The Tan Delta Test (Dissipation Factor Test) evaluates the insulation condition of a transformer by measuring dielectric losses under AC voltage.
tanδ = Resistive Current/Capacitive Current
Why Tan Delta is Important?
- Early detection of insulation aging.
- Identifies moisture or contamination.
- Tracks insulation condition over time.
- Prevents unexpected failures.
- Improves transformer reliability.
Understanding Tan Delta (tan δ)
- Indicates insulation losses.
- Higher value → insulation deterioration.
- Sensitive to moisture, aging and contamination.
- Measured under AC voltage conditions.
Acceptance Criteria
- New Transformer → tan δ ≤ 0.5% at 20°C.
- Service-Aged Transformer → tan δ ≤ 1.0% at 20°C.
Tan Delta Test – Two Winding Transformer
Test Modes & Measurements
| Test Mode | Cable Connection | Measured |
|---|---|---|
| UST | H.V → H | CHL |
| GST-GNT | Red → H | CHG |
| GST-G | H.V → Test | CHL + CHG |
| UST | H.V → Test | CLH |
| GST-GNT | Red → H | CLG |
| GST-G | H.V → Test | CLH + CLG |
Tan Delta Test – Three Winding Transformer
Test Modes & Measurements
| Test Mode | Cable Connection | Measured |
|---|---|---|
| UST-R | H.V → H | CHX |
| GSTg-R & GSTg-B | Red → 1X1(X), Blue → 1X1(Y) | CHG |
| UST-R | H.V → H | CHG + CHX |
| GSTg-R & GSTg-B | Red → 1X1(X), Blue → 1X1(Y) | CXY |
| UST-R | H.V → H | CXY |
| GSTg-R & GSTg-B | Red → 1X1(X), Blue → 1X1(Y) | CXG |
| UST-R | H.V → H | CXG + CXY |
| GSTg-R & GSTg-B | Red → 1X1(X), Blue → 1X1(Y) | CYG + CYH |
The Tan Delta Test is a critical diagnostic tool in transformer maintenance that:
- Detects insulation deterioration early
- Helps in preventive maintenance planning
- Ensures system reliability and safety
- Reduces risk of costly failures
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