Analysis of DGA Data of Wind Farm Transformers Filled with Synthetic Ester

DGA data as normal, needs attention, and need intervention. A summary of the existing guideline limits given for synthetic ester filled transformers in different standards is provided and it is noted that mostly the standards provide the 90th percentile as guideline threshold for normal or need attention values. The impact of the sample size was considered on the derived key statistical parameters, and it was found, for our database, that 500 data entries are sufficient to give representative statistics. The traffic light threshold proposal was made using the following method. First, the data were sorted in ascending order, and outliers above the 98th percentile were excluded. Second, plots of the data indicate that the distribution resembles exponential trend and so database characterisations were provided in the form of the following percentile values, 70th, 80th, 90th, 95th, and 98th percentiles. In addition, both statistical mean and standard deviation are calculated for each DGA gas. Third, for the traffic light system, normal, or green, threshold was calculated using the average of all data within the 90th percentile value. Similarly for the yellow and red zones threshold values are calculated using the average of data points within the 95th and 98th percentiles respectively. Proposed traffic light system was then compared to various 90th percentiles criteria as demonstrations using DGA samples results from 10 transformers. The comparison showed that the proposed traffic light system is more conservative, but it gives earlier signals to assets managers aiding in categorizing DGA results as normal, need attention, or need intervention. Finally, it is discussed in this contribution that the distribution of the database influence whether the 90th percentile can be used as a guideline limit, and it is suggested that it would be specific to the application and to the transformer fleet. Further studies are needed to assess this claim.