Internal Arc Testing is Becoming‘ Hot’
Current should ideally move along defined, well-engineered paths. But the power we wish to transport actually flows outside the conductor, carried instead by electromagnetic fields. Only for low frequency applications – fortunately power frequency belongs here – can we justify the assumption that conductors carry power and utilize Kirchoff’ s simple junction and mesh-rules instead of Maxwell equations.
However, in the event of a fault( typically due to a some insulation defect), electrons go their own way. The available power simply heats the air and current continues‘ happily’ along the new conductive path, i. e. the highly ionized medium, or arc. In fact, nature acts like this in any medium. Even if the gap-to-bridge is vacuum, the current creates its own medium by injecting plasma( created from contact metal vapour) into the vacuum gap. Indeed, that’ s how vacuum switching devices operate.
Free-burning arcs are intense heat sources used in the early days of electrical power for lighting and these days to power arc furnaces in metallurgy. Temperatures inside the arc reach between 20,000 and 50,000 K( consider that human tissue is destroyed if exposed for but a second to 70 ° C).
In power engineering, free-burning arcs are a danger not only to the public but to personnel and to any installation as well. According to IEC terminology, the event is called‘ internal arc’ if it appears inside a metal-enclosed installation; in U. S. literature, free-burning arcs are usually called‘ arc flash events’. The author of a new book on arc flash hazard analysis and mitigation( ISBN 978-1-118-16381) claims“ the arc flash hazard analysis has taken the industry by storm as evidenced by a spate of technical papers in the literature”. Today’ s increasing demand for internal arc withstand tests supports his assertion.
Since internal arcs can never be ruled out, equipment and system designers devote much effort to minimizing their impact, both directly and indirectly. Equipment designers reinforce metal enclosed switchgear( i. e. make it‘ arc resistant’) and also find ways to safely control over pressure and hot gases through ducts and pressure relief devices; system engineers similarly search for fast protection.
An important part of such hazard mitigation is verification of internal arc withstand capability by means of testing. During such tests, typically carried out on metal-enclosed MV switchgear panels, the fault arc is initiated by a thin wire. The aim is to verify that personnel in the vicinity of switchgear panels or cubicles will not be injured by indirect effects, especially superheated exhaust gases. To verify this, a checkerboard of cotton indicators, representing clothing, is placed on racks at accessible locations to the test object. None of these are allowed to ignite due to hot gases escaping by rapid pressure rise inside the installation. In the relevant standard( IEC 62271-200), a number of defined criteria must be fulfilled to pass, but based on hundreds of such tests, ignition of the indicators is the most common reason for not passing. Just over 20 % of internal arc tests result in failure to pass and ignition of indicators accounts for over half this proportion.
What cannot be tested, however, is pressure rise inside the room with the switchgear. Given the accelerating pace of installing higher power in ever more compact switchgear rooms, users are becoming aware of possible structural damage due to pressure rise that is not properly released. Indeed, CIGRE Working Group A3.24 plans to issue a Technical Brochure on calculation of pressure rise inside and outside a switchgear panel due to internal arcs and one of the main issues will be impact of insulation medium. It has been established that an internal arc in air behaves differently from one in SF 6 with tests and simulations concluding that arcs in air lead to faster and higher pressure rise in the arcing compartment. Conversely, due to the ability of SF 6 to store thermal energy far more efficiently, pressure rise in neighbouring exhaust compartments or in the switchgear room itself can be higher than for arcing in air. Nevertheless, the IEC standard allows replacing SF 6 with air for environmental reasons and MV internal arc testing with SF 6 is thus avoided.
High power test circuits need to be adequate for internal arc testing since fault arcs generate several hundred volts across the arc body. In LV applications, this arc voltage naturally limits arc current: the supply source simply cannot overcome the high arc voltage. However, in MV applications, arc current is barely affected by the arc voltage counteracting the supply voltage. Therefore, test circuits need not only supply the fault current but also to supply it at a voltage not too much below the rated equipment voltage.
More challenging still are internal arc tests of HV equipment with the concern here being that the fault arc burns through the GIS enclosure. Incidents have been reported of severe chemical reactions involving aluminium with SF 6 products in the presence of the arc. Nevertheless, for technically correct testing, the GIS must be filled with SF 6
. This has major environmental repercussions and, in practice, tests can only be performed in large containers that collect all escaping contaminated SF 6 gas and by-products.
So, health, safety and environmental concerns on the one hand and technically / economically acceptable solutions on the other will no doubt continue to fuel heated debate on what’ s best.
42 20
YEARS
Q2 2013
Dr. René Smeets Rene. Smeets @ dnvkema. com