Arc Resistance Switchgear

Design of a New Generation of Internal Arc Resistant Switchgear
Nirmal Deb AREVA T&D Switzerland Abstract
Although rare in service, an internal arc fault cannot be ruled out completely, and so manufacturers are now designing equipment to withstand internal arc faults, and to minimise the consequences of an internal fault. The standards for the requirements differ considerably, as 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. The design of a new range of withdrawable metal-clad cubicle must provide the maximum safety to operating personnel and cope with a number of different demands, including the ability to limit the consequences of a fault to the compartment in which it occurs. These cubicles shall be built with segregated compartments, doors and front covers designed to withstand severe stresses without allowing the effects of the arc to come outside. The paper describes the design of MV switchgear with respect to internal arc, both from an operational and from an operator safety point of view.

Patrick Bailly AREVA T&D France

Thierry Tricot AREVA T&D France

Leslie T Falkingham AREVA T&D UK

Contact
Dr Leslie T Falkingham, AREVA T&D Products, AREVA Technology Centre PO Box 30 Lichfield Rod Stafford ST17 4LN Telephone: [phone] E-mail: [e-mail]

1. Introduction :
An internal arc fault, though rare in modern, well conceived, properly planned substations, cannot be ruled out completely. Such a fault might occur due to one of several reasons which are difficult to control, including failure of insulation or contacts due to ageing, failure of instrument transformers and overvoltages in system because of switching or lightning surges, pollution due to environmental conditions, maloperation or insufficient maintenance. An important fact is that the probability of occurrence of an internal fault is very much reduced by

Bibliography: 1)P. Chévrier, Etude de l 'interaction arc/matériaux d 'un défaut interne dans une cellule moyenne tension Schneider Electric, Centre de Recherche A2. 38050, France – Journées d’études SEE : MATPOST Novembre 1999 Lyon. 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)F.Chu, G. Ford, C.Law, Estimation of burn-through probability in SF6 insulated substations IEEE transactions on power apparatus system no. 6 June 1982 5) G. Harz, M. Niegel, Störlichtbogen im Griff. Alstom Sachsenwerk, Elektrotechnik 60 (1978) 6)H. Kindler, W. Schels, Systematische Untersuchungen zur Störlichtbogensicherheit von Schaltfeldern und schaltanlagen, Alstom Sachsenwerk, Technische Mitteilungen 65 (1975) Annex: Comparison of the main points of IEC and ANSI regarding internal arc; IEC62271-200 IEEE C37.20.7 Accessibility Accessibility Two classes defined with identification of front, Three classes are defined, 1) at freely accessible lateral or rear side front and 2) at freely accessible exterior (front, Class A: restricted to authorised personnel back and sides). 3) for equipment designed to compartmentalise components Class B: unrestricted How the test object is to be equipped, how test How the test object is to be equipped, how test specimen is to be chosen: Same as in column 1 specimen is to be chosen The specimen shall be fully equipped , mock-ups Any opening created in the equipment as a result of internal components are permitted provided they have the same volume and external material of manufacturing, assembly or modification that have an intentional covering, may have that as original item covering installed. Openings without intentional Each compartment of a functional unit shall be coverings may not be blocked for the test. tested. Each variation in bus phase spacing and In case of modular functional units, the test clearance to ground should be tested, except for specimen shall consist of 2 functional units connected together as in service. When there is a configurations where only the size and/or the quantity of bus changes. In these configurations, a substantial difference in strength between the representative enclosure may be tested using the joining sides of adjacent units and the side smallest physical bus size to produce the greatest forming the end of the switchboard, 3 functional phase to phase and phase to ground clearances. units shall be used and the test of different compartments repeated in central and lateral A typical section may be used to perform the test positions. provided its compartment represent the smallest internal volume and/or most restrictive method for relief of overpressure. Room simulation Room simulation The room shall be represented by a floor, ceiling The test arrangement should simulate room and 2 walls perpendicular to each other and conditions in a manner that consider the following: exhaust ducts simulated cable access. -distance to adjacent walls 7 Unless manufacturer states a larger minimum clearance, the ceiling shall be located at a distance of 500 mm from the upper part of the test specimen. The lateral and rear wall shall be placed at 100 mm from the lateral and rear side of the test specimen in case of non accessible lateral or rear side. For accessibility of rear and lateral sides the walls shall be placed at a distance of 800 mm If the design is with exhaust duct the tests shall be performed with simulation of such exhaust ducts. Indicators These are pieces of black cotton clothes (150g/m2 so arranged that their cut edges do not point toward the test specimen. Care shall be taken to see that they can not ignite each other. Indicators shall be fitted at all accessible sides of the metal unit vertically up to a height of 2 m. The distance from the indicators to the switchgear shall be 30 cm.(for class A) and 10 cm (for class B). Indicators shall also be placed horizontally at a height of 2m above the floor on all accessible sides between 30 and 80 cm (accessibility A) or between 10 and 80 cm (accessibility B) from the unit. When ceiling is placed at a height of 2m above the floor, no horizontal indicator is needed. The length of mounting frame shall be bigger than the test specimen to take into account the possibility of hot gases escaping in slant direction (up to 45°). The area occupied by the indicators shall be at least 50% of the global area reserved to the indicators positioning. -ceiling height -any obstruction located near the equipment that may deflect hot gas into an area defined by the accessibility type -any opening beneath the equipment (cable vaults) which may allow hot gas to escape into an area defined by the accessibility type. If the design incorporates an exhaust system that will vent pressure directly out of the room, no room simulation is necessary. Indicators Indicators are pieces of black cotton cloth (150g/ m2) so arranged that their cut edges are not exposed to the test sample and each indicator is isolated from each other to prevent multiple ignitions from a single source. This is obtained by fitting them in a special mounting frame. Vertical indicators are to be located from floor to a minimum height of 2m and at a distance of 100 mm from the switchgear. If the equipment is intended for mounting on an elevated base, indicators should be placed below the base of the test sample to monitor gas escape at the floor level. Horizontal indicators are to be located at a minimum height of 2m from the floor and horizontally 0.8m from the sample. The internally mounted indicators for accessibility type 3 are to be located at 100 mm from the interior surface being evaluated. Internal indicators are mounted in any applicable plane, parallel to the surface being evaluated. There is no height restriction for internally mounted indicators. They should be mounted for the applicable surfaces up to full height of the equipment. Test connection The tests should be carried out 3 phase. If a lower voltage is applied it should not be less than 60% of the rated voltage and is not recommended for equipment rated 5 kV and below. The maximum value of the AC component during the test shall not exceed the minimum value of the ac component during the test by more than 15% Where fast active protective devices will limit the rated duration of test arc to 50 ms or less, the frequency at the beginning of the test shall be rated frequency of the equipment ±10%. For a rated duration greater than 50 ms, the frequency at the beginning of the test shall be the rated frequency of the equipment ±20%and the frequency of waveform should not deviate from the initial value by more than 8% for the duration of the test. Duration of arc fault Preferred value 0.5 sec Criteria to fulfil a test For qualification as arc resistant switchgear followling conditions are to be met: No 1 secured doors, do not open. Bowing or other 8 Test connection The test should be carried out 3 phase. The short circuit current applied during the test may be lower, if specified by the manufacturer, than the rated short time current withstand. A lower voltage than the rated may be applied if the following conditions are met: -the current remains practically sinusoidal -the arc is not extinguished prematurely The specified short circuit current should be set within +5% -0% tolerance. This tolerance applies to the prospective current only if the applied voltage is equal to the rated voltage. The current should remain constant. Frequency deviation is between 48-62 Hz for a rated frequency of 50/60 Hz For a rated frequency different from 50/60 Hz, the frequency at the beginning of the test should not deviate from the rated value by more than 10% Duration of arc fault Standard recommendations are 1 sec, 0.5 sec and 1 sec Criteria to fulfil a test IAC 1 class switchgear is qualified if following criteria are fulfilled : No 1 whether correctly secured doors, covers do not open. Deformations are accepted provided that no part come as far as the position of the indicators or the walls in every side. It is admitted that the switchgear does not comply anymore with its IP code No 2 whether no fragmentation of the enclosure occurs within the time specified for the test. Projection of small parts (upto 60g) are accepted provided they stay within the position of the indicators. No 3 arcing does not cause holes in the accessible sides upto a height of 2 m No 4 indicators don 't ignite due to the effect of hot gases No 5 whether all earthing connections are still effective distortion is permitted except on doors, covers which may have devices (relays). No sign of distortion which could cause these devices to be ejected from the equipment is acceptable. No 2 Parts are not ejected in the vertical plane defined by the accessibility type- Parts large enough to be hazardous do not eject from top of equipment. No 3 assessment of burn-through -accessibility type 1 arcing does not cause holes in the freely accessible front of the enclosure -accessibility type 2 arcing does not cause holes in the freely accessible front, sides and rear of the enclosure -accessibility type 3 arcing doesn 't cause holes in the freely accessible front, sides and rear of the enclosure or in the walls separating the vertical sections in an assembly or between compartments of a vertical section No 4 indicators do not ignite as a result of escaping gases or particles. No 5 whether all earthing connections are still effective. 9