Tan Delta is an effective tool in determining the
insulation condition of power cables using a VLF-TD System
Tan Delta measures the resistive component of a cable
system; a perfect cable is all capacitance and has no
resistance. The resistive component is caused by voids in
the insulation, electrical trees, water trees etc. The
resistive element causes the voltage not to lead the
current by 90° (something less) This is difference in angle
(less than 1° ) is the loss angle. The tangent of this loss
angle is the Tan Delta / or the tangent of the loss angle.

Tan Delta is an effective tool in determining the
insulation condition of power cables using a VLF-TD System
Tan Delta measures the resistive component of a cable
system; a perfect cable is all capacitance and has no
resistance. The resistive component is caused by voids in
the insulation, electrical trees, water trees etc. The
resistive element causes the current not to lead the
voltage by 90° (something less) This is difference in angle
(less than 1° ) is the loss angle. The tangent of this loss
angle is the Tan Delta / or the tangent of the loss angle.

tan delta testing is usally carried out to check the
healthiness of insulation.so this testing is usally carried
out in cb ,ct, bushings etc ,for all these equipements one
standard value will be there,for breakers this value will
be 0.00073, if any devation in this value we have to
suspect about it insulation healthyness.

Ideally an insulation (be it across a cable or a electrical
switch gear) should be of infinite resistance. That is
ideally if voltage is applied across insulation no current
should pass through (considering resistive current only).
Now there is another aspect of an insulation. An insulation
is also a capacitor (two plates seperated by dielectric)
and hence there must be some current (opf course ac
current) that will flow if voltage is applied acroos
insulation (this is capacitive current). Now the ration of
the resistive current (which shoud ideally be zero) and the
capacitive current is called loss tangent or tan delta and
is measurable. So if tan delta is measured we are actually
get an idea of the resistive current through the
insulation. A high tan delta indicates high resistive
current and that means insulation is detoriating.

tan angle is the ratio of resistance current and
capacitive current ..it is used to estimate the life of
insulation by comparing periodical tandelta value of the
same insulation..pure insulation has only capacitive
current but with the insulation deteriorationor cavity
formation(may occur due ambient condition temp or cont.
over current flow) resistance is added hence resistive
current increase wich leads to increase in tandelta value..

Ideally an insulation (be it across a cable or a electrical
switch gear) should be of infinite resistance. That is
ideally if voltage is applied across insulation no current
should pass through (considering resistive current only).
Now there is another aspect of an insulation. An insulation
is also a capacitor (two plates seperated by dielectric)
and hence there must be some current (opf course ac
current) that will flow if voltage is applied acroos
insulation (this is capacitive current).& IDEAL CAPACITIVE
CURRENT IS ALWAYS 90 DEG AHEAD THAN VOLTAGE AS PER R-C
CIRCUIT VECTOR DIAGRAM.BUT DUE TO AGING EFFECT &
DETORIATION OF INSULATION , CAPACITIVE CURRENT IS SLIGHTLY
LESS THAT 90 DEG & ONE ANGLE CREATED BET IDEAL CAPA.
CURRENT & ABOVE SAID CURRENT IS CALLED TAN DELTA ANGLE.
HENCE WE CLEARLY SAY THAT TAN DELTA IS the ration of
the resistive current (which shoud ideally be zero) and the
capacitive current is called loss tangent or tan delta and
is measurable. So if tan delta is measured we are actually
get an idea of the resistive current through the
insulation. A high tan delta indicates high resistive
current and that means insulation is detoriating.

In general for electrical component, Ten delta means that the tangent value of insulating material. what is tangent value of IM? it is nothing but the dissipation factor. what is dissipation factor? The ratio of Power/ Voltage-Ampear (W/VA). so finally in simple words Tan delta testing is nothing but to find out the health of insulation OR insulating material.The test can be performed on Cables, power TR's or any electrical component.