what is the significance of tan delta testing?explain the
theory?

Answer Posted / m r bhanderi

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.

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