A very low-frequency test voltage is applied across the equipment whose insulation to be tested.įirst, the normal voltage is applied. The cable, winding, current transformer, potential transformer, transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. Now, from the diagram above, it is cleared, the ratio, I R to I C is nothing but tanδ or tan delta. Now, total leakage electric current I L(I c + I R) makes an angle δ (say) with y-axis. resistive component of leakage current, I R will also be along x-axis.Īs the capacitive component of leakage electric current I C leads system voltage by 90 o, it will be drawn along y-axis. In the vector diagram above, the system voltage is drawn along the x-axis. Sometimes it is also referred to as dissipation factor. This ratio is commonly known as tanδ or tan delta. For good insulator, this ratio would be quite low. In another way, the healthiness of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component. Hence, it is needless to say that, for good insulator, this resistive component of electric leakage current is quite low.
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