Diagnostic Investigation & Correlation In-Service HV & EHV Current-Transformers Exhibiting High Levels of Hydrogen

High levels of hydrogen have been observed in some current transformer (CT) units of the 72.5kV to 330 kV free standing-type using paper-oil insulation and inhibited mineral oil. Several techniques have been used by Trench and Hydro Québec (HQ) to investigate the possible causes of this high hydrogen formation, such as partial discharge tests, dissolved gas analysis, moisture content, dielectric dissipation factor and compatibility tests of the oil with metal parts. The most probable cause, stray gassing (slight overheating around 100°C) of the oil, occurred during the early operation of the units then disappeared, did not damage paper and oil insulation, and is not a concern for the normal operation of CTs. Indeed, all units with initially high hydrogen levels have continued to operate without any sign of electrical or thermal problems, and with hydrogen slowly diffusing out with time. Similar observations of normal operation with high hydrogen levels have been reported at CIGRE in CTs, bushings and small Padmount power transformers. CTs with high hydrogen contents due to this same cause could thus reasonably be kept in service rather than replaced. [3]