CABLE DIAGNOSTICS TESTING

CABLE DIAGNOSTICS

Insulation Resistance Test

Conductor Resistance Test

Over Voltage Pressure Test

Definition: SANS 10198-13:2016

“AC voltage testing method utilizing AC signals at frequencies in the range 0,01 Hz to 1 Hz, As per SANS 10198 Cables will be subjected to VLF 0.1Hz Over Voltage Acceptance or Maintenance Pressure Test for medium voltage cables of up to 33kV.60min per phase. Each Phase is tested individually while the other phases are connected down to ground."

Tan Delta (TD)

The TD test ensures that an initial footprint of the cable is taken which points out the overall condition of the cable insulation. On an annual or routine basis this test should be repeated to check cable insulation and related degradation. If the TD results are above the normal standard and / or indicate cable dielectric deformation, the cable should be subjected to PD test.

Tan Delta Stability (STD Stability)

Tan Delta Stability is a very important criteria in analyses of the possible causes of insulation degradation. It is used to pick the presence of wet joints and partial discharge activity in the cable system.

Delta Tan Delta (ΔTD)

Delta Tan Delta is defined as the difference in Tan Delta values taken at 1.5U0 and 0.5U0. This provides a good indication of partial discharge activity and presence of water trees in the cable system.

Tan Delta Connection Diagram

Online Partial Discharge (PD) Testing

Non-destructive online partial discharge measurements are measured using highly sensitive inductive sensors. Partial discharge activity is recorded during a full cycle of plant events. Recorded data is then analysed to check for any harmful PD.

Offline Partial Discharge (PD) Testing

Definition:

“IEC 60270, partial discharges are localized electrical discharges that only partially bridge the insulation between conductors and which can or cannot occur adjacent to a conductor."

Partial Discharge diagnostics allows a precise analysis of cables and their joints and terminations. However, PD measurements are influenced by the type and location of the defect(s), operating and testing voltage magnitude, circuit operating conditions, type of insulation material, ambient noise etc. Therefore accurate interpretation of PD data requires sound knowledge of temporal PD behaviour. The basic goals of PD interpretation are to:

Distinguish true signals from background noise,

Establish that the PD signals are located within the cable being tested,

Confirm that the PD poses a risk to the cable system.

There different type of PD that can be found within a cable and or it accessories, as explained below:

Internal PD - is embedded in the insulation material, e.g. a cavity in the cable insulation

Interface PD - is embedded in the insulation material but originates in the junction of two different insulation materials, e.g. cable termination / joint

External surface PD - is external PD occurring on the surface of termination, spacers or bushings, normally due to pollution and humidity

Corona PD - is discharges in air initiated due to HV connections and due to sharp edges of conductors or grounded metallic casings or metallic objects.

External Disturbances - are discharges coupled into the measurement circuit from external signals.