Tan Delta Testing of Insulating Oils in Transformers

Jun 1, 2018
Posted by Staff

Monitoring and testing of oil quality is a critical part of maintenance routines of oil-filled electrical equipment to ensure that their operation is continuous and reliable.  Oil performs several important functions, including insulation, cooling, and minimising contact of oxygen with cellulose and other materials in transformers that are susceptible to oxidation. 

One of the fundamental tests of oil quality is the dielectric dissipation factor test, otherwise known as Tan Delta, which determines the overall quality of the oil by indicating contaminates. Results of the test can form the basis for whether a transformer is suitable to continue in its service, or if replacement or regeneration of transformer oil is needed.

Tan Delta is performed by applying an AC voltage to a test cell of known gap, measuring the total current flow through the oil, and separating and comparing the reactive and resistance portion of the current passing through the oil.  If the insulating oil is free from contamination, the oil and the electrodes that it separates (e.g. the transformer HV windings and the grounded tank) closely exhibit the properties of a perfect, parallel plate capacitor.

Megger has developed a new insulating oil tester – Oil Tan Delta (OTD) – which measures Tan Delta, relative permittivity and specific resistivity of insulating oils in transformers.  Productivity in the laboratory is maximised, as the OTD was designed with the focus on reducing the test cycle time by forced cooling and inductive heating, together with an oil drain facility negating the need to move the oil vessel. User safety is also paramount and with this in mind, Megger have developed independent and dual redundant high voltage cut-off circuitry. If for any reason the user needs to abort the test, this can be done so by pressing any key, immediately terminating the test and removing the high voltage.

Each test that the OTD performs adds a piece of the puzzle combining to reveal an overall picture of the condition of the insulating oil, but the tests themselves are effective at identifying the presence of particular types of contaminants, including humidity, polar contaminants, oxidation, metal compounds, aldehydes, ketones and alcohols.  Testing for contaminants is an important process when purchasing unused new oil, determining the progress of the oil purification process and for quality control of oil in service.

The IEC and IEEE provide test standards that are applicable to specific groups of transformer oil, namely New Oil, Oil in Service and Reclaimed Oil.  Each of these test standards specify how to interpret results of Tan Delta by comparison of measured results with published recommended levels that are transformer specific.  Further details of these standards are detailed in Megger’s application note: Oil Tan Delta – Concepts, standards and testing.

To find out more information on Megger’s new Oil Tan Delta test set, visit our website.


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