Technical Reference
Partial Discharge Reference Guide
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What is Partial Discharge?
Partial discharge (PD) is a localized electrical discharge that only partially bridges the insulation between conductors. It's an early indicator of insulation degradation in high-voltage equipment.
IEC 60270 Definition: "A localized electrical discharge that only partially bridges the insulation between conductors."
Example Severity Levels by Equipment
These levels are illustrative examples, not from a standard. IEC 60270 defines how to measure apparent charge in pC; it does not set severity limits. Acceptance levels come from the equipment standard or the manufacturer. For TEV, compare against background readings, against similar cells and against the trend over time.
Power Transformers (Oil-Filled)
| PD Level (pC) |
Severity |
Action |
| <100 | Normal | Continue monitoring |
| 100-500 | Low | Increase monitoring |
| 500-1,000 | Moderate | Investigate, DGA test |
| 1,000-2,500 | High | Plan intervention |
| >2,500 | Critical | Urgent investigation |
Switchgear (Metal-Clad)
| TEV Level (dBmV) |
Severity |
Action |
| <-5 | Normal | Continue monitoring |
| -5 to +5 | Low | Investigate next outage |
| +5 to +15 | Moderate | Schedule investigation |
| +15 to +25 | High | Priority investigation |
| >+25 | Critical | Urgent action |
PD Pattern Recognition
| Pattern |
Phase Position |
Likely Defect |
| Near zero crossing | 0°, 180° | Internal void |
| Near voltage peak | 90°, 270° | Surface discharge |
| Negative half only | 180°-360° | Corona (protrusion) |
| All phases, erratic | Random | Floating electrode |
Measurement Methods
| Method |
Frequency |
Application |
| Conventional (IEC 60270) | 100 kHz - 1 MHz | Lab testing, commissioning |
| UHF | 300 MHz - 3 GHz | GIS, transformers |
| Ultrasonic (40 kHz) | Acoustic | Transformers, switchgear |
| TEV | Surface | Metal-clad switchgear |
Sources
- EA Technology — W. G. Higginbotham, «Non-invasive PD testing of switchgear», NETA World, Summer 2017
