Functional performance criteria for comparison of less flammable transformer oils with respect to fire and explosion risk

Sammanfattning: Power grids worldwide are expanding not only driven by ambitious clean energy, but also because of the rising need for reliable energy. Key components of these power grids are transformers. Transformers are traditionally filled with mineral oil, to serve as a coolant and dielectric insulator. Now globally a rising trend is observed towards the adoption of less flammable, biodegradable transformer liquids at ever increasing voltages and power ratings. The objective of this report is to discuss past, current and future attempts to quantify the fire and explosion risk in less flammable liquid filled high voltage transformers. Testing procedure standards that give a reliable assessment of the fire behavior of electro technical insulating liquid based on relevant physical characteristics of the fluids are currently under development, such as IEC 60695-8-3. However more effort is required in order to provide meaningful information concerning the relation between small-scale tests and large-scale tests and that between the tests and failure scenarios in real life applications. The experimental focus of this report, small-scale comparative tests in the Cone Calorimeter and other settings, is limited to pool fires. Spray and vapour/gas cloud fires and explosions, even though of great importance, are not considered. In total 5 liquids were tested: mineral oil, silicone liquid, synthetic ester and 2 natural esters. The comparative tests display a wide range of fire properties for the respective liquids. The higher the fire point the longer it takes for a liquid to ignite. Polluting the samples with 3% mineral oil decreased the time to ignition, especially for the natural esters. The heat release rate calculated from the cone experiments show analogies with the heat of combustion values tabled, except for the silicone liquid where a crust formation on the liquid’s surface impeded combustion. Heat losses from the burning surface to cooler liquid below or boundaries greatly also affect the burning behaviour. These complexities result in the fact that great care should be taken when scaling this small scale burning behaviour to use in fire safety applications.