Electricity + Control March 2019

HAZARDOUS AREAS + SAFETY

Once ignited, a liquid will sustain fire if the heat of combustion is greater than the heat required to reignite. Since the heat of combustion of conven- tional mineral oil greatly exceeds that of re-igni- tion, it will not self-extinguish, but more important- ly will propagate fire across the complete surface of a pool. For high fire point liquids the heat of combustion is not sufficient to sustain ignition or propagate a pool fire. Even if ignited, once the heat source is removed, the fluid self-extinguishes. When arcs occur under oil, only a small amount of the energy goes to raise the liquid temperature. Heat is concentrated at the arcing point, but the larger pool of fluid serves as a heat sink and only a small quantity of the liquid sees a large rise in temperature. Most of the arc energy is consumed, tearing apart the fluid molecules and producing combustible gases. Failure modes in transformers In order to understand how less-flammable liquids reduce risk, it is necessary to understand trans- former failure modes and the physical and chemi- cal nature of the materials used in manufacturing transformers. Many transformers fail because their insulation systems are no longer able to withstand stresses created during naturally occurring events (such as lightning, switching impulse, overloading, ferro-res- onance, secondary short circuit, line fault, etc.). Events such as those outlined above cause the insulation system to experience localised stress; the insulation system is unable to withstand the stress, and a failure occurs (typically somewhere in the paper). From this failure, a low or high re- sistance arc generates, and continues until it is extinguished (either the source is removed or the arc-gap distance becomes so large that the dielec- tric liquid quenches it). Much depends on how long the arc survives. During the life of the arc, most of the energy as-

sociated with the arc (approximately 95%) is con- suming/destroying the materials surrounding the arc. The remaining energy is dissipated by heating the surrounding materials. For the dielectric liquid, this means the arc is tearing apart the molecules, generating combus- tible gases (acetylene for example) at a substantial rate. As long as the arc survives, pressure quickly continues to build inside the transformer tank. The rapid buildup of pressure and the ability of the tank and components to withstand the physi- cal challenges being placed upon them determine the potential impact of the catastrophic failure. If the arc is extinguished quickly, and the tank and components withstand the pressure such that no venting occurs, the transformer simply stops operating. If, however, the arc is sustained, the pressure builds to a point where the tank or com- ponents cannot withstand the stress. The weak- est part of the tank will be compromised (i.e. a bushing is dislodged, or a weld gives way, etc.) and volatile gases will escape. Mixed with air and an ignition source, the gases will explode, causing substantial damage. When this occurs, heat released from the in- itial burning of combustible gases may vaporise and burn a dielectric liquid that is close to its flash point. If the heat of combustion continues to va- porise the liquid, a sustained fire on top of the liq- uid will result. It is at this point that the characteristics of the dielectric liquid become paramount. The pre- dominant dielectric coolant is mineral oil. Mineral oil, while exhibiting reliable dielectric properties, typically does not provide an adequate margin of fire safety during transformer failure. Mineral oil, with a fire point of approximately 160°C, requires significantly less energy to bridge normal operat- ing temperatures outlined in IEEE C57.12.00 and IEC 60076-1 to its fire point as compared with less-flammable liquids.

In small and large scale tests, the fire resistance properties of natural ester have been shown to be superior to those of other K-class fluids.

Electricity + Control

MARCH 2019

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