tan delta test of a power transformer

Criteria tan d acceptance value for transformer is less than 0.5 at 20 deg. Celcius.

Regards,

Refer to what standard <0.5 at 20 deg celcius? What i understand that in IEC 60250 only explain about the calculation and how we do the test.

Can you please inform in what standard which saying above acceptance value?

Thx a lot.

IEC 60250 gives everything required to be known about tan delta test. The basic knowledge should be an understanding of tan delta. If you know that everything is clear.A pure insulating material act as a capacitance. The current will be 90 degree ahead of voltage. Depending on the impurity there will be a resistive component resulting an angle and the tangent of the angle gives the degree of contamination in the insulating materiel which is measured and IEC stipulate a safe limit value of the insulation which is considered acceptance level.