**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.

For Transformer Specifications visit :-

http://electrialstandards.blogspot.com/2015/10/transformer-technical-specifications.html

For Transformer Specifications visit :-

http://electrialstandards.blogspot.com/2015/10/transformer-technical-specifications.html

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(tanδ)= Ir/ Ic= (V/R)/ (Vx
2πfC)

=1/(2πfCR)

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.

.