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
Dr Leslie T Falkingham, AREVA T&D Products, AREVA Technology Centre PO Box 30 Lichfield Rod Stafford ST17 4LN Telephone: +44 1785 274 650 E-mail: Leslie.firstname.lastname@example.org
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 special design of easy handling of the equipment, the choice of selected and tested materials, proper and adequate insulation, use of electrical and mechanical interlocks. Moreover rigorous factory testing on components and products after final assembly also helps a great deal to improve the quality 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. One possible approach is to use quick arc detecting device and quick fault clearing relays, which will certainly help to keep the damage from such a fault to a minimum, and these are available as options on this equipment. However these are active systems to prevent damage, and as such there is a possibility that they may fail to perform, or operate. Whatever we do, the possibility of a fault occurring still remains, and so it is necessary to develop switchgear which can cope with an internal arc in a safe way, using passive design techniques to minimise damage within the unit as well as to the building and other adjacent units. Such a modern arc resistant metalclad medium voltage air insulated switchgear is now available to cope with even this rare occurrence of internal fault, covering voltage and current ranges from 3.3kV to 27 kV, 630A to 5000 A, and with a short circuit capacity up to 50 kA. Several standards presently exist or are being currently developed to cover internal arcing, and for information the key differences between the new IEC and ANSI standards are listed in an Annex.
2. The physical phenomena:
To understand the effects of an internal fault one should consider the physical process a little more in 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...