When assessing the health and reliability of electrical cables, particularly those used in power distribution and transmission, understanding insulation quality is crucial. Two key parameters often referenced in insulation testing are Tan δ (also known as the loss angle) and the dissipation factor. These metrics enable engineers to assess the condition of cable insulation and predict potential failures before they occur.
When an AC voltage is applied to an insulating cable, it behaves not just like a perfect capacitor but also shows some resistive properties due to the imperfect insulation. This resistive component causes energy dissipation within the insulation as heat, which is undesirable.
Tan Delta, like Power Factor, testing provides an overall assessment of the condition of the load tested, in this case, cable systems. What is the dielectric health of my cable from point A to point B? If cables are to be replaced if faulty, the goal is to test hundreds of similar cables, rate their health by several criteria, compare all the results, and develop a prioritized "hit" list of where we go next with cable system improvement efforts.
The cable is isolated from its load with the ends open and insulated from ground. An AC voltage source, usually from a VLF hipot, is applied to the cable in several step voltages, typically 0.5 Uo, 0.75 Uo, 1.0 Uo, 1.25 Uo, 1.7 Uo, 1.8 Uo, 2 Uo. for the voltage source, VLF @ 0.10 Hz or 0.05 Hz. is normally used for TD testing, although use other AC voltage sources can be used, with interpolation of the results required since frequency dependent: 50/60 Hz, AC Serious, or Resonant Systems Damped AC OWTS.