View of Atmospheric DGA Gases in Sealed Wind Turbine Transformers Filled with Synthetic Ester - Early Leak Detection Indicators

Transformers used in wind turbines are subjected to unique environmental conditions which render their assessments and diagnostics more challenging. Assessment includes visual checks for any abnormal conditions, such as leaks or corrosions, liquid sampling, and DGA assessment, along with oil level switches or pressure sensors for leakage detection. As transformers are subjected to operational and mechanical stresses, it is important to identify any leaks which permit oxygen and moisture ingress into the transformer. The absolute values and the ratios of the atmospheric gases may indicate sealing system failure. However, the dynamics of moisture migration and the impact of ageing processes could hinder the ability to detect abnormal conditions. This paper aims to provide a review of normal atmospheric gas levels in synthetic ester filled transformers used in onshore wind energy generation. The data presented is for a group of transformers which were sampled during operation for DGA measurement. The assessment includes two types of sealed transformers systems: hermetically sealed transformer with a corrugated tank and sealed flat-wall tank with a nitrogen blanket. In the nitrogen blanket design, the liquid quickly becomes saturated with nitrogen after the start of operation. The analysis involved presenting typical statistical means and standard deviations for oxygen and nitrogen for 100 transformers in each design group. The normal distribution of the atmospheric gases along with the oxygen to nitrogen ratio is presented for the two groups. Finally, specific cases are presented where leaks were reported looking at DGA data.