What is TAN DELTA testing of transformer?

Tan Delta is also known as the 'loss angle' or 'dissipation factor'. It is used to test the level of degradation in insulation materials of electrical machines and power cables. The winding insulation of an electrical machine, or a cable free from defects, will act as a perfect capacitor, that is, it will only store energy and not dissipate it.
In an ideal capacitor, the voltage and current are out of phase by 90 degrees and the current flowing through the insulation is purely capacitive Ic. However, dielectric losses within the insulation due to deterioration or other factors, will result in the reduction of the resistance in the insulation. This will in turn increase the resistive current Ir. The phase angle between the current and voltage is now less than 90 degrees, and the extent to which this is less than 90 degrees gives the level of degradation in the insulation. So this 'loss angle' is required to be measured.
If the angle is Delta, then the tangent of Delta can be calculated by dividing is 'opposite over adjacent', which is Ir/Ic.
There are various methods of how this is tested and industrial standards, such as the IEC provide acceptable values of Tan-delta for different types of insulation.

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