The key components of oil-immersed transformers
1. Core (of a transformer or electromagnet)
The core is the most important part of the equivalent circuit in oil-immersed transformers. It is generally composed of hot-rolled or cold-rolled steel or iron oxide magnetic cores with a silicon content of about 5% and a thickness of 0.35 or 0.5 mm, coated with anti-corrosion paint on the surface. These cores are assembled by stacking silicon-containing steel or iron oxide magnetic sheets. The core is divided into two parts: the core column and the yoke. The core column is equipped with windings, and the yoke serves to close the equivalent circuit. The basic forms of the core structure are the heart type or the shell type.
2. Winding
The winding is a part of the circuit of an oil-immersed transformer. It is generally made by winding insulated layer flat copper wire or round copper core wire on a winding mold. The winding sleeve is placed on the iron core column of the oil-immersed transformer. The low-voltage winding is on the inner layer, and the high-voltage winding is placed on the surface layer of the low-voltage winding. Between the low-voltage winding and the iron core, and between the high-voltage winding and the low-voltage winding, threaded caps made of insulating layer materials are used to separate them, which is beneficial for the insulation layer.
3. Transformer oil
The composition of transformer oil in oil-immersed transformers is quite complex. It is generally composed of cycloalkanes, ethane and aliphatic hydrocarbons. In oil-immersed transformers used in power distribution equipment, the oil in the transformer plays two key roles: Firstly, it acts as an insulating layer between the windings and the core, as well as between the windings and the iron core, and within the oil tank. Secondly, when the oil is heated, it causes thermal convection, which helps conduct heat to the iron core and windings of the transformer. Common types of oil-immersed transformer oil include 10#, 25# or 45# sizes. The model indicates the temperature at which the oil begins to solidify when it is at zero degrees Celsius. For example, "25#" oil indicates that this type of oil begins to solidify at -25℃. The oil specification should be selected based on local natural conditions.
4. Oil Tank
The oil reservoir is installed on the hood of the engine oil tank. The volume of the oil reservoir is approximately 10% of the weight of the engine oil tank. There is a pipeline connection between the oil reservoir and the engine oil tank. When the volume of the oil-immersed transformer changes with the temperature of the liquid and expands or contracts, the oil reservoir functions as an oil storage tank and for refilling oil, ensuring that the core and windings are immersed in oil; and due to the installation of the oil reservoir, the contact area between the oil and air is reduced, thereby lowering the deterioration rate of the oil.
On the side of the oil reservoir, there is an oil level indicator. Near the glass tube, there are the relative height reference lines of the oil level at -30℃, +20℃ and +40℃, indicating the relative height of the oil level that an unused oil-immersed transformer should reach; the reference lines can effectively reflect whether there is sufficient remaining oil during the operation of the oil-immersed transformer under various conditions.
Breathing holes are used on the oil reservoir to allow the upper indoor space of the oil reservoir to communicate with the atmosphere. When the oil in the oil-immersed transformer expands and contracts due to heat, the gas in the upper part of the oil reservoir can enter and exit through the breathing holes, causing the oil level to rise or fall, thus preventing the oil tank from deforming or even being damaged.
5. Insulation tube
This is an important insulating layer equipment on the oil-immersed transformer box. The majority of oil-immersed transformers use porcelain insulating tubes for insulation. Through high and low voltage insulating tubes, the high and low voltage windings of the oil-immersed transformer are connected from the oil tank to the outside of the oil tank, providing insulation for the winding-to-ground voltage (shell and core) of the oil-immersed transformer. Moreover, it is the main component for connecting the fixed wires to the external circuit. The high-voltage porcelain bushing is relatively tall and large, while the low-voltage porcelain bushing is relatively shorter.
6. Split taps
For oil-immersed transformers, changing the tap device of the high-voltage winding and adjusting the tap position can increase or decrease the number of turns in a portion of the primary winding, thereby changing the voltage ratio and achieving voltage regulation. Oil-immersed transformers that are taken out of operation and disconnected from the power grid are adjusted in voltage by manually switching the tap switch area, which is called no-load voltage regulation.
7. Gas relay
The gas relay is connected to the middle of the rubber tube between the oil tank and the oil reservoir of the oil-immersed transformer, and is connected to the control circuit to form the gas protection device. The upper contact of the gas relay and the light gas signal form an independent control circuit, and the lower connector of the gas relay is connected to the external circuit to form the heavy gas protection. The heavy gas position causes the high-voltage universal circuit breaker to issue a heavy gas action signal.
8. Explosion-proof tube
The explosion-proof tube is a safety protection device for oil-immersed transformers. It is connected to the upper part of the cover of the oil-immersed transformer. The explosion-proof tube is in communication with the atmosphere. When a fault occurs and generates heat, the oil in the oil-immersed transformer will vaporize, triggering the gas relay to send out an alarm signal or shutting off the power supply to prevent the oil tank from cracking.
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