Partial Discharge in Cables
Partial Discharges in the insulation are the main cause of ageing in high voltage paper insulated cables. The discharges are responsible for much of the deterioration seen in the insulation wall, such as pinholing and electrical treeing, and eventually lead to the breakdown of insulation.
Off-Line Cable Mapping
| On-Line Cable Monitoring
|All 3 phases can be tested Individually||Cable must be removed from service for testing||Cable can be tested in service||Unable to test individual phases to earth|
|Testing is between phases as well as to earth||am radio interference if close to a transmitter||Unable to test between phases|
|Inception voltage can be recorded if below working Voltage||Can only test at working voltage|
|Test voltage can be raised above working voltage to anticipate tap changes||Cable boxes must be insulated from cable sheath|
|No need for insulating cable sheath from cable box||Cables on ringmains require sectionalizing|
|Can be used to commission new cable installations||Signals can originate from other feeders connected to the busbar and/or switchgear|
PD Cable Mapping
The technique uses a Very Low Frequency (VLF) power supply to energise long lengths of cable from a standard single phase mains outlet. The 40kV peak VLF power supply is van mounted and has a fully variable voltage and frequency output allowing testing of HV cables up to 33kV with 30% overvoltage.
Associated Instrumentation provides information on any PD activity which is analysed using a pc based software package and is finally displayed as a partial discharge map. The map highlights potentially problematic areas of cable and allows comparisons to be made between different phases and joint positions.
Typical Test Criteria
- Cable isolated and open circuit at remote end
- Can be tested from switchgear spouts, cable box or pole termination in yard
- PILC circuits up to 3km can be tested, usually to 20% overvoltage
- Ideal for commissioning new XLPE circuits, 8km successfully mapped, 30% overvoltage
- Test frequency of 0.1, 0.2, 0.5Hz normally used dependant upon cable length
- Discharge occurring from half way and confined to within 10-20% of Cable length, in this case 37-74m, damage most likely caused during installation or by a 3rd party contractor.
- Note the low inception voltage indicating that discharge is normally always present, and the relatively high levels recorded for each phase both average and maximum values.
- Finally, a failure was reported nearly 2 years after test as shown on this map demonstrating the unpredictable nature of Partial Discharge and its damaging influence on cable systems.
Paper cable circuit No 1
- The most obvious feature here is the 200m section of discharge approx 100m out from End 1, particularly on Blue phase.
- Note also the high number of joints in this location indicating continual failure / ineffective repairs.
- This circuits reliability can be greatly improved with the replacement of this small damaged section of cable while removing the number of potentially problematic joints at the same time