On-Line Partial Discharge Detection within MV and HV Cables
Background
The On-Line Partial Discharge (PD) testing of Medium Voltage cables (6.6kV to 36kV) and High Voltage Cables (66kV to 400kV) originated in the UK in the late 1990's through the use of split-core High Frequency Current Transformer (HFCT) and TEV sensors which are attached around the earth strap or cable core of the cable at the switchgear/transformer with the equipment 'live'.
The techniques employed in the new On-Line PD test techniques have evolved from Off-line PD testing of cables (with a VLF or other external HV Power Supply) which was pioneered in the UK in the 1980's. The move across to On-line pd testing over the past few years has been led by customer requests to avoid cable outages for insulation condition testing (because Off-line tests need the cable to be isolated at both ends and a portable HV power supply applied).
The test set-up is inherently safe as the HFCT sensor is connected either around the earth strap after it is 'brought-off' the cable.
Partial Discharge in Power Cables
- Effects all cables and cable accessories (joints and terminations) above 6.6kV
- Indicates the points of degradation in the cables and cable accessories
- Can be used for pre-fault detection of incipient faults to provide an 'early warning' against failures
Why carry out partial discharge testing and predictive maintenance?
- MV and HV Plant Commissioning Tests
- Ageing population problem
- To focus plant replacements and to avoid unplanned outages
- For networks with increasing failure rates or problems with a particular plant type
- Troubleshooting and failure investigations
- Safety (mostly with cable terminations, switchgear & sealing ends/outdoor HV plant)
- Quality Assurance checks
Partial Discharge Test Methods
- On-Line
- In-service under normal working conditions
- No need to isolate the circuit
- Circuit loaded when tested
- Economical and Non-invasive
- Teed circuits can be tested
-
Off-Line
- Energised with external supply: VLF, 50Hz, OWTS, and Resonant Test
Pros and Cons of On-Line vs Off-Line
On-Line and Off-Line PD test methods can be considered as being complementary; a combination of both will provide the most robust solution:
| ON-LINE: Advantages | OFF-LINE: Advantages |
|---|---|
| No need to isolate the circuit | Proven technology |
| Circuit loaded when tested | Better sensitivity than On-line |
| Economical and Non-invasive | |
| Teed circuits can be tested | |
| ON-LINE: Drawbacks | OFF-LINE: Drawbacks |
| Data interpretation can be difficult | Circuit not loaded during testing |
| Earthing Pre-requisites | An Outage is required |
| Expensive and Time-consuming | |
| Teed circuits cannot be tested easily |
Examples of Cables that can be PD Tested
- Polymeric (XLPE, PVC)
- Paper (PILC, MIND)
- Rubber (EPR)
- Both 3-Core and Single-Core Cables
- 'Mixed' cables with transition joints
For further detailed information, visit our Applications Notes page.