. In such applications, low-resistance tower footings, driven poles, reinforced concrete guideways or separate earth rods must be used as earth terminations, depending on local track conditions. Where the rails have been laid without any additional insulating measures, however, they generally exhibit only minimal leakage resistance and can be used as earth terminations. In such cases, however, the discharge current is conducted to earth via the rails, which poses a risk for electrical and electronic equipment close to or actually in the track. An effective means of protecting against overvoltages resulting from this process is to provide additional surge arresters in the actual equipment. Substation earthing arrangements are unproblematical because the structure’s own earthing system serves as an efficient, low-resistance (≤ 2 Ω) earth electrode for lightning protection measures. Fig. 6: Protection of the overhead contact line by A1 surge arresters (from [3]). 4.2. Use of A1 Surge Arresters to Protect Overhead Contact Lines Fig. 6 shows that to provide comprehensive protection for overhead contact lines, “A1” type outdoor surge arresters to VDV 525 designation should be installed at every feed point, at the ends of feed sections and dead-end feeders, at tie points and at power extraction points. Additional A1 surge arresters are recommended for sections of track that are particularly prone to lightning strikes, for example bridges or free overland stretches without trees. A favoured method of installation in order to keep the electrical connection to the overhead contact line as short and straight as possible is to mount the A1 surge arrester at the same height as the overhead contact line. The earth connection should be connected to the earth electrodes by the most direct route possible and the cable used for this purpose must itself be insulated against the overhead contact line tower or the structure’s earth, as shown in Fig. 7. Fig. 7: Connection of A1 surge arresters for earthed and non-earthed rails (from [3]) 4.3. A1 and A2 Surge Arresters for Substation Protection The wiring of supply and return lines in the substations with surge arresters is an important element of a lightning protection concept for a railway power system. Two differently dimensioned surge arresters are used in this case and their principal mode of action is described in Fig. 8: The A1 surge arresters are connected between the line circuit-breakers or cable terminals and the return line, where they effectively limit overvoltages on line circuit-breakers and power rectifiers when positive lightning strikes occur and simultaneously protect the associated control and measurement equipment. The currents from negative lightning strikes are conducted through the -5 - diodes of the power rectifier, which is not itself damaged thanks to its substantial current carrying capacity. The unavoidable rise in potential due to a lightning impulse current is limited by the A2 surge arresters between return line and the structure’s earth. As in the case of the A1 surge arresters, these connection cables should also be as short as possible and also short-circuit-proof. References [1] IEC 60099-4, Edition 1.2, 2001-12 Metal-oxide surge arresters without gaps for a.c. systems [2] Fien H., Hinrichsen V., Pelmer J Hochspannungs-Metalloxid-Ableiter: Auslegungs- und Konstruktionskriterien. Elektrizitätswirtschaft, Vol. 93 (1994), Issue 19, pp. 1148-1156 [3] VDV Schriften 525 12/01: Schutz der Fahrstromversorgungs-anlagen von Gleichstrombahnen bei Blitzeinschlag , Verband Deutscher Verkehrsunternehmen (VDV), Karnekestraße 37-39, 50672 Cologne Fig. 8: Protection of a substation using A1 and A2 surge arresters EBS: equipotential bonding strip 4.4. Protection Following Failure of an A1 Surge Arrester Although MO surge arresters are extraordinarily reliable items of equipment with a failure rate significantly below 0.1%/year, they do sometimes break down under adverse conditions. Such failures are mostly due to non-specification loadings, e.g. direct lightning strikes of unusually high current intensity or a voltage overspill, both of which make the A1 surge arrester permanently conductive. If the running rails exhibit a low leakance per unit length, the earth electrode can absorb an inadmissibly high fault voltage. However, connection of an additional A2 surge arrester with a low continuous voltage (120 V ≤ Uc ≤ 300 V) between the earth electrode and the return conductor (as shown in Fig. 9) limits this fault voltage and trips the line circuit-breaker by backfeeding from the overhead contact line system. 5. Conclusions Metal-oxide surge arresters are an indispensable element of lightning protection concepts for the power systems of electric railways. The live electric element consisting of MO varistors can be enclosed in a porcelain, polymer composite or directly moulded polymer housing. With each of the various forms, it is important to consider their different mechanical properties and failure performance, so as to optimise the level of protection provided for personnel and property. VDV publication No. 525 gives recommendations on the use of metal-oxide surge arresters for the protection of railway power systems. A concept using A1 and A2 surge arresters to protect against inadmissibly high potential rises or contact voltages - both in the substations and in items of track-side equipment - is presented for the first time. Fig. 9: Protection of a contact line using A1 and A2 surge arresters (from VDV 525) -6 - Dr.-Ing. Kai Steinfeld (38) studied electrical engineering at Berlin Technical University and was awarded his doctorate there from the Institute of Electrical Engineering (HighVoltage Department). He is Head of Development at the Surge Arrester and Limiter Unit at Siemens AG. Fon: +49(0) 30 386 2 46 55, Fax: -2 49 08 Email: [email protected] Dipl.-Ing. Reinhard Göhler (47) studied electrical engineering at Brunswick Technical University. He is in charge of the Test Laboratory (and also Sales Director for Special Surge Arresters) in the Surge Arrester and Limiter Unit of Siemens AG. Fon: +49(0) 30 386 2 33 90, Fax: -2 67 21 Email: [email protected] Address: Siemens AG, PTD H 42, Nonnendammallee 104, D-13629 Berlin