Basic structure of transformer

The basic structure of a transformer is that the transformer, as the core component of the power system, is a static electrical equipment. Its main working principle is to transfer electrical energy through electromagnetic coupling between different windings, achieving changes between different voltage levels. The structure of transformers is relatively complex and consists of many components, including the transformer body, transformer oil tank, transformer cooling device, transformer non electrical quantity protection device, and transformer outlet device.

(1) The transformer body includes an iron core, winding, insulation structure, leads, and tap changer. Below is a brief description of each part: (a) The iron core section has a structure as shown in Figure 1. Most of the iron cores are three column type, with three-phase windings A, B, and C placed separately. Therefore, the iron core is a “day” structure, forming a closed magnetic circuit. It is an important structure for the electromagnetic performance and mechanical strength of the main transformer. In order to reduce magnetic resistance, the iron core is stacked with layers of silicon steel sheets, and a nearly circular cross-section is formed by combining multiple layers of laminations. The iron core layers are tightly fastened by clamping components. In order to reduce eddy current losses in fasteners and iron choke laminations, the fastening screws, clamps, shells, and iron core are generally insulated. When the transformer is put into operation, the entire transformer will generate uneven electromagnetic fields. At this time, the iron core and iron core clamps at different positions in the electric field will induce different potentials, resulting in a floating potential. Due to the potential difference generated by the floating potential, spark discharge will occur. To avoid spark discharge, the iron core and all metal clamps need to be reliably grounded, so that the iron core potential is zero potential. At the same time, the iron core cannot have two different grounding points, Because this will create a circulation of tens of amperes, causing local overheating of the iron core.

(b) The transformer winding and insulated transformer winding are the main circuit parts of the transformer. They are made by winding insulated wires wrapped in insulating paper one by one, and the insulation layer and wires are combined to form a cylindrical whole. The structure is shown in Figure 2. The inter turn insulation of the winding is the insulation between a single wire and a wire in the same coil. Multiple turns of the winding are combined into a bundle, and each winding forms a coil, Cake to cake insulation refers to the insulation between each cake, and the cylindrical shape formed by winding is formed by wrapping high, medium, and low voltage windings together to form a concentric circle, and finally wrapped together on the iron core column. Generally, there is a voltage regulating winding on the outer layer of the high-voltage winding, and there will be a voltage regulating tap. The other end of the tap is connected to the tap switch for small-scale voltage regulation.

The insulation of a transformer mainly refers to three parts: the main insulation of the transformer, the longitudinal insulation of the winding, and the end insulation of the transformer. The main insulation of transformers refers to the insulation between transformer windings, between windings and oil tanks, between windings and iron core columns, and between windings of different voltages in the same phase, as well as between windings of different phases. The insulation structure between different phase windings basically belongs to a relatively uniform electric field. Therefore, an oil partition structure that divides the large oil pitch into small oil pitches is used to form the main insulation of transformers; The longitudinal insulation of transformer windings mainly refers to the internal insulation of each winding itself, including inter turn insulation, inter cake insulation, inter layer insulation, inter segment insulation, and insulation between line segments and electrostatic plates; Transformer end insulation refers to the insulation between the ends of the winding and the upper and lower iron yokes. Due to the geometric shape of the upper and lower iron yokes, the electric field in this area is extremely uneven. The end of the winding often has to withstand high amplitude power frequency and impulse voltage. Due to the difference in electrode shape, it is necessary to increase the distance between the electrodes. The increase in insulation distance at the end will increase the height of the transformer iron window, and the volume and weight of the transformer will also increase accordingly. Therefore, it is required to minimize the insulation distance at the ends as much as possible without increasing costs or reducing insulation strength. In the operation of the main transformer, in order to ensure its safe and reliable operation under atmospheric lightning overvoltage and operating overvoltage, new requirements are put forward for the mechanical strength, heat resistance strength, and electrical strength of the winding. In recent years, with the requirements of the State Grid for the short-circuit resistance of transformers, new requirements have been made for the mechanical strength of windings. The mechanical strength of windings refers to the instantaneous effect of the electric force generated by the maximum short-circuit current that the coil can withstand during a short circuit. (c) Transformer tap changer is mainly divided into load tap changer and no-load tap changer. It is mainly used to adjust the voltage of the load center, adjust reactive power, and adjust load current. Among them, the electrical contact system of the non excitation tap changer is composed of moving contacts, fixed contacts, and supporting components. Usually, the non excitation tap changer, along with the operating mechanism, is installed in the main oil tank of the transformer. The tap changer diagram of the transformer is shown in Figure 2.3. Figure 3 (a) shows the off load tap changer, and Figure 3 (b) shows the on load tap changer. Figure 4 shows the swinging structure of the tap changer. Due to the complex structure of tap changers, some have independent boxes, and the oil chamber of the switch is not connected to the oil tank of the oil immersed transformer, making it relatively independent。

(2) The oil tank of an oil immersed transformer plays a role in mechanical support, cooling, heat dissipation, and insulation protection. It is a container for oil immersion, the outer shell of the transformer, and the framework for the placement of other components inside the transformer. Mainly divided into: 1) Bucket type fuel tank, which is easy to manufacture, saves materials, and has good strength. But when repairing or inspecting the transformer body, a larger crane must be used to lift the body out of the oil tank. For this reason, it is necessary to have a high maintenance room on the installation site and install a crane with a large lifting capacity. This will increase a lot of construction investment. Therefore, when the transformer capacity increases to a certain limit, the operating department does not want to use this structure; 2) The bell jar type oil tank, which uses a transformer with a bell jar oil tank, only needs to lift the bell jar during maintenance, and the body can be exposed in place without moving. This does not require the installation of heavy lifting equipment at the operating location. The typical structure of the fuel tank. To meet the external transportation requirements, the top is made into a sloping surface, forming a “ridge” shape. The height of the lower box is relatively small, only containing a portion of the lower yoke. After removing the bell cover, the coil part can be completely exposed. When using a strong oil circulation guided oil cooling structure, two long sleeved reinforced channel steels on the bottom of the box are often used as oil guide channels. The oil conservator (oil storage tank) of transformers is divided into two types: the main body (oil tank) and the tap changer oil conservator (oil storage tank). Require the oil level of the main body to be slightly higher than the tapping oil level to prevent oil from the tapping oil tank from seeping into the main body oil tank. At present, the main body oil conservator adopts two types of structures: internal oil type and external oil type.

(3) The main cooling methods for transformers include self cooling, air cooling, forced oil air cooling, and forced oil water cooling. Transformers with a rated capacity of 63000kVA and below can generally use oil immersed self cooling, while transformers with a capacity of 8000kVA and above can use air-cooled cooling.

(4) The non electrical quantity protection device for transformers mainly includes a pressure release valve. The valve seat at the top of the oil tank is sealed by a metal cover with a spring. This sealing pressure can be improved by improving the quality of the spring, screening the spring, and controlling it within a small range. When the internal pressure of the transformer is 54000 ± 10% Pa, the pressure release valve opens to release pressure. When the pressure is less than 30000Pa, the pressure release valve resets.

(5) The transformer bushing is an insulation device that connects the high-voltage and low-voltage leads in the transformer to the outside of the oil tank, and plays an important guiding role in the current load inside the device. Transformer bushings are mainly divided into three types: oil and oil bushings, oil and gas bushings, and oil paper capacitive bushings. The structure is shown in Figure 6. Among them, the oil paper capacitive sleeve is currently the most widely used, and the current carrying method is to use a cable through type. The connection of the sleeve in the transformer is completed by combining the axial compression force caused by multiple sets of pressure springs. In general, sleeves above 110 kV are equipped with a heart clamp structure at the connection between the porcelain component and the connecting sleeve, which can significantly improve the sealing effect of the sleeve. During the connection process, the sleeve is equipped with a tapping device, an oil extraction valve, a vent plug, etc. Each structure has a different function. There is a small sleeve connected to the sleeve flange, which is connected to the capacitor core. Generally, the grounding copper plate on the small sleeve is used to ensure grounding. At the same time, it plays a role in maintenance and testing during the operation of the transformer, such as dielectric loss detection, insulation detection, etc.