A transformer is a crucial component in the electrical power system that plays a pivotal role in altering voltage and electric current between different voltage levels within the transmission and distribution systems. Optimal maintenance is essential to ensure the transformer operates efficiently and reliably. Transformer oil holds a central role in cooling, insulating, and safeguarding the internal components of the transformer, also serving as an electrical insulator. The quality of the oil affects the long-term performance, lifespan, and reliability of the transformer. However, over time and due to operational conditions, the oil's quality can degrade due to oxidation, hydrolysis, and contamination. The accumulation of degradation products diminishes electrical insulation and cooling efficiency of the oil, necessitating periodic monitoring to uphold transformer performance. Based on the results of Dissolved Gas Analysis (DGA) testing, the transformer oil condition shows elevated levels of Carbon Dioxide and Carbon Monoxide. Moreover, the Key Gas method indicates indications of excessive heat on the cellulose (Overheat of Cellulose). Furthermore, the Roger’s Ratio analysis indicates the presence of heat symptoms below 150°C in the transformer oil. Similarly, in the Duval Triangle analysis, there are indications of Partial Discharge disturbances and thermal failure with temperatures exceeding 700°C in Station Service Transformer Unit 1 (Thermal Fault of Temperature >700°C).