Abstract
The article describes a case study when the voltage collapse during lightning impulse tests of new power transformers was noticed and when the repeated tests finished with a positive result. The step-by-step process of reaching the conclusion on the basis of dissolved gas analysis (DGA) as a key method of the investigations was presented. The considerations on the possible source of the analysis showed that the Duval triangle method, used in the analysis of the concentration of gases dissolved in oil samples taken from bushings, more reliably and unambiguously than the ratio method recommended in the IEC 60599 Standard, indicated a phenomenon which was identified in the insulation structure of bushings analyzed. Additionally, the results from DGA were found to be consistent with an internal inspection of bushings, which showed a visible trace of discharge on the inside part of the epoxy housing, as a result of the lightning induced breakdown.
Highlights
The dissolved gas analysis (DGA) is the most widely used method in the diagnosis of power transformers
The data concerning DGA indicated the occurrence of partial discharges (PDs) in bushings which were confirmed by on-site PD tests
Based on the above considerations, the authors decided to present further in this paper a case study when the voltage collapse during the lightning impulse tests of new power transformers was noticed and when the repeated tests finished with a positive result
Summary
The dissolved gas analysis (DGA) is the most widely used method in the diagnosis of power transformers. The data concerning DGA indicated the occurrence of partial discharges (PDs) in bushings which were confirmed by on-site PD tests.
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