Sunday, October 11, 2015

Tan delta test; loss angle test; dissipation factor test

Tan Delta Test
Tan delta test is also known as loss angle test and dissipation factor test.

This method works on principle that any insulating material acts as a capacitor in its pure state when connected across power supply. In a pure capacitor current leads the applied voltage by 90 degree. But there is hardly any pure insulator as over the period of life there is always a resistive component in insulators. Also due to ageing dirt and moisture enters into these insulators or insulating materials leads to resistive component. Thus by knowing ratio, of resistive component of current to capacitive component of current ,life of insulator or insulating material can be determined.

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Tan delta test is conducted on Insulation of cables, windings, Bushings of Transformers. This test is uses to check the life of cable and deterioration of insulation . By conducting this test it will gives idea about ageing process of cable, winding, bushing which will useful for prediction of life of cable, winding and bushings.

Tan Delta Testing Methodology
There are following steps in Tan delta testing:-
1.     In first step, Insulating material or insulator (Cable, winding, CT, PT, Transformer bushing) undergoing Tan delta test is first isolated from the circuit.
2.     In second step, a very low frequency voltage is applied across equipment whose insulation is under test. This voltage is equal to normal voltage is applied.
3.     If in second step value of Tan delta is good enough then applied voltage level is raised to 1.5-2.0 times of rated voltage.

4.     Tan delta controller used of measurement of Tan delta values take the readings of tan delta.
5.     A loss angle analyzer is connected across tan delta controller to compare values of tan delta at different voltage levels.
This should be kept in mind that test voltage applied to be at very low frequency.

There is following reason to keep frequency to very low:-
When voltage is applied at high frequency then capacitive reactance of insulator becomes very low, which will leads to high capacitive component of current. Resistive component doesn’t depend on frequency instead depends on applied voltage and conductivity of insulator. Thus at high frequencies capacitive current is large and leads to high amplitude of vector sum of capacitive and resistive components of currents. This will leads to high apparent power for conducting Tan delta test which is practically impossible. That is why tan delta test is done at low frequencies.
Usually frequency range for conducting this test is from 0.1 to 0.01 HZ.

Tan delta test; loss angle test; dissipation factor

Tan delta(tanδ)= Ir/ Ic= (V/R)/ (Vx 2πfC)
Thus we see that tanδ  1 / f

Thus we see that lower will be frequency higher will be the tanδ value, so measurement values will become easier.

Test results

From tan delta test we will be able to ascertain the condition of insulation due to ageing by following:-
1.     If we have previous tanδ results then new tests values will be compared with previous results to ascertain condition of insulation.
2.     If tanδ  values will come same for all test voltages then insulation will be perfect. If there is weakness in insulation then tanδ values will increase with voltage level.