How to Test a Bushing: Tan Delta and Power Factor Guide

Tan delta (dissipation factor) testing is one of the most important and economical ways to assess the insulation condition of high-voltage bushings. By measuring active losses in the dielectric, it reveals whether the insulation is damp, aged or suffering partial discharge.

Test principle

In an ideal capacitor, current leads voltage by 90°. When the insulation has losses, the phase angle is less than 90°, and the tangent of the deviation angle δ is tan delta. A larger tan delta means greater losses and poorer insulation condition.

Tan delta vs power factor

For high-voltage insulation, angle δ is small, so tan delta and power factor (PF) are numerically almost equal and can be used interchangeably.

Typical limits

• New bushings: tan delta should normally be below 0.5% (varies slightly by voltage class and standard).
• In-service bushings: compare against factory values or historical data; a significant increase (e.g. doubling) warrants attention.
• The trend matters more than any single absolute value.

C1 and C2 tests

• C1: capacitance and losses of the main insulation (conductor to test tap).
• C2: capacitance and losses of the tap insulation (test tap to flange).
• Testing C1 and C2 separately helps localise the defect more precisely.

Test precautions

• Clean the porcelain surface before testing to avoid surface leakage.
• Record ambient temperature and humidity — temperature affects tan delta.
• Use a dedicated dissipation-factor test set (e.g. Doble-type).
• Compare data only under the same batch, model and temperature.

Interpreting results

• Stable tan delta close to factory values → good condition.
• Slowly rising tan delta → possible moisture or aging; increase monitoring.
• Sudden increase → investigate immediately with DGA and partial discharge tests.

Include tan delta and capacitance in your routine test plan and benchmark against the factory test report to build a complete insulation-condition history.