How the transformer works
The primary winding and the secondary winding of the transformer are equivalent to two inductors. When an alternating voltage is applied to the primary winding, an electromotive force is formed on the primary winding, generating an alternating magnetic field, and the secondary winding is subjected to the primary winding. The induced electromotive force (voltage) is also generated in the same manner as the primary winding magnetic field, so that the secondary winding outputs an alternating voltage, which is the transformer’s variable voltage process.
The output voltage of the transformer is related to the number of turns of the winding. The ratio of the output voltage to the input voltage is generally equal to the ratio of the number of turns N2 of the secondary winding to the number of turns N1 of the primary winding, ie U2/U1=N2/N1; the output current of the transformer In inverse proportion to the output voltage (I2/I1=U1/U2), the output voltage of the step-down transformer is usually reduced, but the output current is increased and has the ability to output a strong current.
The main components and functions of the transformer
The most basic structural components of a transformer consist of a core, windings and insulation. In addition, for safe and reliable operation, a fuel tank, a cooling device and a protection device are also installed.
(1) Iron core
The iron core of the transformer is the path of the magnetic flux line, which acts to concentrate and strengthen the magnetic flux, and at the same time to support the winding.
(2) Winding
The winding of the transformer is the path of the current, the current is passed through the winding, and the induced electromotive force is generated by the electromagnetic induction.
(3) Fuel tank
The oil tank is the outer casing of the oil-immersed transformer. The main body of the transformer is placed in the fuel tank, and the tank is filled with transformer oil.
(4) Oil pillow
The oil pillow is also called auxiliary oil tank. It is a round barrel-shaped container made of steel plate. It is horizontally installed on the fuel tank cover of the transformer. It is connected with the fuel tank by a curved connecting pipe. The oil level is provided at one end of the oil pillow. The volume of the oil pillow is generally 8% to 10% of the volume of the oil contained in the transformer tank. Its function is that the inside of the transformer is full of oil, and because the oil level in the oil pillow is within a certain limit, there is room for manoeuvre when the oil expands and contracts at different temperatures, and the position of the air pillow is small, so that the oil and air are in contact with each other. The possibility of oil moisture and oxidation is reduced. In addition, the oil in the oil conservator is much lower than the oil in the upper part of the tank, and hardly convects the oil in the tank. A gas relay is installed on the connecting pipe of the oil pillow and the fuel tank to reflect the internal fault of the transformer.
(5) Respirator
The respirator contains a desiccant, ie silica gel, to absorb moisture from the air.
(6) Explosion-proof pipe
The explosion-proof pipe is installed on the fuel tank cap of the transformer. The top end of the explosion-proof tube is equipped with a piece of glass. When a fault occurs inside the transformer, a high pressure is generated, and the gas inside the oil breaks through the glass sheet and is discharged to the outside of the tank to release the pressure, thereby protecting the transformer tank from being damaged.
(7) Thermometer
The thermometer is installed in the side temperature cylinder on the fuel tank cap to measure the temperature of the upper oil in the fuel tank.
(8) Bushing
The bushing is an insulating device that leads the leads of the high and low voltage windings of the transformer to the outside of the fuel tank. It is both the insulation of the lead to the ground (the outer casing) and the role of the fixed lead.
(9) Cooling device
The cooling device is a device that dissipates heat generated by the transformer during operation.
(10) Oil purifier
also known as temperature difference filter. Its main part is a cylindrical net oil tank welded by steel plate, which is installed on one side of the transformer tank. The tank is filled with adsorbent such as silica gel and activated alumina. In operation, due to the temperature difference between the upper oil and the lower oil, the transformer oil flows from top to bottom through the oil purifier to form convection, and the oil is in contact with the adsorbent, wherein the water, acid and oxide are absorbed, so that the oil Get purified. Extend the life of the oil. The oil purifier of the strong oil circulation transformer relies on the oil flow pressure difference to make the transformer oil flow through the net oil pump to achieve the purpose of purification.
Use of transformer
A power transformer (referred to as a transformer) is an electrical device used to change the magnitude of an alternating current voltage. According to the principle of electromagnetic induction, it exchanges a certain level of AC voltage into another level of AC voltage to meet the needs of different loads. Therefore, transformers play a very important role in power systems and power supply systems.
The output voltage of the generator is usually 6.3KV, 10.5KV and up to 20KV due to the insulation level of the generator. It is difficult to carry out long-distance transmission with such a low voltage. Because when the power is supplied with a certain amount of power, the lower the voltage, the larger the current, and the electric energy may be mostly consumed on the resistance of the power line. Therefore, only the step-up transformer can be used to raise the terminal voltage of the generator to tens of thousands of volts to hundreds of thousands of volts in order to reduce the current transmission and reduce the energy loss on the transmission line without increasing the cross section of the conductor to transmit the power over a long distance.
After the transmission line delivers tens of thousands of volts or hundreds of thousands of volts of high-voltage electrical energy to the load zone, the high-voltage must be reduced by a step-down transformer to a low voltage suitable for use in electrical equipment. For this reason, in the power supply and demand system, a step-down transformer is required to convert the high voltage delivered by the transmission line into voltages of various grades to meet various composite needs.
Transformer types and characteristics
(1) by purpose
1. Power transformer for boosting or stepping down the power system.
2. Test the transformer to generate high voltage and conduct high voltage test on the electrical equipment.
3. Instrument transformers, such as voltage transformers, current transformers, for measuring instruments and relay protection devices.
4. Special-purpose transformers, electric furnace transformers for smelting, rectifier transformers for electrolysis, welding transformers for welding, and voltage regulating transformers for testing.
(2) According to the number of phases
1. Single-phase voltage regulator for single-phase load and three-phase transformers.
2. Three-phase transformer for the rise and fall of three-phase systems.
(3) According to the form of winding
1. Autotransformer for connecting ultra-high voltage, large capacity power systems.
2. Double winding transformer for connecting two voltage grade power systems.
3. Three-winding transformer for connecting three voltage grade power systems, generally used in regional substations of power systems.
(4) According to the core form
1. Core transformer for high voltage power systems.
2. Shell-type transformers, special transformers for high currents, such as electric furnace transformers and electric welding transformers; or for power transformers such as electronic instruments and televisions, radios, etc. The shell structure can also be used for large capacity power transformers.
(5) According to the cooling medium
1. Oil-immersed transformers, such as oil-immersed self-cooling, oil-immersed air-cooled, oil-immersed water-cooled, forced oil circulation air-cooled and water-cooled.
2. Dry type transformer, relying on air convection to cool. This kind of transformer with low voltage and no oil is usually cooled by fan, which is suitable for fire prevention and other occasions. In the factory low-voltage transformers in the 600 MW unit, dry-type transformers are commonly used for fire protection requirements.
3. Inflatable transformer, using special gas (SF6) instead of transformer oil to dissipate heat.
4. Evaporate the cooling transformer and replace the transformer oil with special liquid for insulation and heat dissipation.
(VI) “Power Transformer Operation Regulations” DL/T572-1995 classifies transformers into three categories according to capacity
1. Distribution transformer, transformer with voltage of 35kV and below, three-phase rated capacity of 2500kVA and below, single-phase rated capacity of 83kVA and below, with independent winding and natural circulation cooling.
2. In the transformer, the three-phase rated capacity does not exceed 100MVA or the capacity of each column does not exceed 33.3MVA, with independent winding, and the rated short-circuit impedance Z meets the requirements.
3. Large transformer, three-phase rated capacity above 100MVA, or its rated short-circuit impedance Z meets the requirements.
Transformer fault statistics
(1) Failure of various types of transformers
According to the statistics of transformer faults displayed by the relevant departments on the type of transformers, one can see that power transformer faults always occupy the dominant position.
(2) Transformer failure of different users
Transformers are used in different sectors and the failure rates are different. To analyze the risk of transformer failure, users can be divided into 11 separate types: (1) cement and mining; (2) chemical, oil and gas; (3) power sector; (4) food processing; Medical care; (6) manufacturing; (7) metallurgical industry; (8) plastics; (9) printing; (10) commercial construction; (11) pulp and paper industry.
When considering the frequency and degree, the risk of the power sector is the highest, and the metallurgical industry and manufacturing are ranked second and third respectively.
(3) Failure of transformers of various service life
According to the transformer designer, the service life of the transformer under “ideal conditions” can reach 30 to 40 years, which is obviously not the case in practice. In the 1975 study, the average life of the transformer at the time of failure was 9.4 years. In the 1985 study, the average life of the transformer was 14.9 years.
The service life of transformers in the power industry should be given special attention. China has experienced a period of rapid industrial development since the reform and opening up, and the large-scale development of basic industries, especially the power industry. These devices, which were installed from the early 1980s to the 1990s, are now in the aging stage, depending on their design and operation. Today, these devices have been in operation for almost 25 years, so special attention must be paid to the possibility of failure of installed transformers.
Analysis of the causes of transformer failure
(1) Line inrush current
Line inrush (or line interference) is listed first among all factors that cause transformer failure. This category includes operational overvoltages, voltage spikes, line faults/flashovers, and other transmission and distribution (T&D) anomalies caused by misoperations, transformer de-parallel, and on-load tap-changer tapping. This type of cause accounts for the vast majority of transformer failures.
(2) Insulation aging
In the past 10 years, insulation aging has been ranked second in the cause of the failure. Due to the insulation aging factor, the average life of the transformer is only 17.8 years, much lower than the expected life of 35 to 40 years! In 1983, the average life of the transformer was 20 years.
(3) damp
This category of moisture includes flooding, pipeline leakage, roof leakage, moisture intrusion into the tank along casing or fittings, and moisture in the insulating oil.
(4) Poor maintenance
Insufficient maintenance is listed as the fourth leading factor in transformer failure. This category includes protection devices without transformers or incorrect installation, coolant leakage, fouling, and corrosion.
(5) overload
This category includes faults that are determined to be caused by overload, and only those transformers that have long been in a small horse-drawn cart that exceeds the power rating of the nameplate. Overloading often occurs when the power plant or the power sector continues to slowly increase the load. Eventually the transformer is overloaded, and too high temperatures lead to premature aging of the insulation. When the insulation board of the transformer ages, the insulation strength of the insulation paper decreases. Therefore, the impact of an external fault may cause insulation damage and malfunction.
(6) Lightning strike
Lightning waves appear to be less than previous studies because of changes in the classification of causes. Now, unless it is explicitly a lightning strike, the general impact fault is classified as “line inrush.”
(7) Three-phase load imbalance
Due to the long-term overload of a phase caused by the imbalance of the three-phase load, the temperature of the phase is high and the insulation is aged, resulting in an inter-turn short circuit or a phase-to-phase short circuit.
(8) loose connection
Loose connections can also be included in under-maintenance, but there is enough data to list them separately, so it is different from previous studies. This category includes manufacturing processes and maintenance in electrical connections. One of the problems is the improper cooperation between metals of different natures, although this phenomenon has decreased in recent years. Another problem is that the fastening between the bolted joints is not appropriate.
(9) Poor process/manufacturing
In HSB’s 1998 study, only a small percentage of failures were attributed to process or manufacturing defects. For example, the outlet end is loose or unsupported, the block is loose, the welding is poor, the core is poorly insulated, the short circuit strength is insufficient, and foreign matter remains in the fuel tank.
(10) Destruction and vandalism
This category is usually identified as apparent vandalism. The United States has not reported on transformer failures in this area for the past 10 years. In China, this phenomenon has occurred from time to time, such as theft and vandalism.
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