DAMAGE ASSESSMENT FOR TRANSFORMER WINDINGS SUBJECTED TO CORROSIVE OIL EXPOSURE USING DIELECTRIC SPECTROSCOPY

SUMMARY Corrosive oil has been blamed as cause of several failures in power transformers and shunt reactors therefore extensive work worldwide has been performed to understand the chemical process of Copper Sulphide (Cu2S) formation. Reports identify the dibenzyl disulphide (DBDS) as the root cause and therefore several techniques for its removal had been presented together with the use of mitigation techniques that include metal passivators [1]. However, little work has been performed to assess the damaged caused in the transformer and shunt reactor insulation once exposed to Cu2S formation and before it undergoes a failure. In this paper, the damage assessment of specimens removed from real power transformers and shunt reactors that underwent a failure caused by corrosive oil is presented. The damage assessment technique relay on a model named Montes, which was developed as a suitable way to evaluate paper insulated copper conductors using dielectric spectroscopy techniques and compensating for moisture content and temperature [2]. The Montes model introduced in the paper demonstrates that it is feasible to correlate the dielectric stress reduction on inter-turn insulation caused by Cu2S accumulation in the different layers of paper in windings. This will support the decision as to proceed with passivation or consider a full rewinding in extensively damaged windings. The Montes model employs simple electrode geometries arranged in small sections of real windings and demonstrates that dielectric variables to detect effects from Cu2S are easily measured at frequencies below 1000 Hz. However, temperature and moisture content affects strongly the results in frequencies below 1 Hz. To reduce the impact of moisture, the model uses conductivity values obtained at frequencies above 1 Hz and below 1000 Hz, compensating for temperature to assess degradation.