Tan Delta Testing of Power Transformer

Tan Delta, also called Loss Angle or Dissipation Factor testing, is a diagnostic method of testing electrical equipment to determine the integrity of the insulation. This is done to try to predict the remaining life expectancy of the equipment.

If the insulation free from defects, it approaches the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being the two plates separated by the insulation material.

In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation,for example, moisture, the resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees. It will be something less than 90 degrees. The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination, hence quality/reliability. This "Loss Angle" is measured and analyzed.

Below is a representation of an insulation. The tangent of the angle δ is measured. This will indicate the level of resistance in the insulation. By measuring IR/IC (opposite over adjacent – the tangent), we can determine the quality of the insulation. In a perfect insulation, the angle would be nearly zero. An increasing angle indicates an increase in the resistive current through the insulation, meaning contamination. The greater the angle, the worse is the insulation.

Insulation power factor is the angle 90-δ. If the insulation is excellent, the tan delta or dissipation factor is equal to the power factor. Essentially, both tan delta and power factor are just the same.