Tan Delta Test: Working Principle, Need, Modes & Very Low Frequency Testing

What is Tan Delta?

Tan Delta, also known as Loss Angle or Dissipation Factor, is a cable diagnostic method used to determine the quality and health of cable insulation. It helps predict the remaining life of the cable system and assists in scheduling maintenance, replacement, or rejuvenation.

In an ideal insulator, only capacitive current flows through the insulation because the dielectric is perfectly pure. However, real-world insulating materials contain impurities such as dirt and moisture that increase with ageing. These impurities create a resistive leakage path, causing a resistive component to appear in the leakage current.

The ratio of the resistive component to the capacitive component of leakage current indicates the condition of the insulation. A low ratio means healthy insulation, while a higher ratio indicates deterioration. This ratio is known as tan δ (tan delta) or the dissipation factor.

Formula:

tan δ = I_R / I_C

Where:

• I_R = Resistive component of leakage current
• I_C = Capacitive component of leakage current

Need for Tan Delta Testing

The primary purpose of the Tan Delta test is to assess transformer and cable insulation health to ensure safe and reliable operation. By calculating the dissipation factor and capacitance, technicians can detect:

• Insulation ageing
• Moisture ingress
• Partial breakdowns in bushings
• Winding displacement or deformation
• Hot spots caused by insulation degradation

The test helps identify insulation deterioration early so that corrective action can be taken before a failure occurs.

Method of Tan Delta Testing

Before testing, the cable, windings, transformers, CTs, PTs, and bushings are disconnected from the electrical system. A very low-frequency (VLF) voltage is applied across the insulation under test.

The testing steps are:

• A standard test voltage is applied.
• If tan delta values are within acceptable limits, the voltage is increased to 1.5–2 times the normal operating level.
• The Tan Delta measuring unit records values at different voltages.
• A loss angle analyzer compares values at normal and elevated voltages.

Applying test voltage at very low frequency is essential for accurate results and manageable power requirements.

Why Very Low Frequency (VLF) is Used?

At higher frequencies, the capacitive reactance of the insulation decreases, causing a significant increase in capacitive current. Because the resistive current remains almost constant, the total current increases dramatically, requiring high apparent power to conduct the test.

Using very low frequency solves this problem by maintaining manageable current levels.

Key relation:
Tan δ ∝ 1/frequency

At lower frequencies, the tan delta value increases, making it easier to measure accurately. Therefore, Tan Delta tests are typically performed at frequencies between 0.1 Hz and 0.01 Hz.

Different Modes of Tan Delta Test

1. GST Guard Mode

Measures only the current leaking to ground. Leakage through red or blue test leads is eliminated. Grounded edges are bypassed and not included in the calculation. Only leakage to ground is captured.

2. UST (Ungrounded Specimen Test) Mode

Used to measure insulation between ungrounded leads of equipment. The isolated portion of insulation is tested separately without influence from grounded components.

3. GST Mode

This mode measures all leakage paths. The test apparatus calculates current, capacitance, watt-loss, GST Guard, and UST contributions. The sum of GST Guard and UST should match GST values under normal conditions.

If these values do not match, the test setup may have wiring errors or a problem in the testing terminal arrangement.

Conclusion

The Tan Delta test is an essential diagnostic method that assesses insulation behavior, detects ageing, identifies moisture or partial breakdown, and ensures the health of cables and transformer bushings. It provides valuable early warnings, improving reliability and preventing costly failures.