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Transformer oil testing

Transformer oil testing is a key part of any preventative maintenance programme and should be conducted on a regular basis.

Electrical transformer

Consider it an early warning system that helps set priorities, schedule work, and arrange for outside services and order parts and materials. Listed below are some of the most common tests for transformer oil:

Dissolved Gas-in-Oil Analysis

A Dissolved Gas-In-Oil Analysis (DGA) is the most important test for transformer oil and can give an early indication of abnormal conditions. This test analyses the type and quantity of gases dissolved in the oil by using a small sample that is examined in the laboratory. Certain quantities and combinations of gases typically found in the oil can indicate insulation overheating/overloading, liquid overheating, partial discharge (corona) or arcing in the transformer.

Screen Testing

The screen test is a collection of physical, electrical and chemical tests for transformer oil. Typically, a one-litre sample of oil is collected. No single test will indicate the true condition of the liquid, so it is suggested that all tests be performed:

  • Di-electric Breakdown
  • Interfacial Tension
  • Colour
  • Acidity
  • Power Factor (dissipation factor)

Other Tests

The water content test detects the moisture content in parts per million of the liquid insulation. A test to determine polychlorinated biphenyls (PCBs) on a percentage value, or as a parts-per-million value, is widely available from commercial laboratories.


It is recommended that DGA is carried out every six months on critical transformers. Screen tests should be carried out every three years.

Case Study

As a regular part of an insurance contract, a HSB engineer was carrying out DGA oil tests on large transformers at an electric utility. On the strength of the test results, the engineer recommended that one 156MVA transformer should be taken out of service to permit an internal inspection. This revealed burned contacts on the no load tap changer – a serious condition if left untreated. The transformer was repaired on site and returned to operation.

This quick response to the machine’s own danger signal saved the utility the cost of a replacement transformer; estimated at £600,000.

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