Insulating liquids in electrical engineering - General presentation

Electrical insulators were originally unimpregnated natural products that oxidized rapidly in direct contact with air. It was neither possible, nor obvious, to use liquids, although in 1854, windings were immersed in turpentine [1] , increasing their dielectric strength and operating life, and demonstrating that solid insulation could be protected from direct oxidation.

The first induction coils, primitive transformers and industrial transformers, built as early as 1884, were of the dry type. It wasn't until 1891 that petroleum oil was used on an experimental basis to insulate three-phase transformers; but the end of the 19th century still saw air-cooled devices rated at 1,000 kVA at 30 kV, and all ventilated from 50 kVA upwards. With the multiplication of electrical power distribution networks and the increase in installed power, air transformers became enormous. To reduce their volume, the use of mineral oil became widespread in 1905. In 1917, the first cable with a hollow conductor and fluid oil was produced [2] and in 1918, mineral oil-impregnated capacitors were introduced [3] . In France, the first 70 kV power distribution line appeared in 1921, followed by the 220 kV network in 1932 and the current 400 kV network in 1960 [4] . While higher voltages exist abroad (550 and 700 kV) or are planned (1,500 kV), voltages have not changed in France; on the other hand, unit powers have doubled every six years [5] .

This increase in power, while voltages remain unchanged, shows that the primary role of insulating oil is more to evacuate energy losses than to perform its dielectric function. This is true for transformers and reactors, but not for capacitors or oil-filled cables. For these materials, the dielectric role predominates. At the end of the 1920s, with the advent of askarel, a synthetic liquid, the need to use safety liquids revealed the possibility of creating suitable molecules, with permittivities greater than those of mineral oils, for example.

Gradually, the electrical parameters became more precise, under the influence of various constraints on use. Askarels, suspected as early as 1966, were progressively banned in all countries. This led to a flowering of new liquids, which posed the problem of choice and the definition of selection criteria [6] .

In capacitors, they were replaced by a first generation of synthetic...