Oil-immersed transformers have several isolated independent oil systems. During operation, the oils in these independent systems are not interconnected, and their quality and operating conditions differ. Gas chromatography analysis of the oil content must be performed separately to determine the presence of potential faults.
(1) Main Body Oil System: This system connects to the oil surrounding the windings. It includes the oil in the cooler or radiator, the oil in the conservator, and the oil in the 35kV and below oil-filled bushings.
(2) Oil in the On-Load Tap Changer Switch Chamber: This section has its own protection system, including a current relay, conservator, and pressure relief valve. The oil in this switch chamber serves as insulation and current extinguishing. The oil will be released when the switch cuts off the load current. This system requires excellent sealing performance, even during arcing pressure generated during switching.
(3) Fully Sealed System for 60kV and Above Voltage Levels: This system primarily serves as insulation or increases the electrical strength of the insulating paper in the oil-capacitive bushing. When filling the main body with oil, the bushing end terminals should be sealed to prevent air ingress.
(4) Oil in the high-voltage outgoing line box or the oil in the ignition-type outgoing line box. The high-voltage outgoing lines of a three-phase 500kV transformer pass through a corrugated insulating isolation oil system. This oil system mainly serves an insulating function.
(5) When conducting various insulation tests on an oil-immersed transformer, the first step is to vent the gas by releasing any potentially stored gas through the vent plug. Gas chromatography analysis of the oil in each system can help predict potential faults.