The following Case Study is conducted on two power transformer failures as presented by authors Shanker et al. in the IEEE Asia-Pacific Power and Energy Engineering Conference in 2017 [1]. This study provides further dissolved gas analysis to confirm the Low Energy Degradation Triangle (LEDT) method on published transformer data and case studies already published.
Background
This case study is based on the dissolved gas analysis (DGA) tests conducted on two transformers in a thermal power station in India. The transformer ratings and dissolved gases are listed in Table 1 [1].
| Case Study 1 | Case Study 2 |
| Transformer Rating: 250 MVA, 15.75 kV/420 kV, 3-Phase, Oil Forced Air Forced (OFAF) cooling | Transformer Ratings: 290 MVA, 18 kV/240 kV, 3-Phase, Oil Forced Air Forced (OFAF) cooling |
| The dissolved gas test results: Hydrogen (H2): 8653 ppm Methane (CH4): 817 ppm Ethane (C2H6): 208 ppm Ethylene (C2H4): 3 ppm Acetylene (C2H2): 2 ppm Carbon Monoxide (CO): Not Detected | The dissolved gas test results: Hydrogen (H2): 44 ppm Methane (CH4): 250 ppm Ethane (C2H6): 116 ppm Ethylene (C2H4): 1989 ppm Acetylene (C2H2): Not Detected Carbon Monoxide (CO): Not Detected |
For case study 1 the paper considered the key gas method which confirmed electrical partial discharge with both Rogers Ratio Method and Doernenburg’s Ratio Method also confirming partial discharge.
For case study 2 the paper identified that the prevalent dissolved gas in oil was Ethylene with the fault type indicated by this key gas being a thermal-oil fault. The Doernenburg Ratio Method indicated a thermal fault present in the transformer with Rogers Ratio Method also confirming the fault present being a ‘thermal fault’ with fault temperatures >700°C.
Dissolved Gas Analysis in this Study
The following analysis is now made in this study by supplementing the published paper analysis with the LEDT Method, Duval’s Triangle 1, Duval’s Triangle 4 or Duval’s’ Triangle 5 and Duval’s Pentagon.
LEDT Method
The LEDT method is presented in Figure 1. It can be seen that the LEDT method identifies a fault condition for both case study 1 and 2. The diagnosis for case study 1 is Partial Discharge and the diagnosis for case study 2 is a Thermal Fault, t > 700°C.
Duval’s Triangle 1
Duval’s Triangle 1 as displayed in Figure 2 . Duval’s Triangle 1 method identifies a fault condition for both case study 1 and 2. The diagnosis for case study 1 is Partial Discharge and the diagnosis for case study 2 is a Thermal Fault, t > 700°C.
Duval’s Triangle 4
Duval’s Triangle 4 is presented in Figure 3. The one precondition is that Duval’s Triangle 4 can only be used after it has been identified by Duval’s Triangle 1 that the fault type is that of either PD, T1, or T2. For case study 1 the fault identified is on the border. It is identified that for case study 2 there is carbonization.
Duval’s Triangle 5
Duval’s Triangle 5 is presented in Figure 4. Duval’s Triangle 5 should be used only for faults identified first with Duval’s Triangle 1 as faults T2 or T3. It should never be used in case of electrical faults D1 or D2. It is also found that for faults O, S, and PD, Duval’s Triangle 4 must be used.
Analysis can only be made for case study 2 where it is confirmed that there is a Thermal fault, t > 700°C
Duval’s Pentagon
Duval’s Pentagon is presented in Figure 5. Duval’s Pentagon identifies a fault condition for both case study 1 and 2. The diagnosis for case study 1 is on the border of T1 and D2 fault and the diagnosis for case study 2 is a Thermal Fault, t > 700°C.
Discussion of Results
A summary of the results is presented in Table 2.
From the results it is confirmed that all methods were able to identify a fault in both case studies. Partial Discharge was the consistent diagnosis for all methods except Duval’s Pentagon which identified a Thermal Fault, t < 300°C.
For case study 2 it was consistent with all methods that a thermal fault was present in the transformer. Duval’s Triangle 4 identified carbonization present.
The paper also confirms that the power utility did confirm that the failure types for case study 1 was partial discharge and for case study 2 it was a thermal fault.
Conclusion
From the case study presented above it has once again proven that dissolved gas analysis of the oil is a very good method in identifying faults in power transformers. The basic key gas methods can be used first to identify possible faults and then ratio methods can confirm the fault details.
The LEDT method has also been proven to provide an accurate diagnosis of fault conditions in a power transformer.
References
- T. B. Shanker, H. N. Nagamani, D. Antony and G. S. Punekar, “Case studies on transformer fault diagnosis using dissolved gas analysis,” 2017 IEEE PES Asia-Pacific Power and Energy Engineering Conference (APPEEC), Bangalore, India, 2017, pp. 1-3, doi: 10.1109/APPEEC.2017.8309010.