Delta Testing of Power Transformer

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