“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."
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 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.
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.
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:
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.
Sheath Test / Serving Test