What is partial discharge, and how does it lead to transformer failure?
Partial discharge (PD) is a localized electrical breakdown that does not completely bridge the insulation. It occurs in voids, gas bubbles, oil gaps, or along insulation surfaces where the local electric field exceeds the dielectric strength.
The physics of PD:
• Electron bombardment - breaks polymer chains in paper/oil.
• Localized heating - up to several hundred °C, carbonizing insulation.
• Chemical degradation - produces H₂, C₂H₂, CO (detectable by DGA).
• Acid formation - cellulose decomposition accelerates aging.
|
Stage |
Activity |
Detection Method |
Time to Failure |
|
Inception |
Micro-voids discharge intermittently |
UHF / HFCT (sensitive) |
Years |
|
Development |
Discharge becomes stable & repetitive |
PD monitoring + DGA |
Months |
|
Acceleration |
Surface tracking / treeing forms |
DGA (H₂, C₂H₂ rising) + PD trending |
Weeks |
|
Pre-failure |
Severe PD, imminent breakdown |
Multiple alarms |
Days to hours |
Why PD is dangerous:
• A transformer can operate for months with active PD before eventual failure.
• The damage is cumulative and irreversible - carbonized paper cannot be restored.
• ~30–50% of in-service transformer failures originate from undetected PD (CIGRE).
Key takeaway:
PD testing is a TRENDING tool. Rising magnitude or repetition rate demands immediate investigation.