Case Study on Fault Detection in Generator Step-Up Unit Transformers of Mae Moh Coal-Fired Power Plant Unit 14 Using Dissolved Gas Analysis

Generator step-up unit transformers (GSUTs) are critical components in large generating units, where early fault detection supports reliable operation. During early operation of Mae Moh coal fired power plant unit 14, the dissolved gas analysis (DGA) online system on GSUT phase A detected a gradual increase in methane, ethane, and ethylene while the unit operated steadily at approximately 600 MW net output. The gas behavior differed from phases B and C, indicating abnormal phase specific behavior. The DGA online trend served as the primary diagnostic indicator and guided a structured investigation that included gas ratio interpretation, Duval graphical analysis, furan testing, infrared thermography, internal inspection, magnetic screening, and metallurgical examination. DGA interpretation consistently indicated a thermally driven gas generation mechanism, while furan analysis confirmed that cellulose insulation degradation was not involved. Subsequent magnetic screening and laboratory material analysis identified an unintended ferromagnetic carbon steel component at the low voltage bushing turret of phase A. The combined evidence demonstrates that localized eddy current and hysteresis losses within this component were responsible for the observed hydrocarbon gas generation. This case study highlights the effectiveness of DGA based fault detection, supported by targeted verification techniques, for identifying latent material related heating mechanisms in GSUTs.