Answer / dinesh lokhande

Transformer bushings OIP (Oil impregnated paper) consist of
paper foil surrounig conductor which termiantes at test tap
and oil is filled in between paper. This arrangement is
equivlent to no of capacitances in series.

To check healthiness of deielectric we conduct Capacitance
Tan detla test from specially designed apparatus which
gives values of capacitance and tan delta which can be
compared to Factory test results and designed values.

Vector disagram is as shown below.

Ia* *Ic
* *
* *
*D *
* *
**
*************** V

In above vector we can explian as follows.
From property of ideal capacitor currnet vector Ia leads
Voltage vector V by 90 deg.

However due to dielctric losses, Calculated or charging
current Ic lags behind actual current by angle delta (D -
dielctric loss angle).

Hence Tan delta calculated is dielectric loss.

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