Oil-immersed transformers can be classified by phase into three-phase transformers and single-phase transformers. In three-phase power systems, three-phase transformers are generally used; however, when the capacity is excessively large and transportation constraints apply, a bank of three single-phase transformers may also be employed.

　　Introduction to Oil-Immersed Transformers

　　Distribution transformers are critical equipment in the power supply and distribution systems of industrial and mining enterprises as well as civil buildings. They step down the 10 (6) kV or 35 kV network voltage to the 230/400 V busbar voltage used by end users. These products are suitable for 50 (60) Hz AC operation, with a maximum rated three-phase capacity of 2500 kVA (the maximum rated single-phase capacity is 833 kVA; single-phase transformers are generally not recommended). They can be installed indoors or outdoors, and when the rating is 315 kVA or less, they may also be pole-mounted.

　　Classification of Oil-Immersed Transformers

　　1. Based on their construction, transformers are classified as core-type or shell-type. If the windings surround the core, it is a core-type transformer; if the core surrounds the windings, it is a shell-type transformer. The two types differ only slightly in structure and are essentially identical in principle. All power transformers are core-type.

　　2. Based on the number of phases, transformers can be classified as three-phase or single-phase. In three-phase power systems, three-phase transformers are generally used; however, when the capacity is excessively large and transportation constraints apply, a bank of three single-phase transformers may also be employed.

　　3. Based on the number of windings, transformers can be classified as two-winding or three-winding transformers. Most conventional transformers are two-winding types, featuring two windings on the core: a primary winding and a secondary winding. Three-winding transformers are typically used for larger capacities (above 5,600 kVA) to connect three different voltage transmission lines. In special applications, Satons transformers with even more windings may also be employed.

　 　The principle of oil-immersed transformers

　　The components of an oil-immersed transformer include the core assembly (core, windings, insulation, and leads), transformer oil, the tank and cooling system, the voltage regulation device, protective devices (desiccant breather, pressure relief valve, gas relay, oil conservator, and temperature measuring devices, etc.), and bushings.

　　Oil-immersed transformers are essential equipment in power systems, primarily used to change voltage and transmit electrical energy, forming the foundation for the safe and economical operation of the grid. They operate mainly on the principle of electromagnetic induction. On a closed iron core, two electrically insulated windings are wound: the side connected to the power source is called the primary winding, and the side delivering electrical energy is called the secondary winding.

　　When an AC supply voltage is applied to the primary winding, an AC current flows through it, producing an alternating magnetic flux in the core. This alternating flux links both the primary and secondary windings, inducing electromotive forces E1 and E2 in each winding. If the secondary winding is connected to a load, a current will flow into the load, resulting in power output from the secondary winding.

　 　Structure of Oil-Immersed Transformers

　　A transformer mainly consists of a core, windings, an oil tank, an oil conservator, insulating bushings, a tap changer, and a gas relay.

　　1. The core is the magnetic circuit of a transformer. During operation, it generates hysteresis and eddy current losses, causing it to heat up. To reduce these losses and minimize size and weight, the core is constructed from cold-rolled grain-oriented silicon steel sheets with high permeability and a thickness of less than 0.35 mm. Depending on how the windings are arranged relative to the core, transformers are classified as either core-type or shell-type.

　　In large-capacity transformers, cooling oil passages are often incorporated into the core to ensure that heat generated by core losses is efficiently removed by the circulating insulating oil, thereby achieving effective cooling.

　　2. Both windings and the core are essential components of a transformer. Because the windings themselves have resistance, and because contact resistance exists at joints, heat is generated according to the I²Rt relationship. Therefore, windings must not carry currents exceeding their rated value for extended periods. Furthermore, during a short-circuit condition, large electromagnetic forces develop in the windings, which can damage the transformer. The basic winding arrangements include concentric and interleaved types.

　　3. The main faults of transformer windings are turn-to-turn short circuits and short circuits to the enclosure. Turn-to-turn short circuits primarily result from insulation aging, or from mechanical damage to the insulation caused by transformer overload or through-faults. They can also occur when the oil level inside the transformer drops, exposing the windings; additionally, during through-faults, overcurrent-induced deformation of the windings can mechanically damage the insulation, leading to turn-to-turn shorts. In the case of a turn-to-turn short circuit, the current in the shorted winding may exceed its rated value, while the total winding current remains within limits. Under these conditions, gas protection will operate, and in severe cases, differential protection may also trip. Short circuits to the enclosure are likewise caused by insulation aging, moisture ingress into the oil, falling oil levels, or lightning and switching overvoltages. Furthermore, during through-faults, overcurrent-induced winding deformation can also produce a short circuit to the enclosure. For such faults, gas protection and earth-fault protection typically operate.

　　4. The core (windings and core) of an oil-immersed transformer is housed in an oil tank filled with transformer oil; the tank is welded from steel plates. For medium- and small-sized transformers, the oil tank consists of a tank shell and a tank cover. The transformer core is placed inside the tank shell, and the core can be lifted out for inspection and maintenance by opening the tank cover.

　　Characteristics of Oil-Immersed Transformers

　　Oil-immersed transformers feature a fully sealed design, isolating the transformer oil from air to slow down oil degradation. Their surfaces are coated with electrostatic powder coating, providing an attractive appearance and excellent corrosion resistance. Under severe overload conditions, the winding insulation resists aging and thermal breakdown, maintaining outstanding electrical and mechanical properties throughout their service life. Redesigned coil oil passages reduce hotspot humidity, while the redesigned tank increases the heat-dissipating surface area, ensuring that severe overloads do not cause oil degradation.