INTERNAL ARC RESISTANT SWITCHGEAR

Transcription

1 INTERNAL ARC RESISTANT SWITCHGEAR Nirmal DEB Patrick BAILLY Thierry TRICOT Alstom T&D. - Switzerland Alstom T&D. - France Alstom T&D. - France nirmal.deb@tde.alstom.com patrick.bailly@tde.alstom.com thierry.tricot@tde.alstom.com Summary As an internal arc fault cannot be ruled out completely, manufacturers are designing equipment to withstand internal arc faults. The standards for the requirements differ considerably for example the requirements of ANSI are not the same as for IEC. But the standards are continuing to evolve as this issue becomes more understood. A new range of withdrawable metal-clad cubicle with segregated compartments, doors and front covers designed to withstand severe stresses without allowing the effects of the arc to come outside, named PIX, has been developed to provide the maximum safety to operating personnel with a worldwide approach to cope up with different demands. The paper describes the development with respect to internal arc, both from an operational and and an operator safety point of view. The PIX takes two approaches to deal with internal arcs. Firstly in the event of an internal arc this new design takes control of the situation and keep the effect of the arc confined to the place of occurrence and thus provides continuous operation. Secondly fast operating relays and arc detection systems help to keep the damage and shut out time to a minimum. 1. Introduction An internal arc fault, though extremely rare in modern substations which are well conceived, properly planned and with protection systems secured, cannot be ruled out completely. Such a fault might occur due to several reasons difficult to control, such as failure of insulation or contacts due to ageing, failure of instrument transformers, overvoltages in system because of switching or lightning surges, pollution due to environmental conditions, maloperation or insufficient maintenance. The design of a modern arc resistant air insulated switchgear for medium voltage metalclad applications, which covers voltage and current ranges from 3.3kV to 36kV, 630A to 5000 A and with short circuit capacity up to 50kA has to be designed to cope up with this possible occurrence of internal fault. To reach this goal, two important aspects have to be taken into account for the design: - The physical phenomena initiated by the arc - National and international standards for the switchgear and controlgear and their evolution However, on their own these rules are not enough: the know how and experience of the manufacturer is also vital to insure the best design of the switchboards. Finally, the type tests provide the proof of the safety of the switchboard for this aspect, in so far as the rules of installation are respected. An example of a medium voltage range of switchboard which comply in particular with the requirements of the current IEC 60298, ANSI C and the future requirements of the IEC project is given at the end of this paper. 2. The physical phenomena: To understand the effects of an internal fault one should consider the physical process in more detail. The first phase (compression) starts with the arc ignition and ends after reaching the maximum pressure in the corresponding compartment. The enclosed air in the compartment will be heated depending on the arc energy with the pressure relief flaps closed. The pressure in the compartment rises directly proportionally to arc fault current and the length of the arc, and indirectly proportional to the volume of the chamber in which the fault occurs. The duration of the compression phase and the maximum pressure rise depends on arc energy (arc fault current and arc voltage) and the volume of the chamber containing the fault and also other factors such as the place and position of ignition, and also air circulation openings. The second phase (expansion) is when maximum pressure peak is reached and the pressure relief system operates to relieve the pressure. In this phase the compressed gas is ejected from the chamber through a special duct In the third phase (emission) the remaining air will be heated further and ejected with the pressure relief flaps open. In this stage the remaining air in the compartment will reach the arc temperature. In this stage almost all of the air in the compartment is expelled. The fourth and final phase (thermal) in the process now lasts up to the end of arc current duration. In this stage the arc energy is applied completely towards the fixed parts inside the compartment. This results in melting and vaporisation of copper connections, feeders, switching devices, metal parts of the enclosure as well as any plastic and insulating materials. The erosion of material depends mainly on the duration of this period and is also dependent on arc current value, specific thermal characteristic of material used as well as the distance of the switching equipment from the arc root. ALS_Deb_A2 Session 1 Paper No 2-1 -

2 The complete process of internal arc inflicts two heavy stresses on the equipment which can be shortly described as follows: - Mechanical stress The pressure rise affects the compartment in which the fault occurs in bending of the main and partition walls. The fixing elements as bolts and nuts or the hinges or the fixing of doors or covers or flaps are stressed for strain or shearing. To tackle this problem specially and carefully chosen materials and special bending forms of sheet metal are necessary which will withstand these stresses without undue damage or deformation. - Thermal stress For the whole duration of the fault material will be eroded and vaporised at the arc root. Mainly the copper parts in the vicinity of the arc will be eroded. But other materials such as metal walls or partitions may also be melted and vaporised thus allowing hot gases to escape outside. The plastic and insulating parts used in the switchgear equipment are also heated up and may under the circumstances be vaporised. It is important that the insulating material should be selected so that they do not continue to burn after the extinction of arc and also that they do not release any toxic or corrosive elements which may increase the indirect damages. 3. The relevant standards and their evolution: In the course of mid sixties it was evident that a common need for equipment with internal arc withstand capability existed and therefore the necessity of a common standard was felt. In IEC a standard for metal enclosed switchgear IEC 298 published first in 1960 there was no mention of internal arc phenomena. At that period the problems and consequences of the internal arc fault was not given much attention as the problem was not properly understood. But within the next decade this phenomenon attracted much attention and PEHLA in Germany came up with a standard which was immediately hailed as a new theme in the field of metal clad equipment. IEC also adopted the requirements of this PEHLA which was introduced into the IEC standard as amendments. However today the standard IEC is accepted world-wide as the key international standard though there are still appreciable differences with standards prevalent in USA and in some other nations. But still today the demands of the standards are generally restricted to agreement between manufacturers and users. IEC is now proceeding towards a new update of its standard IEC 298 (to be effective from 2004 onwards as IEC ) where a new product can be defined with internal arc class and a type test has been asked for this classification of switchgear. The IEC evolution is to define a purpose-based classification to fit user need. The standard takes into account the customers needs in terms of service continuity and safety during both operation and maintenance. For this aspect a primary selection (arc fault proof or not) has to be done. This primary selection is defined by IAC class : when selecting a metal enclosed switchgear and controlgear, the probability of internal faults should be properly addressed with the aim to provide an acceptable protection level for operators and also, when appropriate, for the general public. IAC class metal enclosed switchgear and controlgear is to be type tested for all its compartments. It seems today that with the implementation of the updated standard in 2004, IEC will be recognised as the most acceptable standard in the world market. 4. The design The new design of a metal-clad switchgear has to take care of the internal arc faults and keep the effects of arc confined in the place of occurrence in order to thus provide continuous operation of the remaining functions not associated with the fault. Moreover quick operating relays and arc detecting systems can help to keep the damage, and as a result the shut out and replacement time, to a minimum. To fulfil the demand of different standards special measures have to be taken and the switchgear has to be conceived with separate segregated compartments for each function. Special attention has to be given to the design of the doors and the front covers which will withstand severe stresses and cannot allow any damaging effects outside. Care also has to be taken to design the pressure relief flaps of circuit breaker and busbar compartments so that when the flap of circuit breaker compartment opens it will not contaminate the busbar compartment or vice versa. Apart from this, measures have to be taken to avoid any burn through in order to provide ultimate safety in the accessible parts. Moreover the inspection windows require special care in design. Measures also have to be taken not to allow any part or piece of the equipment to fly away during such a fault. Such new design has to be available with walkways on front, rear or on sides as desired. The ultimate goal is to provide the maximum possible safety to personnel in the substation, and as the results of the internal arc tests in an independent laboratory show this can be achieved to a high level. An example given in 6 shows the practical application of these rules. The aim of such a design is to use very simple technologies, that allows world-wide manufacturing possibilities together with a good level of quality and without sophisticated tools. To reach this goal, all the resources and skills as well as the experience available in different countries in ALS_Deb_A2 Session 1 Paper No 2-2 -

3 different continents in respect to handling, operation, and maintenance of switchgear were employed to develop a new arc resistant switchgear range. Moreover the comments from the customers and utilities have been given due attention to reach the present solution. This new metal-clad medium voltage switch gear range called PIX has now been developed by ALSTOM incorporating these best practices and can meet not only the requirements of ANSI and IEC, but will also fulfil the specific requirements of standards of many different nations all over the world (China, Australia, South America etc.) giving the maximum possible safety for the users. Last but not the least, it is to be remembered also that the probability of occurrence of an internal fault is very much reduced by special design for, easy handling, the choice of selected and tested materials, proper and adequate insulation, together with the use of electrical and mechanical interlocks. Moreover rigorous factory testing on components and products after final assembly also help a great deal to contribute to the safety of the product. A good documentation of all functions including instructions of proper handling will also help to reduce the occurrence of such faults to a great extent. Apart from all these points, several choices of quick arc detecting device and quick fault clearing relays are also offered which will certainly help to keep the damages of such a fault to an absolute minimum. 5. Tests Tests have to be performed at independent laboratory considering the different aspects of the product and the different demands of standards. Care has to be taken for the choice of functional unit and test arrangement which covers the different and most difficult cases of applications. Thus, for the example of PIX, cubicles not only at the ends but also in the middle of a switchboard panel were tested. Similarly all compartments were also tested separately until satisfactory results were obtained. Busbar compartments, segregated or non-segregated, were also tested rigorously. Figure 2 : Oscillogram Figure 3 :Test arrangement/ sectionnal view A typical test arrangement is shown in figures 3 and 4 where the test object consists of 3 cubicles and a test in the circuit breaker compartment with an arc fault current of 40 ka/1s was carried out in the end cubicle (cubicle on left side) with accessibility on front, left and rear side. The test arrangement in the photo (figure 1) immediately after the test shows all the indicators intact. The oscillogram (figure 2) of the 3 phase arc fault test 40 ka/1s shows arc currents and arc voltages in all the three phases, pressure as a function of time and the instant of opening of the pressure relief flap. The energy developed measured 38 MJ. Figure 1 : Test arrangement just after arc fault test ALS_Deb_A2 Session 1 Paper No 2-3 -

4 a) Doors and front covers: Special measures are taken to design these parts to be able to withstand severe stresses. After closing they slide laterally into position to enable a larger fixing surface. b) Rear wall : This is designed in such a way that it allows a certain amount of deformation to absorb the high stresses in the cable compartment without bursting. Its strength and design allows back to wall or with rear corridor installation. Figure 4 : Test arrangement/ front & top view To conclude this section it has to be said that the results of the tests will be representative of the real life of the product only in so far as the conditions of installation specified by the manufacturer are respected. As stated before, good documentation is very important to prepare an installation for complying with this requirement. 6. Dominant features of internal arc resistant switchgear Example of new switchgear PIX with all the main criteria described above (figure 5) : c) Pressure relief flaps: These are designed to allow an early pressure relief (within approximately 10 ms of occurrence of a fault) to avoid any mechanical bursting of the corresponding compartment. They are light in weight and provided with special hinge system so that they do not fly off. Moreover if the pressure relief flap of circuit breaker compartment opens due to a fault in the circuit breaker compartment it is designed to cover the air vent opening of the busbar compartment flaps and thus protects the busbar compartment from contamination. The function of busbar compartment flap acts in the same way towards circuit breaker compartment. d) Position of pressure relief flaps: The position of flaps is so designed that when open the flaps remain within the cubicle and thus do not require any consideration during installation. This gives an extra advantage to keep the height of ceiling or the distance to wall to a minimum. e) Flaps to close air ventilation openings in cable compartment: These are designed to close quickly the air ventilation outlets on occurrence of a fault and thus avoid any outlet of hot gas or molten material outside the unit. f) Segregated busbar compartment: This is designed in such a way that the arc remains confined within the compartment where the fault initiated. g) Spouts: These are made of epoxy or polyester depending on current rating and do not produce any exotic gases when subjected to internal arc and are not easily flammable. Figure 5 :New switchgear PIX h) Special measures for higher arc fault current: Use of additional pressure relief in the cable compartment, use of additional sheet metal lateral wall, and use of extra light weight pressure relief flap. ALS_Deb_A2 Session 1 Paper No 2-4 -

5 i) Separating space between two adjacent cubicles: The design of the side frame of the cubicle is such that when assembled in rows side by side there is a free space of about 5 mm left between the side walls of two adjacent cubicles. This gives an extra security for the neighbouring cubicle. If a burn through did take place in the cubicle with the fault, the neighbouring cubicle is not affected and remains intact. One other advantage of this design is to have a minimum of friction between a faulty cubicle and those on each side when extracting it for repair. The time of maintenance is therefore reduced. j) Separate cable duct for low voltage wiring: The cable duct is concealed within a protected sheet steel enclosure which allows an extra security against the effects of thermal overloading due to the very high temperature arising from an internal arc in the compartment. This provides an extra security for the monitoring and controlling instruments in the low voltage compartment, and significantly reduces the probability of damage to these circuits. k) Inspection windows: They are specially designed and located in frames inside the door or front cover. These are designed so that the bending of the door or front cover due to high pressure during an arc fault is not transmitted to the frame of inspection window and thus protects the glass from bending stresses. Moreover, these inspection windows allow a visual control of the position of the circuit breaker withdrawable part and of the earthing switch and cables lugs that can prevent in some cases the possibility of an internal arc fault due to maloperation or ageing. l) Blastshields: These are designed to retain the hot gases as well as the molten materials in the near vicinity of the installed equipment so as not to endanger the operating personnel. 7. Bibliography: 1) P. Chévrier, Etude de l'interaction arc/matériaux d'un défaut interne dans une cellule moyenne tension Schneider Electric, Centre de Recherche A , France- Journées d études SEE : MATPOST, Lyon Nov 99 2) J. Gauthier, G: Sonzogni, L'essai de non coupure sur les tableaux MT : une amelioration de la securite des personnes et des biens Laboratoire d'essais des Renardières, Electricité de France 3) F. Lutz, Ein Verfahren zur Berechnung der Druckentwicklung in Mittelspannungs-Schaltanlagen bei Störlichtbögen Dissertion 1981 an der Institut für Allgemeine Elektrotechnik und Hochspannungstechnik der RWTH Aachen 4) Krenicky, U. schmitz, G. sonderegger, Lichtbogenfehler in SF6-isolierten, gekapselten Hochspannungs-Schaltanlagen Brown Boveri Mitteilungen ) F.Chu, G. Ford, C.Law, Estimation of burnthrough probability in SF6 insulated substations IEEE transactions on power apparatus system no. 6 June ) G. Harz, M. Niegel, Störlichtbogen im Griff. Alstom Sachsenwerk, Elektrotechnik 60 (1978) 7) H. Kindler, W. Schels, Systematische Untersuchungen zur Störlichtbogensicherheit von Schaltfeldern und schaltanlagen, Alstom Sachsenwerk, Technische Mitteilungen 65 (1975) m) Tunnel: This feature is available as option to be able to divert the exhaust outside the building in order to have the substation uncontaminated after the occurrence of a fault. ALS_Deb_A2 Session 1 Paper No 2-5 -