Latest Findings at Transformer Bushings Condition Evaluation by Dielectric Response Methods

Transformer bushings have to feed the high voltage from the portal field in the substation to the winding in the transformer. For this reason the bushing has a high responsibility for the save operation of a transformer. If any problem arises in the insulation system a shut-off in minor case or a consequential damage on the transformer can occur.
Especially at aged bushings the failure probability is significant high. For the detection of failures in bushings the ordinary diagnostic inspection by oil analysis is not sensitive enough. Additional dielectric response measurements have to be applied. With these kinds of measurements the humidity of the insulation system can be determined in a proper way, but there were also other ageing parameters.
To minimize the risk of transformer outages the utilities often keep spare bushings stored. However the storing conditions over a long period often result in degradation due to humidity absorption of the insulation system.
In this paper the applicability of different dielectric response measurement circuits for on-site
testing was investigated as well as the effect of different storing conditions of transformer bushings.