Electricity + Control March 2019

HAZARDOUS AREAS + SAFETY

K-class fluid and the prevention of fire in transformers

By Roberto Ignacio da Silva, Cargill (BR)

Dielectric liquids with a high fire point, also called K-class fluids, are defined as having a fire point greater than 300°C. K-class fluids offer significantly better fire re- sistance than conventional mineral oil with a typical fire point of 160°C. The ability of K-Class fluids to reduce the initial intensity of a transformer catastrophic failure and prevent an oil fire has been confirmed by test results.

Take Note!

The higher fire point of K-class fluids makes them almost impossible to ignite under realistic transformer conditions of operation. Once ignited, a liquid will sustain fire if the heat of combustion is greater than the heat required to reignite.

1

2

T he fire point is the most important property for determining fire safety of a transformer fluid. When a liquid-filled transformer op- erates at full (100%) load in a 40°C temperature ambient, the oil temperature is approximately 100°C. In case of failure of electrical protection devices, heating from an internal arcing fault or chronic overloading can increase the fluid temper- ature and put conventional mineral oil at risk of ig- nition. The higher fire point of K-class fluids makes them almost impossible to ignite under realistic transformer conditions of operation. In the more than 40 years of use of silicone, high molecular weight hydrocarbons, natural es- ter, or synthetic ester liquids, no transformer fire has been reported. Specifically, natural ester has the best-in-class fire point (natural ester FR3 fluid with 360°C versus synthetic ester with 316°C) as shown in Figure 1 . In small and large scale tests, the fire resistance properties of natural ester have been shown to be superior to those of oth- er K-class fluids. Also, natural ester has a flawless fire safety record in over two million transformers and 23 years of use. Liquids ignition To ignite, a liquid must reach its fire point and be exposed to an ignition source in the presence of oxygen. As a liquid is heated, vapour concentra- tion above it increases. Vapours ignite initially and

combustion is sustained on the fluid surface. The temperature at which a liquid ignites depends on the geometry of the test. Standardised fire point tests use a defined geometry and ignition source. In the Cleveland Open Cup test method, a small volume of liquid is heated using a standard test cup, heater and thermometer. A gas flame is pe- riodically passed over the surface. Flash point is defined as when a flame appears on the full flu- id surface before self-extinguishing. Fire point is measured when the complete surface of the fluid stays ignited for at least five seconds. Lower fire point correlates directly to easier ignition. Auto ig- nition temperature is usually around 50°C higher than the open cup fire point.

Dielectric fluid fire point

400

360

350

316

300

˚ C

250

200

160

150

Mineral oil Synthetic FR3 fluid

Figure 1: Dielectric fluid fire point comparison.

22 Electricity + Control

MARCH 2019