The following power transformer case study is based on the paper as presented at the 2020 International Conference on Power, Energy and Innovations (ICPEI), October 14-16, 2020, Chiang Mai, THAILAND [1].
The LEDT method was not applied at that stage by the authors and this publication is based on the data presented to assess what analysis the LEDT method would have provided.
Background Information
The dissolved gas data from two urban distribution transformers were sampled on 8 September 2015, 22 October 2019 and 30 October 2019. Transformer No.1 ratings are as follows; 24/32 MVA, 69/24 kV, 3-phase, Oil Natural Air Forced(ONAF) and was manufactured in 1992. Transformer No.2 had the same ratings and was operated in parallel. The dissolved gas data as extracted from the publication is included in Table 1.
When failure occured on transformer No.1 it was taken out of service and all the load was driven through transformer No.2 which resulted in it also moving towards failure.
LEDT Analysis
The Low Energy Degradation Triangle (LEDT) was developed based on the three combustible gases Hydrogen (H2), Methane (CH4) and Carbon Monoxide (CO). More details on the LEDT Method can be found at this link.
For transformer No.1 the following is the LEDT plot.
From the LEDT method it can be seen that the intial sample was in the Normal region. Subsequent samples were then found in the D1 region indicating discharges of low energy. This was definitely a change that would suggest a fault condition.
For transformer No.2 the following is the LEDT plot.
From the LEDT it can be seen that the intial sample was in the T1 region indicating a potential thermal fault of low energy (<300 degrees Celsius). Subsequent samples were then found in the D1 region indicating discharges of low energy (sparking). With the related high levels of dissolved combustible gases it would suggest immediate disconnection for further investigation.
Conclusion
From the paper in reference [1] it was concluded that the Doernenburg ratio and the Duval triangle 1 showed that partial discharge occurred. The Rogers ratio method provided no diagnosis.
The LEDT method from this assessment does provide a diagnosis. In this case it was identified as D1 discharges of low energy (sparking) for both transformer No.1 and transformer No.2. The LEDT method also identified that transformer No.2 had an initial T1 thermal fault which then progress to D1 when it was loaded with the full load of transformer No.1
References
- N. Chattranont, S. Woothipatanapan and N. Rugthaicharoencheep, “Case Study on Power Transformer using Dissolved Gas Analysis Technique,” 2020 International Conference on Power, Energy and Innovations (ICPEI), Chiangmai, Thailand, 2020, pp. 165-168, doi: 10.1109/ICPEI49860.2020.9431480.