Under Load Tap Changer Diagnostics Based on Transformer DGA and DC Resistance Tests

Under load tap changers (ULTC) have the most share of defects in transformers causing enormous expenses to utilities. ULTCs have been a weak part of any transformer as they deteriorate over time due to mechanical problems or contact wear from repeated operations. Erosion of the contacts over time is expected due to the nature of their function. Coking of the contacts causes overheating, which can cause thermal runaway. Diagnostics of ULTC is a key for any utility. The goal of diagnostics is to provide equipment owners with a ranking that prioritizes maintenance activities. This goes beyond the simple good/bad distinction, to provide some grading to permit different management options. Historically, DGAs in ULTCs have been considered rather insignificant due to the large amount of gases normally generated by the arcs. This has however been reconsidered the recent years and the opinion today is that quite a lot of information is gained by DGAs of ULTC oils. Dissolved gas analysis (DGA) has been proven to provide relevant information about transformer and tap changer health or faults. On the other hand, winding DC resistance measurement can be a complementary method for identification of defective connections triggered by DGA detected fault. In this paper, the DGA analysis is employed to analyze the gases generated in the transformer tank by a possible fault at selector switch tap. Diagnostics performed using both DGA and DC winding resistance tests for a 30MVA 63/20kV three-phase transformer with an in-tank ULTC. The results showed that combination of DGA and DC winding resistance is a simple and effective diagnostic technique to detect coking and carbonized contacts of the ULTC tap selector contacts. Trending of transformer tank DGA data and DC winding resistance proved to be reliable to trigger internal inspection, overhaul and repair of ULTC tap selector.