Three-phase Oil-filled Transformer

Three-phase oil-filled transformers (6–35 kV) are electromagnetic devices in which the core and windings are immersed in mineral or synthetic insulating oil within a sealed steel tank. The oil serves as both dielectric medium and heat transfer agent, circulating through internal ducts to dissipate losses to external radiators or coolers via natural convection or forced flow. These units are equipped with conservators, dehydrating breathers, pressure-relief valves, and Buchholz relays to accommodate oil volume variations and detect incipient faults. Insulation systems comply with IEC 60076 and GB/T 6451, with thermal class A (105°C) rating. They are deployed in pole-mounted installations, pad-mounted enclosures, and distribution substations, covering voltage ratios from 6 kV to 35 kV and ratings typically from 30 kVA to 10 MVA. Cooling methods include ONAN, ONAF, and OFAF. The tank structure is designed for positive-pressure sealing at 0.1–0.3 MPa. Routine maintenance requires periodic testing of oil breakdown voltage, acidity, and moisture content to sustain dielectric integrity over the service life.