OIL CORROSION AND CONDUCTING CU 2 S DEPOSITION IN POWER TRANSFORMER WINDINGS

SUMMARY Copper corrosion and Cu 2 S deposition, has recently led to failures of transformers & shunt reactors during service. Units from several different operators and several OEMs have been affected. Analysis of actual failure cases showed that these transformers were well within established industry standard practice regarding design as well as operation, and used oil that fulfilled international material standards. Sealed units with rubber sack appear primarily to have been affected. Final failure has occurred as a turn-to-turn breakdown in the HV windings, and inspection of the failed windings showed deposits in the winding consisting of Cuprous Sulphide, Cu 2 S, an electrically conducting compound. The deposits varied within the winding, usually with more deposits in the upper part of the windings, but the complex deposition pattern could not be explained simply by the temperature distribution. We have carried out comprehensive corrosion research aiming at a thorough understanding of the phenomenon, and have reproduced the corrosion as well as the deposition of Cu 2 S in our laboratories. The influence of the gas content of the oil, and other parameters have been studied under very well controlled circumstances, and a number of different oils have been evaluated at temperatures ranging from 80 to 150 °C. The main finding is that Cu 2 S deposition occurs only for certain oils, which can cause corrosion and deposition under the typical environment of a sealed power transformer. With such oils deposition has been demonstrated over a wide temperatures range, from as low as 80 °C, and on a variety of solid materials. These oils further show a very strong oxygen dependence of the deposition, which explains much of the observed complex deposition patterns in the failed units. Real failures have all occurred with transformer oils that passed the corrosion tests in the international standards, DIN 51353 and ASTM D1275 respectively. Improved test methods are clearly needed that can take into account the observed oxygen dependence, an important factor present in real transformers. A Covered Conductor Deposition (CCD) test has demonstrated a superior selectivity for identifying the corrosive oils that have caused actual failures in transformers, and is thus a very promising type of test