Transformer oil

Transformer oil

Transformer oil

Insulation and cooling are the primary functions of transformer oil. Dielectric strength must be high, thermal conductivity must be high, and chemical stability must be high.

 Typical specifications are: flash point 140° C or greater, pour point 40° C or lower, dielectric breakdown voltage 28 kV (RMS) or greater.

It is possible to circulate convection cooling oil through external radiators on a large tank filled with oil. Power transformers with capacities of thousands of kVA may additionally have cooling fans, oil pumps, and even oil-to-water heat exchangers. Power transformers are dried using either electrical self-heating or vacuum treatment to ensure that the transformer is 100% dry before adding insulating oil.

The result is the prevention of corona formation and subsequent breakdown of the electrical system. Transformers with an oil reservoir may have a Buchholz relay on the gas detector (oil reservoir).

These safety devices detect gas accumulation in transformers due to an electrical arc, corona discharge, or overheating. These devices can trip an automatic circuit breaker when gas is accumulating or a rapid pressure rise occurs. With no conservator, the transformer typically includes sudden pressure relays that serve a similar purpose to the Buchholz relay.

When a transformer is in use, the oil is under electrical and mechanical stress. Also present in high temperatures are contaminations caused by chemical interactions between insulation and windings. Over time, the chemical properties of transformer oil change, taking away its effectiveness for the intended purpose. Transformer oil is used in electrical apparatus of various sizes.

To ensure its suitability for further use, it is constantly tested for electrical and chemical characteristics. Some oils are treated and filtered in order to improve their condition. There are four types of tests:

  1. Dissolved gas analysis
  2. Furan analysis
  3. PCB analysis
  4. General electrical & physical tests:

Tests can be conducted using any of the methods identified in standards published by IEC, ASTM, IS, and BS. Tests like Furan and DGA aren’t meant for test oil quality. They are intended to determine any anomalies in the transformer’s windings or paper insulation that would not otherwise be detectable without a comprehensive overhaul. Recommended intervals for these tests are:

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