What is Tan Delta and why do we measure it?

The most common technique used to assess the insulation of primary equipment like transformer, rotating machine, cables, circuit breaker, is an AC test.

AC test is used to check aging of the insulation and mechanical deformation. What kind of electrical insulation is used today for electrical insulation?

  • Solid: Celulose (Paper or Pressboard), cast resin, porcelain
  • Liquid: mineral oils, silicone oils, synthetic compounds
  • Gaseous: Sulphure Hexafloride (SF6), air and vacuum, nitrogen

What is an electrical insulation?

IEEE defines electrical insulation as “material or a combination of suitable non-conducting materials that provide electrical isolation of two parts at different voltages”.

Now let us focus on the fact that we talk about two options - first: one material functions like an insulator and second: two or more different materials are used to function like an insulator. 
An example of having only one material as insulation is resin cast in dry type transformers, while an example for complex insulation could be an insulation system of power transformer, where we could use for example mineral oil, paper and pressboard to insulate. 
The advantage of measuring the Tan Delta is that  this method allows you to measure the status of the WHOLE insulation system, not just a material but the electrical insulation as a whole. We need to emphasize this because there are techniques that measure the status of only one insulating material within electrical insulation systems, for example - only oil and dielectric breakdown tests.
That particular test (dielectric breakdown oil test) is also good for its purposes, but the limit of that test is that it gives you olny the information of the state of the oil insulation and you would like to have the information on the whole - integral insulation system, not just an element of the insulation system.
Hence, when measuring the Tan Delta on a Power Transformer you measure the state of the insulation as a whole. What is also important to note here is why these aspects are emphasized. It is because in a Power Transformer - water - which is one of the main causes of insulation failure - usually resides in paper and pressboard (cellulose) and not in oil. Water molecules are more drawn to the cellulose insulation and if you only test the oil to see how much water you have in your insulating system, testing just the oil, will not give you a full information. That is why we do Tan Delta test. To see the full picture.