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Capacitance and Tan Delta

What is capacitance?

Two conductors which are separated by a distance can store an electric charge between them. Capacitance describes the ability of two conductors, separated by an insulating material, to store charge. Thus all the insulating materials are dielectric and are supposed to behave as capacitor when connected to line and earth.

Ideally the 100% pure insulating material which is also dielectric in nature should only have capacitive component in the electric current passing through it. There is no resistive component as there is no impurity. Capacitive current component leads the voltage by an angle of 90°, whereas the resistive component is in line with the voltage.

What is tan-delta or dissipation factor?

But as we know, practically a 100% pure insulator is not possible. And also due to age, continuous electrical stress, moisture, dirt and all such impurities and factors a conductive path for current is formed in an insulator resulting to leakage current. This leakage current also have resistive component.
This is needless to say that for good insulator this resistive component of electric leakage current is quite low. We can also say that the healthiness of an electrical insulator can be determined by the ratio of the resistive component to the capacitive component. For good insulator, this ratio would be quite low. This ratio is commonly known as tanδ or tan delta. It is also referred to as dissipation factor (DF).

In the vector diagram above, the system voltage is drawn along the x-axis. Conductive electric current i.e. resistive component of leakage current, IR will also be along x-axis.
As the capacitive component of leakage electric current IC leads system voltage by 90o, it will be drawn along y-axis.
Now, total leakage electric current IL(Ic + IR) makes an angle δ (say) with y-axis.
Now, from the diagram above, it is cleared, the ratio, IR to IC is nothing but tanδ or tan delta.

Note: This δ angle is known as loss angle.

This test is very important for High-Voltage Bushings, Power Transformers, Generators, Power Capacitors, H.T. cables etc. Power Electronical manufactures a variety of Tan-Delta test systems to serve the different applications of testing Transformers, Oils, Generators, XLPE Cables, and Capacitors etc. Our products such as PE-ACDF-1 [12kV, 200mA], PE-HVTD-12/100 [12kV, 100mA], and PE-HVTD-XX/YYY [customizable up to 15kV, 500mA] are mainly used for measuring the Capacitance and Tan Delta.

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