Transformer Insulation tests include:
- Insulation Resistance test between transformer windings and tanks
- Tan-delta and capacitance tests of the transformer and bushings.
Insulation resistance tests measures the DC resistance between various insulated parts of the transformer and usually is performed bewteen:
- HV connectors to LV connectors
- HV connectors and Ground
- LV connectors and Ground
Typical values range between 100MΩ and up to 1000GΩ. It is performed by a high voltage resistance tester that can increase the clamp voltage up to 10,000vdc, while measuring the weak current through the insulating medium.
A variation of the resistance check is a resistance check over time, called the polarization index (PI). PI test consists of dividing the resistance value after 10 minutes to that of after a minute. The high voltage across the resistance path creates polarization of dipole particles, such as moisture. The more dipoles are formed between the electrodes after the given time, the higher the PI, the better the insulation. A PI value below 1.5 is considered questionable.
Tan Delta and capacitance test
This AC test is based on checking the phase angle created between a High voltage injected to one clamp and the resulting current flowing from the other clamp. Under ideal insulation, the angle between the voltage and current is perfectly 90 degrees. Any resistance will alter that value to somewhat smaller and the tangent of the resulting angle is the tan-delta (or power factor) value, and is given in %. An acceptable value is usually below 1% for transformer and 0.3% for bushing insulators.
Tan-delta is complemented by capacitance value, and the following checks are performed on-site:
- HV and Ground
- LV and HV
- LV and Ground
- HV tip and Bushing tap (for each bushing)
- Bushing tap and Ground (for each bushing)