Tan delta

Dear Sandeep,

Tan delta is the ratio of the power dissipated in the insulation material of the transformer (transformer oil)to the power applied. Equal to the tangent of the loss angle, or the cotangent of the phase angle. The dissipation factor can be calculated using:
D = tan δ = cot θ = 1 / (2π f R_pC_p)
where δ is the loss angle,
θ is the phase angle,
f is the frequency,
R_p is the equivalent parallel resistance,
and C_p is the equivalent parallel capacitance.

Should also note that for small loss angles of less than 10 degrees, the tangent of the loss angle is essentially the same as the cosine of the phase angle which is the power factor. For small power factors, C tan delta is simpler, and more accurately measured by means of a Schering or modified Schering Bridge.

dear...its useful information. our trf. is 6.3/8 mva and its vector group is ynyn0. so i guess phase angle is 0. but i dont know loss angle? wat is it? and how we calculate ?

As mentioned by Blue stone , Loss angle is measured by modified schering bridge or by any automated dissipation factor measuring instruments (EX: Automated C, L & tan δ (Dissipation Factor) Measuring Bridge by Haefely test AG instruments).

Ok.....but how we define loss angle?

Any One else??

Loss angle is the angle by which the resultant current (leakage current in transformer oil) differs from ideal 90 degrees phase angle relative to the voltage.