Computational methods for the analysis of explosions in oil-insulated electrical equipment

Abstract

Hardy S e n i o r Member I n s t i t u t de r e c h e r c h e d'lydro-Qu6bec (IREQ) 1800 montse S t e -J u l i e Varennes, QuBbec, Canada JOL 2PO Abstract -Over t h e last few y e a r s , t h e problem of e x p l o s i o n s i n o i l -i n s u l a t e d equipment as a r e s u l t of low-impedance f a u l t s h a s t a k e n on i n c r e a s i n g proport i o n s , mainly because of t h e h i g h e r c u r r e n t l e v e l s now a v a i l a b l e and t h e aging of t h e equipment.The problem may become c r i t i c a l when t h e e x p l o s i o n is n o t confined t o t h e t a n k and sends o i l o r p r o j e c t i l e s f l y i n g i n t o t h e environment, j e o p a r d i z i n g t h e s a f e t y of surrounding equipment. T h i s paper p r e s e n t s two methods f o r a n a l y z i n g t h e dynamic response of e l e c t r i c a l equipment t a n k s comp l e t e l y f i l l e d w i t h o i l under f a u l t c o n d i t i o n s l i a b l e t o c a u s e e x p l o s i o n s . Both methods s i m u l a t e t h e same i n f i n i t e c y l i n d r i c a l model, s u b j e c t e d t o an e c c e n t r i c a r c , but d i f f e r e n t i r e l y in t h e i r approach. The f i r s t i s based on an a n a l y t i c a l f o r m u l a t i o n of a thin-walled s h e l l c o n t a i n i n g an i n c o m p r e s s i b l e l i q u i d ; t h e s o l u t i o n i s obtained by superposing o r t h o g o n a l components and n u m e r i c a l l y i n t e g r a t i n g t h e e q u a t i o n s of motion.The o t h e r approach i s more g e n e r a l and i n v o l v e s t h e use of t h e h i g h l y s p e c i a l i z e d s o f t w a r e PISCES, adapted f o r arc-induced e x c i t a t i o n .An u n d e r l y i n g i n t e r e s t of t h e proposed methodology i s t h a t i t can be a p p l i e d , a f t e r m o d i f i c a t i o n s , t o t h e a n a l y s i s of p a r t i a l l y f i l l e d equipment such a s d i s t r ib u t i o n t r a n s f o r m e r s .Comparison of t h e computer r e s u l t s p r o v i d e s a means of m u t u a l l y v a l i d a t i n g t h e two methods and t h u s paves t h e way f o r an extended use of PISCES t o complex models w i t h a more r e p r e s e n t a t i v e t a n k c o n f i g u r a t i o n . A few p a r a m e t r i c s t u d i e s performed with t h e a n a l y t i c a l model have pinpointed t h e c r i t i c a l f a c t o r s t o be cons i d e r e d i n t h e d e s i g n of explosion-proof t a n k s f o r e l e c t r i c a l equipment. pBgv1ouswIlRKI n c r e a s i n g concern over t h e problem of e x p l o s i o n s i n o i l -i n s u l a t e d e l e c t r i c a l equipment h a s r e s u l t e d in a s e r i e s of t h e o r e t i c a l and e x p e r i m e n t a l s t u d i e s , m o s t l y on d i s t r i b u t i o n t r a n s f o r m e r s , and f u r n i s h e d a means of i d e n t i f y i n g and u n d e r s t a n d i n g t h e fundamental mechanism t h a t may l e a d t o a c a t a s t r o p h i c f a i l u r e .In some c a s e s , approximate q u a n t i t a t i v e r e s...

Numerical Method for the Investigation of Fault Containment and Tank Rupture of Power Transformers

“…The corresponding mass inflow rate is given as (2) where is the total fault energy and is its duration. The injection velocity is set at every time step from the mass inflow-rate definition (3) where is the total area of the faces used in the mesh to inject the gas and is calculated by (1). Once the fault duration is complete, the velocity is set to zero.…”

“…2) Air and Gas From Oil Decomposition by the Arc: Air and the generated gas from oil decomposition by the arc are modeled as perfect gases following the gamma equation of state given by (1). The constant used is 1.4 for the air and 1.352 for the gas.…”

“…The density is calculated at every time step with the pressure of the cell where the gas is injected, using the perfect gas relationship (1) The velocity is calculated from the mass inflow rate of gas which later is assumed to be constant to simulate an arc of constant power throughout the duration of the fault. To determine the mass inflow rate, the chosen value of generated gas quantity from the arc is 85 cm per kiloJoule of fault energy at standard pressure and temperature.…”

Numerical Methods for the Analysis of Power Transformer Tank Deformation and Rupture Due to Internal Arcing Faults

“…Mahieu assumed that the oil is incompressible, based on which the amplitude of pressure induced by an arc in a gas blanket located at the top of the tank was computed. In 1988, a more accurate method was invented by M. Foata, M. Iordanescu, and C. Hardy, who built a 2D potential flow model and used finite element method to study the dynamic response of transformer tanks under overpressure regardless of the fact that the oil is compressible and viscous [ 3 ]. Later in 2003, J.…”

Design and testing of an arc resistant power transformer with a reinforced turret

“…Our experimental method assumes a test equipment containing an ideal gas undergoing an adiabatic process, with no heat exchange with the outside, and that all the energy leaving the pressure vessel enters the test object at the end of the injection system. Considering these assumptions, the change in internal energy can be express as ΔE (J) [27]:…”