Partial Discharge Testing of Power Cables
A cable can pass a withstand test and still be dying. The withstand test proves it can hold voltage today. Partial discharge testing tells you whether it will still hold voltage in three years.
1. What Partial Discharge Is
Partial discharge is a localized electrical breakdown in the insulation — a spark that bridges only part of the insulation gap, not the full cable. It happens inside:
- Voids — air pockets left during manufacturing or jointing
- Contamination — foreign particles in the insulation
- Water trees — the classic XLPE aging mechanism in wet environments
- Degraded joints and terminations — the most common PD site in practice
Every spark erodes the insulation a little more. Left unchecked, PD sites grow into full faults — typically without warning.
2. Why PD Testing Is So Valuable
PD testing is the only routine method that detects active discharge sites — the actual precursor of failure — rather than measuring bulk condition. A cable with high PD activity will fail; the question is only when.
3. Offline vs Online PD Testing
Offline PD testing — the cable is de-energized and energized with a test source (typically VLF at 0.1 Hz or damped AC), while PD is measured. Benefits: repeatable, controlled conditions, PD source can be localized. Standard reference: IEC 60270 for conventional PD measurement.
Online PD testing — sensors (HFCT, capacitive couplers) are clamped on while the cable runs in service. Benefits: no outage, captures PD under real operating conditions. Limits: noise, and localization is harder.
Practical guidance: use offline PD at commissioning and every 2-5 years for aged cables; consider online PD monitoring for critical feeders where outage cost is high.
4. Interpreting PD Results
- PD magnitude: measured in picocoulombs (pC). Absolute thresholds vary by cable class, but trends matter more than absolute values — rising PD over time is the alarm
- Phase-resolved patterns (PRPD): the pattern of discharges across the AC cycle identifies the defect type — internal voids, surface discharge, corona, and joint defects each have characteristic patterns
- Location: with offline testing, PD source position can be measured — converting "this feeder has PD" into "this joint at meter 350 has PD"
5. Building a PD Program on a Budget
You don't need an online monitoring fleet to benefit:
- Commissioning baseline: run PD test on new cables and record the baseline — cheap insurance
- Periodic campaigns: test the oldest/most critical feeders every 2-5 years
- Pair with TDR baselines: keep commissioning TDR traces alongside PD records — the two methods complement each other
- Prioritize by PD trend: focus repair budget where PD is rising, not where it's stable
6. How TriEdge Helps
While TriEdge specializes in fault location (TDR, surge generators, pinpointers, burnout systems), our product line integrates with a complete testing program:
- A10 / A10 Advanced TDR — waveform storage and baseline comparison track developing faults
- Portable test systems — VLF-class HV capability for withstand and conditioning
- Factory-direct pricing — build a testing program at 40–60% below equivalent Western costs
View A10 → · Full catalog (PDF)
FAQ
What is partial discharge in cables? A localized electrical spark inside cable insulation caused by voids, contamination, or degraded joints. It erodes insulation over time and eventually becomes a fault.
Why test for partial discharge if the cable passes a withstand test? A withstand test proves today's health; PD testing reveals the active defect sites that predict tomorrow's failure. A cable can pass a withstand test and still fail months later.
How is PD measured? In picocoulombs (pC), typically per IEC 60270 for conventional measurement, using coupling capacitors (offline) or HFCT sensors (online).
Can PD testing find where the defect is? Offline PD testing can localize the PD source to a specific distance along the cable, turning "this feeder is bad" into "this joint at meter 350 is bad."
How often should cables be PD tested? At commissioning, then every 2-5 years for aged or critical feeders. Consider online PD monitoring for feeders where an outage is very costly.