Cable diagnosis with TanDelta/loss factor measurement

TanDelta measurement is a non-destructive test that evaluates the condition of the insulation system as a whole. The ratio between the capacitive charging current (Ic) and the resistive leakage current (Ir) is measured. There will always be some leakage current in an insulation system, but an increased value indicates aging of the insulation. For cable systems, the TD values are checked against the criteria given in IEEE 400.2-2013.

How is TD measurement performed?

Using a voltage source, the voltage is increased stepwise from 0.5 to 1.5*U0. At the same time, the TD values are logged at all steps. This allows for the following evaluation criteria:

▪VLF-TD time stability (VLF-TDTS) at U0

▪Differential VLF-TD in mean TD between 0.5 and 1.5 U0

▪Mean VLF-TD at U0

The results of the test will be the following:

▪No action required

▪Further investigation

▪Measures must be initiated

Typical reasons for dielectric losses:

▪ Presence of water trees in XLPE insulation

▪ Moisture in joints and end terminations

▪ Corrosion in laminate with PD

▪ Poor contact between laminate and cable shield

▪ Drying in insulation of paper-insulated pulp-impregnated cables

▪ Damaged insulation in paper-insulated, mass-impregnated cables due to moisture

Neka uses two different technologies for TanDelta testing:

TanDelta diagnosis with Damped AC reveals:

▪Dipole losses are represented by insulation aging which can be detected by TD measurements above 10 Hz

▪Interface losses are represented by surface problems between materials that can be detected by TD measurements at 50/60 Hz

▪Localized high intensity discharging is represented by partial discharges that can be detected with TD measurements above 10 Hz

"DAC TD is particularly useful for detecting defects in paper cables"

TanDelta diagnosis with VLF reveals:

▪Conductive losses are represented by water tree growth in pex cable which can be detected by TD measurements below 1 Hz