Electric Energy T&D - Index

Electric Energy T&D - EEMag March / April 2008 - Index

Prevention of Oil-Filled Transformer Explosions
By
Guillaume Perigaud Sébastien Muller Gaël de Bressy Ryan Brady Philippe Magnier
I. INTRODUCTION
Power transformers are one of the most dangerous electrical equipments
because of the large quantity of oil they contain which is in direct
contact with high voltage elements. Under such circumstances, low
impedance faults that result in arcing can appear in transformer tanks
once the oil loses its dielectric properties. Oil is then vaporized and
the generated gas is pressurized because the liquid inertia prevents
its expansion. The pressure difference between the gas bubbles and
the surrounding liquid oil generates a dynamic pressure peak, which
propagates and interacts with the tank. The dynamic pressure peak
interactions with the tank structure generate reflections, which build up
static pressure. Then, the static pressure rises leading to tank explosion
and possible fire resulting in very expensive damages for electricity
facilities, possible environmental pollution and human life risks.
To avoid such damages, the transformer explosion and fire prevention
strategy presented in this paper, here called Transformer Protection
(TP), is based on the direct mechanical response of a Depressurization
Set (DS) to the tank inner dynamic pressure peak due to an electrical
fault. Since transformers always rupture because of the static pressure
at their weakest point, the DS is designed to be this weakest point in
term of inertia to break with the dynamic pressure peak before the tank
explodes. Thus during a transformer short circuit, the TP is activated
within milliseconds by the first dynamic pressure peak of the shock wave
generated by the electrical fault and before static pressure increases. It
then depressurizes the tank by expelling the oil and gas mixture.
This fast direct tank depressurization method has been experimentally
studied by arcing tests in industrial size oil-immersed transformers. Physical
modeling and numerical tools, validated on collected experimental data,
have been developed in order to test the TP reliability in various operation
conditions. The following sections thus deal with:
• § , brief description of the protection;
• §3, the experimental campaign carried out on arcing in large
transformers;
Transformer Protector Corporation
• §4, the theoretical and numerical developments which results
prove the reliability of the whole prevention strategy.
II. DEPRESSURIZATION STRATEGY DESCRIPTION
Mitigating the effects of a transformer explosion consists in either limiting
the explosion consequences by extinguishing the subsequent fire (fire
walls or sprinklers) or preventing tank explosion by using mechanical
technologies that absorb the high overpressures generated by the
electrical arc, thus preventing the tank rupture and the subsequent fire.
The explosion prevention technology presented in this paper is of the
second sort. This fast-direct-tank-depressurization-based method activates
as soon as the high pressure peak of the pressure wave reaches it.
Figure 1: Transformer equipped with fast direct tank depressurization based method (TP)
Indeed, the electrical fault generates a dynamic pressure peak, which
travels at the speed of the sound inside the transformer oil, 1, 00
Meter per second (4,000 feet per second). This dynamic pressure
peak bursts a rupture disc located in the DS (Item 1 in Figure 1). Oil
and gas are then quickly expelled out of the transformer tank through
the DS (located in ) to an oil gas separation tank (item 3). The
explosive gases are then channeled away to a remote and safe area.
March-April 2008 Issue I
79