Case Studies of PD Measurement Experiences on Site Maintenance and in Workshop Fault Investigations of Oil-filled Power Transformers and Shunt Reactors

Advanced Partial Discharge (PD) measurement techniques, including conventional (IEC 60270) and unconventional (UHF) electrical detection methods, have been developed to detect insulation defects in oil-filled power transformers and reactors in both laboratory and field environments, to understand in which parts or in which insulation materials these defects occur and to determine their location in terms of phase. However, in laboratory conditions, sometimes due to multiple PD sources and in field conditions, especially due to background noises, multiple disturbance signals and multiple PD sources, it can be difficult for non-experts to interpret PD measurement results. Developments in PD source filtering and separation tools such as the Three Phase Amplitude Relation Diagram (3PARD) have significantly increased the diagnostic success for insulation defects in oil-filled power transformers and reactors, and combined use of these tools with UHF and electrical PD tests allows detailed analysis of the results of the measurements. The correlations between the type of discharge found after detailed analysis of the measurements with the combined advanced partial discharge systems and the actual physical damages seen during visual inspections of oil-filled power transformers and reactors emphasize the reliability of these diagnostic tests. The clear agreement between the PD patterns and the visualized damage confirms the effectiveness of PD diagnosis in identifying insulation faults.

This paper highlights the critical aspects of electrical (including UHF) PD testing on oil-filled power transformers and shunt reactors by examining five Case Studies, including field and laboratory measurements, in terms of dissolved gas analysis results and partial discharge tests triggered by DGA. The results show a wide range of PRPD patterns (including all types from main type 1 to main type 6), matching the pattern types given in the six main headings and many subheadings of Cigre Technical Brochure TB676. Where most partial discharge sources were identified using the 3PARD tool except Case 2. The oil-filled transformers and shunt reactors were subjected to maintenance process in the field or opened and investigated in the workshop. In the workshop visual inspections revealed significant insulation damages or workmanship defects. In all cases, the findings were either consistent with or exactly matched typical PRPD patterns, confirming that the assumptions/evaluations and actions based on PD measurements were highly accurate.