Insulating fluids are used in many types of high-voltage electrical equipment, such as transformers, switchgear, capacitors and cables. The insulating liquid has two main functions: dielectric and coolant.
Additional functions of insulating liquid are arc quenching in circuit breakers, protecting the internal insulation from outside air and moisture and providing samples for diagnosing the equipment.
Due to these characteristics, insulation fluid has been widely used for a hundred years. It has high insulating properties for small equipment dimensions, low viscosity for effective heat transfer; pour point suitable for winter conditions. Equally important is the low cost of producing insulating fluid with the desired characteristics, which remain unchanged during long operation. The price of transformer oil, though rather high, is lower in comparison with alternative dielectrics.
Improving the conventional technologies of manufacturing insulating oils (distillates of petroleum, controlling hydrocarbon compounds) lead to improving the quality of these oils, but do not eliminate the main drawbacks of their application, which are related to their origin and manifest themselves in electrical accidents. These accidents occur due to short-circuits inside the equipment, with high electrical currents and high temperatures, sufficient to decompose hydrocarbons in oil forming gaseous products. The consequences of this are equipment damage, oil leaks and fires.
The first flaw of many oil products is a fire hazard. The second flaw is soil pollution with poorly biodegradable products when oil is dumped. And finally, crude oil is not a renewable material (by the way, not all types of crude are suitable for manufacturing insulating oils), and its reserves are gradually running out.
It is logical that the properties of insulating liquid (dielectric and coolant) are related to the design of the equipment where it is used. The equipment type, voltage class, operating conditions and economic factors affect the type of insulating liquid used in it. The liquids are not universal in terms of use, requirements and applications.
The internal insulation is a composition of several dielectrics. Generally, it is a combination of solid, liquid or gaseous dielectrics. At atmospheric pressure, a liquid dielectric (insulation liquid) has an advantage over gaseous, having high dielectric strength, large dielectric constant, and a high thermal conductivity. Therefore, insulating liquids will be used for a long time to come.