The degree of polymerization (DP) test is another means for assessing insulation aging. This test is performed on paper samples. The DP test provides an estimate of the average polymer size of the cellulose molecules in materials such as paper and pressboard. Generally, paper in new transformers has a DP of about 1000. Aged paper with a DP of 150-200 has little remaining mechanical strength, therefore making windings more susceptible to mechanical damage during movement, particularly during extreme events such as through-faults. As insulation aging in transformers can be uneven due to thermal, moisture, oxygen and byproduct concentration, gradient samples from various locations are needed to provide the best diagnosis of the overall insulation condition.
DP testing is recommended in these cases:
- There is other evidence of very accelerated aging of the insulation
- The transformer is > 20 years old and an internal investigation is being performed
- For condition assessment of older transformers for possible refurbishment
- When a partial rewind is being considered
- To assess cause of failure
- For condition assessment of insulation when purchasing a service-aged transformer
- To assess the condition of a transformer after an extreme overheating event