Transformer oil

Transformer oil or insulating oil is a highly refined mineral oil or a low-viscosity silicone oil, which is stable at high temperatures. It is used (for example, in power electronics ) in high voltage engineering in transformers, capacitors and switches for isolation, spark arresting, lubrication (eg on the step switch ) and for cooling.

General

The oil in the transformer oil-insulated transformers housing serves both the insulation of the windings and the cooling. The cross-section of the winding wires is limited due to the limited wrap area. Therefore, current heat is generated inside the coil. Large transformers are indeed much more efficient than small, however, the internal heating of the windings come at them harder to outside. The oil filling increases the heat transfer and also fills all the gaps, so that the insulation is improved.

The oil is from the heat by convection to the transformer housing, which has similar radiators, in turn, to the outside cooling fins or heat exchanger. Large transformers have an additional fan for cooling said heat exchanger.

Insulation fault in the oil are healed to some extent, by continued flow decomposition products. This phenomenon is monitored with a mounted on the expansion vessel Buchholz relay, there formed by gases in the oil are recorded. The Buchholz protection insulation fault can be detected early and the risk of fire can be restricted.

A disadvantage is however, the flammability of the oil at high temperatures. The considerable amount of oil provides a high fire load. Loose oil transformers, so-called dry-type transformers, usually with epoxydharzisolierten windings ( cast resin ) are built. But this isolation has no self-healing and lower stress resistance and poorer cooling properties. Another method to reduce the risk of fire, was for a time ( 1960s), polychlorinated biphenyls (PCB) or polychlorinated terphenyls ( PCT) to use as an insulating or to mix this. These oils have excellent insulating properties, are less or non- combustible, but already banned toxic in contact with skin and why as well as their lack of degradation in the environment since 1999 also in existing systems from a content of 50 ppm.

As an environmentally friendly alternative to PCBs, but also as a replacement for the classic mineral oil, for more than 25 years, synthetic organic esters are used in transformers. These are saturated Pentaerythrittetrafettsäureester that are by a high point (> 300 ° C), flash point (> 250 ° C) and a high ignition temperature ( > 375 ° C) and thus distinguished as PCB difficult to ignite. In addition, the ester fluid is non-toxic, biodegradable and easily classified as not hazardous for water. The insulating properties are comparable to those of mineral oil, which is conventionally used as an insulating oil in the transformer, and with those of the PCB.

Another property of the esters of the liquid in comparison to mineral higher thermal resistance. This is due to the thermally stable ester groups. Due to the higher thermal stability of synthetic esters as organic insulating results in the possibility of building smaller and more compact transformers, and operate with higher performance. There are higher operating temperatures possible, which is especially for traction transformers (eg, locomotive transformers) is important.

The insulation system of a large distribution transformer or machine consists of a solid insulation around the conductor and an insulating paper wound cellulose such as mineral oil or even liquid ester. The water content in a transformer has a significant influence on the dielectric strength and the life or useful life of a transformer. Water results in the transformer for the hydrolysis of cellulose paper and thus to lower the degree of polymerization. For this reason, breathing transformers have a dry cartridge, the ambient air removes water before it comes in contact with the insulating oil. Hermetically sealed transformers have no direct contact with the environment. Despite these measures, all transformers water contained in the ppm range. This originates from the cellulose ( containing in the dry state approximately 1 % water) of the aging process ( oxidation of the insulation material ) and from the ambient air as the drying cartridge in particular in rapid cooling down of the transformer does not remove completely the water from the air. A transformer is closed over the entire useful life never completely isolated from the ambient air.

Due to the different polarities of cellulose ( e = 5.1 ) and mineral oil ( e = 2.2 ) is the most water in the solid insulation. Esters are due to their molecular structure of polar (e = 3.3 ) than mineral oil and have a much higher water solution assets (ester: 2700 ppm at 20 ° C; mineral oil: 44 ppm at 20 ° C). When using liquid ester as an insulating and cooling liquid in the transformer raises another water balance in comparison to mineral oil. It is clearly solved more water from the cellulose by the ester fluid. This reduction of the water in the cellulose hydrolysis, and therefore the aging of the insulating system is slowed down, and extends the operating life of the transformer.