How is the tanδ test applied to bushings, and what physical quantity are we measuring?
For a capacitive bushing (OIP or RIP), the tanδ test measures dielectric losses in the insulation between the conductor and the tap layer, and between the tap layer and the ground flange.
Physical principle:
A healthy bushing behaves nearly as an ideal capacitor. Tanδ = Iᵣ / I꜀ (resistive current/capacitive current).
|
Measurement |
Terminals |
What It Assesses |
|
C₁ & PF/DF (UST) |
Center conductor ⇔ Tap |
Main insulation - paper/oil dielectric between conductor and tap layer |
|
C₂ & PF/DF (GST) |
Tap ⇔ the Ground flange |
Oil gap & external insulation - from tap to grounded flange |
Typical values (new, at 20°C):
• PF for C₁: < 0.5% (oil-impregnated paper)
• PF for C₂: < 1.0% (oil gap)
• Capacitance tolerance: should stay within ±2–5% of nameplate value