Answer Posted / amit shankar choudhary
Tan Delta, also called Loss Angle or Dissipation Factor
testing, is a diagnostic method of testing
cables to determine the quality of the cable insulation.
This is done to try to predict the
remaining life expectancy and in order to prioritize cable
replacement and/or injection. It is also
useful for determining what other tests may be worthwhile.
How Does It Work?
If the insulation of a cable is free from defects, like
water trees, electrical trees, moisture and air
pockets, etc., the cable approaches the properties of a
perfect capacitor. It is very similar to a
parallel plate capacitor with the conductor and the neutral
being the two plates separated by the
insulation material.
In a perfect capacitor, the voltage and current are phase
shifted 90 degrees and the current
through the insulation is capacitive. If there are
impurities in the insulation, like those mentioned
above, the resistance of the insulation decreases, resulting
in an increase in resistive current
through the insulation. It is no longer a perfect capacitor.
The current and voltage will no longer
be shifted 90 degrees. It will be something less than 90
degrees. The extent to which the phase
shift is less than 90 degrees is indicative of the level of
insulation contamination, hence
quality/reliability. This “Loss Angle” is measured and analyzed.
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