How is PD detected and measured in the field and during factory testing?
There are four established PD detection methods:
|
Method |
Sensor |
Freq Band |
Sensitivity |
Pros |
Cons |
|
Electrical (IEC 60270) |
Coupling capacitor / bushing tap |
30 kHz–1 MHz |
1–10 pC |
Quantified in pC; standardized |
Susceptible to noise; requires de-energized |
|
UHF |
Internal antenna / oil valve probe |
300 MHz–3 GHz |
1–5 pC |
Immune to power noise; can localize |
Requires oil valve access; semi-quantitative |
|
HFCT |
Clamp-on CT on ground lead |
1–30 MHz |
5–20 pC |
Non-invasive, clamp-on |
Cannot easily localize |
|
Acoustic (AE) |
Piezoelectric sensor on tank wall |
20 kHz–300 kHz |
~10–50 pC |
Precise localization (triangulation) |
Poor sensitivity for internal PD; signal attenuation |
|
DGA |
Oil sampling port |
Chemical |
ppm H₂/CH₄/C₂H₂ |
Widely available; identifies PD indirectly |
Not real-time; gas lags behind PD |
Recommended combination:
• Factory: IEC 60270 (calibrated pC).
• On-line (transformer): UHF probes + DGA.
• On-line (cable/GIS): HFCT clamps.
• Site localization: Acoustic sensors for triangulation.