# Why is the Tan Delta Test (Dissipation Factor) Important for Transformers?

**URL:** <https://forumautomation.com/t/why-is-the-tan-delta-test-dissipation-factor-important-for-transformers/13984>\
**Category:** Uncategorized\
**Created:** [March 31, 2026, 1:45pm UTC](https://forumautomation.com/t/why-is-the-tan-delta-test-dissipation-factor-important-for-transformers/13984 "2026-03-31T13:45:23Z")\
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**Author:** ![Rabert](https://forumautomation.com/user_avatar/forumautomation.com/rabert/32/6784_2.png) [@Rabert](https://forumautomation.com/u/Rabert)\
**Post date:** [March 31, 2026, 1:45pm UTC](https://forumautomation.com/t/why-is-the-tan-delta-test-dissipation-factor-important-for-transformers/13984/1 "2026-03-31T13:45:23Z")

</div>

The **[Tan Delta Test](https://forumelectrical.com/tan-delta-power-factor-characterization/)** (Dissipation Factor Test) evaluates the insulation condition of a transformer by measuring dielectric losses under **[AC voltage](https://forumelectrical.com/medium-voltage-mv-capacitor-calculator/)**.

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.

 ![image](https://cdn.forumautomation.com/original/2X/c/c8a5a68b286f7881dd36dd50b96aff9d5e5b49b7.jpeg)

## 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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