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Tan δ is a measure of the degree of real power dissipation in a dielectric material and therefore its losses. When a pure insulator is connected across live and earth terminals it acts as a capacitor and only the leakage current will flow. This leakage current leads the voltage by an angle of 90° (capacitive current). A pure insulator also acts as a dielectric. Greater the resistive component bad the insulator is. A good insulator has no or very small resistive component of leakage current. In real, the insulator material cannot be made 100% pure. Also due to ageing of insulator material the impurities like, dirt and moisture enter into it. These impurities provide conductive path to the leakage current. In this way the leakage current passing from line to earth has both capacitive and resistive component.
Tan delta test is performed on the transformer oil and bushings to determine their insulation loss (insulation quality). Oil insulation decreases when impurities like dust, sludge, oxidation products, moisture and polar contaminants present in it. Its insulation also decreases due to aging process.
