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PLN (Persero) is the state-owned company responsible for supplying electrical energy in Indonesia and plays a crucial role in ensuring the reliability and continuity of power distribution. Therefore, all electrical equipment, particularly power transformers, must be maintained in safe and reliable operating conditions. Transformers function to change voltage levels from high to low or vice versa and are essential components in the electric power distribution system. One important transformer component that requires periodic maintenance is the On Load Tap Changer (OLTC). To maintain transformer performance and extend its service life, routine maintenance activities such as OLTC oil replacement are necessary. PT. PLN (Persero) ULTG Rangkasbitung conducts regular OLTC oil replacement twice a year as part of its preventive maintenance program. Transformer oil serves as both an insulating medium and a cooling agent; therefore, its condition greatly affects transformer reliability and operational safety. Degraded oil quality can cause insulation failure and reduce transformer efficiency. This study aims to describe the procedure and implementation of On Load Tap Changer oil replacement at the transformer substation of PT. PLN (Persero) ULTG Rangkasbitung. The research method used is field research, carried out through direct observation and interviews with maintenance personnel. The results of this study are expected to provide a clear understanding of the OLTC oil replacement process, support proper maintenance practices, and emphasize the importance of transformer maintenance in ensuring the reliability and sustainability of the electric power system.

In transformers installed and used in electric power systems, especially at substations (GI) there are often disturbances or problems. One of the problems with the transformer can occur due to the failure of the insulating oil which can be caused by a decrease in the value of the translucent voltage, therefore it is necessary to carry out maintenance to prevent and localize interference. Tap Changer oil is different from oil in the main tank (in terms of placement), in terms of type and function Tap Changer oil with oil in the main tank has similarities from the type of oil and its function as insulation, cooling and protecting windings from oxidation. With the method of observation (observation) in the field of all equipment worked on and with the literature method can be known the process of replacing oil On Load Tap Changer. From the results of the oil translucent voltage test study, it is known that the value of the penetrating voltage of the On Load Tap Changer oil sample used in the transformer is an average value of 38.8 kV, it can be known that the value of the new oil penetrating voltage (sample 1) being filtered is 86.8 kV, the value of the new oil penetrating voltage (sample 2) being filtered is worth 88 kV, and the value of the new oil penetrating voltage (sample 3) being filtered is 98.2 kV.