Magazine | Issue 1 2024 In our workshop’s HV lab, our test engineers started by performing various electrical tests, including a DFR test, but no results were obtained to verify their suspicions of PD activity. PD testing revealed various PD sources As a next step, a series of PD tests were performed with OMICRON’s MONTESTO 200 temporary PD monitoring system using the induced voltage method. The PD tests were done over extended periods of time at different central frequencies and bandwidths to obtain the best signalto-noise ratio. The patterns obtained clearly indicated high PD activity from various PD sources. Thanks to the 3PARD diagram (3-Phase-Amplitude-Relation- Diagram), which is integrated into the MONTESTO 200 monitoring software capabilities, our test engineers could see many clusters, indicating various potential PD sources in the transformer, which could be separated for individual analysis. Analyzing different 3PARD clusters When applying the 3PARD filters on some clusters displayed around the three phases, symmetrical discharges around the zero crossing characterized a predominant PRPD (Phase-Related PD) pattern. The shape of the pattern indicated it was probably related to voids in the insulation material, possibly in the oil. Still, even after applying the vacuum to remove the oil in the transformer tank, the same discharges remained with the same PRPD diagram, which means that the voids might be on or inside the solid insulation. On the other hand, according to CIGRE 676, this PRPD diagram is very close to the one related to voids in glue. Besides, another PD source could be seen very clearly with the 3PARD filtering, and it was most probably due to carbonization on the pressboard barrier – mostly around phase W. Still, it could also be seen in the other phases. Filtering other clusters revealed yet another type of PD related to floating metallic particles around phases U and V. Visual inspection confirms PD testing results Based on the PD test results, our test engineers decided to open the transformer for a visual inspection to identify suspected damaged components. After the transformer was opened, it became clear that our assumptions based on the PD test results were correct, and the extent of the damage was immediately apparent. After removing the oil from the tank, many copper dust particles deposited in different places inside the transformer were visible. This copper dust was likely the floating metallic particles suspected when the oil circulated. Additionally, traces of carbonization were found in almost all the phases (HV coarse, HV fine, LV coarse) on the pressboards and wedges. In addition, our inspection team also found many traces of damaged and burned glue on the upper side of the coarse-tuning windings of phases B and C. The transformer was then repaired in our workshop. All the damaged parts were replaced with new components and materials. PD monitoring verifies the success of repairs Additional PD tests were performed in our workshop’s HV lab after the repairs were made, and no suspicious PD activity was detected. Once back in service, continuous on-site PD monitoring was performed for many months with the MONTESTO 200 device used in our repair workshop. No additional PD activity was recorded that indicated remaining or developing faults in the transformer. This was a verification to us that the repairs were successful. The transformer was back in reliable service. LEARN MORE ABOUT MONTESTO 200 omicronenergy.com/ montesto200 Traces found of damaged and burned glue. 25
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