Tan Delta Test (Loss Angle Test / Dissipation Factor Test)

The Tan Delta Test is also known as the loss angle test or dissipation factor test. It is one of the most reliable methods to evaluate the health of insulation in electrical equipment such as power cables, transformer windings, CTs, PTs, and bushings.

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Principle of Tan Delta Test

Any insulating material ideally behaves like a pure capacitor when connected across a power supply. In a pure capacitor, the charging current leads the applied voltage by exactly 90°.

However, in reality, no insulation is perfect:

• With ageing, dirt, and moisture ingress, a resistive (leakage) current component appears in addition to the capacitive current.

• Therefore, the total current leads the voltage by slightly less than 90°.

The loss angle (δ) is the angle between the total current and its ideal capacitive component.

The tan delta (tan δ) is defined as:

$$\tan δ = \frac{I_r}{I_c} = \frac{(V/R)}{V \cdot 2πfC} = \frac{1}{2πfCR}$$

Where:

• $I_r$ = resistive (leakage) current

• $I_c$ = capacitive current

• $R$ = insulation resistance

• $C$ = capacitance of insulation

• $f$ = applied frequency

👉 Thus, tan δ ∝ 1/f, meaning at lower frequencies, tan δ values are higher and easier to measure.

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Why Low Frequency is Used

Tan Delta Test is always carried out at very low frequency (0.1 Hz – 0.01 Hz) because:

• At high frequencies, capacitive reactance becomes very small, leading to large capacitive current.

• Resistive current is independent of frequency.

• Hence, at high frequencies, capacitive current dominates, making tan δ values extremely small and difficult to measure.

• Low frequency testing ensures measurable tan δ values without excessive power requirements.

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Tan Delta Testing Methodology

1. Isolation: Disconnect the equipment under test (cable, transformer bushing, winding, CT, PT, etc.) from the circuit.

2. Application of Test Voltage: Apply a very low frequency test voltage, starting from normal operating level.

3. Step-up Voltage: If initial results are satisfactory, gradually raise the voltage up to 1.5 – 2.0 times rated voltage.

4. Measurement:

   • A tan delta controller records tan δ values.

   • A loss angle analyzer compares tan δ values at different voltages.

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Interpretation of Test Results

• Stable Tan δ Across Voltages: Indicates good insulation condition.

• Rising Tan δ With Voltage: Suggests ageing, moisture ingress, or partial discharge activity in insulation.

• Trend Analysis: Comparing present results with past records provides insights into the rate of insulation deterioration.

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Applications

• Power Cables – to assess insulation ageing and moisture ingress.

• Transformers – windings and bushings are frequently tested.

• Rotating Machines – insulation of stator windings.

• CTs/PTs – insulation condition monitoring.

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✅ Conclusion:
The Tan Delta Test is a non-destructive diagnostic tool widely used in the field to predict the remaining service life of insulation systems. It is an essential test for condition monitoring, preventive maintenance, and ensuring reliability of electrical equipment.

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