Answer / abhijeet

Tan delta is test carried out to check the insulation
proprties. it is nothing but the ratio of watt loss to
reactive power. insulation is a capacitor in series with
resistor. if we plot the phasor of Ir and Ic, we get delta
angle between Ieq and Ic. this angle should be as close as
to zero. as angle increases, the insulation properties gets
damaged and will conduct the current as in resistors.

Answer / sajid shaikh

Tan delta is called loss angle or dissipation factor
testing,is a diagnostic method of testing winding to
determine the quality of insulation.This is done to try to
predict the remaining life expectancy & in order to
prioritize replacement.

Answer / biswaindu parida

This tests are performed on Transformer Bushings or Ct bushings ( insulators) to check the Insulation properties.
As explained by some of my friends above it is the ratio of Ir/Ic.
Ic- capacitor current of the series of capacitance formed inside the insulators from top to bottom.
Ir- leakage current flowing through it ( which should not be flowing).
Why leakage current?
because no matter in this world is purely capacitive.
We build the insulators such that to perform as a series of pure capacitors. but due to insulation failure or due to dust deposition at the outer surface of bushing during rainy or moisture seasons some leakage current may flow. That is undesirable. thats decreases the performamnce of the insulators.
From Tan-delta test we can know the Ir values in comparision with Ic. It is tolerable upto certain extent, above which we have to take some action to avoid it.

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