In recent years, large power transformers have been increasingly used in residential and public places, which poses certain requirements for the noise of large power transformers and small and medium-sized power transformers. The noise problem of power transformers is receiving increasing attention from people. How to reduce the noise of power transformers has become an urgent problem to be solved in the power transformer manufacturing industry.

Reasons for noise generation in power transformers
The audible noise emitted by power transformers is caused by the magnetostrictive deformation of the iron core and the electromagnetic force inside the winding, oil tank, and magnetic shield. In the past, it was believed that the noise generated by the longitudinal vibration of iron core laminations induced by magnetic fields was the main component of power transformer noise. The amplitude of this vibration is related to the magnetic flux density in the core stack and the magnetic properties of the core material, but not to the load current.
Specifically, there are three sources of noise in power transformers: the iron core, the winding, and the cooler, which is the sum of the noise caused by no-load, load, and cooling system. The reason for the noise generated by the iron core is that the silicon steel sheets that make up the iron core undergo slight changes under the action of an alternating magnetic field, known as magnetostriction, which causes the iron core to vibrate periodically with changes in excitation frequency. The reason for the vibration of the winding is that the current generates electromagnetic force in the winding, and the leakage magnetic field can also cause vibration in the structural components. The electromagnetic force (and vibration amplitude) is proportional to the square of the current, while the emitted sound power is proportional to the square of the vibration amplitude. Therefore, there is a clear relationship between the emitted sound power and the load current.
3 Factors affecting the noise of power transformers
3.1 Factors affecting no-load noise
The main reason for the noise generated by the iron core is that under the action of an alternating magnetic field, the size of the silicon steel sheet will undergo slight changes. Due to the fact that the variation period of magnetostriction is half a cycle of the power supply frequency, the vibration of the power transformer body caused by magnetostriction is based on twice the power supply frequency. Therefore, the vibration of silicon steel sheets is mainly caused by the magnetostrictive properties of ferromagnetic materials.
The magnitude of magnetostriction is related to the material of the silicon steel sheet. The higher the magnetostriction rate, the greater the noise. When the magnetic field strength is the same, the magnetostriction of high-quality silicon steel sheets is also small, so the noise is also low.
The magnetostriction rate is also related to the strength of the magnetic field, the stronger the magnetic field, ε The larger.
The magnetostriction rate is also related to whether the surface of the silicon steel sheet is painted and annealed, because the coating has adhesion to the silicon steel sheet, which can prevent deformation of the silicon steel sheet. The magnetostriction of silicon steel sheets annealed under the same magnetic field strength is much smaller than that of non annealed silicon steel sheets. This is because selecting the optimal annealing process can significantly reduce magnetostriction.
The no-load noise of power transformers is not only related to the material itself, but also to the condition of the joints.
3.2 Other factors affecting no-load noise
(1) The influence of iron core structure. Noise is related to the diameter of the core column and iron yoke, the height of the iron core window, the width of the iron core window, and the quality of the iron core. For every 1 ton decrease in the mass of the iron core, the noise can be reduced by 1/3dB (A) value. For every 0.1 decrease in the ratio of the height of the iron core window to the diameter of the iron core, the noise of the power transformer will decrease by 2-3dB.
(2) The frequency of no-load noise is based on twice the power supply frequency as its fundamental frequency. In addition to the fundamental frequency magnetic flux, there is also high-frequency magnetic flux in the iron core, so there are also high-frequency waves with no-load noise frequency of more than twice. When the natural frequency of the iron core fuel tank is close to the noise frequency, noise resonance will occur, causing an increase in noise. Therefore, the natural frequencies of the iron core and oil tank of power transformers should avoid some high-frequency waves.
(3) In order to prevent resonance of the iron core, the natural frequency of the iron core should also be considered when designing low noise power transformers. When the rated frequency of the power transformer is 50Hz, the natural frequency of the iron core should avoid the following frequency bands: 75-125Hz, 165-235Hz, 275-325Hz, 375-425Hz. If the natural frequency avoids the resonance frequency band, the iron core will not produce resonance.
4 Methods to reduce no-load noise
4.1 Reduce vibration source noise
(1) Using high permeability magnetic materials with magnetostrictive reduction, if 30ZHl20 silicon steel sheet is selected, the noise can be reduced by 4-5dB (A) compared to silicon steel sheets made of other materials.
(2) Reducing the magnetic density of the iron core can reduce noise. For every 0.1T decrease in magnetic density, it can decrease by 2-3dB (A). After reducing the magnetic density, the cross-sectional area of the iron core, the equivalent capacity of the power transformer, and the cost increase accordingly, which is uneconomical in terms of cost. Therefore, reducing magnetic density cannot exceed 10% of the standard magnetic density.
(3) Improve and reduce the seam of the iron core. The iron core adopts multi-level seams. The use of multi-level seam iron cores reduces the noise when unloaded compared to two-stage seams, which can reduce the noise by 4-5dB (A). This is because after using multi-level (four or more) seams, the magnetic flux distribution at the seams is more uniform, and the magnetic density in the air gap is greatly reduced, resulting in a reduction in noise caused by magnetic attraction at the seams. In addition, reducing the seam can reduce the amplitude, as well as the excitation capacity and current of the power transformer, resulting in a significant reduction in sound power, which can reduce no-load noise by 4-5dB (A).
(4) Prevent and reduce mechanical impact on silicon steel sheets during processing and production, otherwise it will increase the magnetostriction of the silicon steel sheets, thereby increasing the noise of the iron core.
(5) Applying epoxy or polyester adhesive to the end face of the iron core can increase the surface tension constraint of the iron core, reduce the amount of magnetostriction, and reduce noise.
4.2 In terms of absorbing sound energy and reducing vibration
(1) A portion of the noise generated by the iron core is transmitted through the bottom of the box and the foundation. If a buffer is added between the body and the fuel tank or between the fuel tank and the foundation, it can attenuate the sound through the buffer. An anti vibration rubber pad can be placed at the foot of the iron core and between the magnetic shielding and the box wall, so that when the vibration of the iron core and magnetic shielding is transmitted to the oil tank, it changes from a rigid connection to an elastic connection, thereby reducing vibration, preventing resonance, and reducing noise. It can reduce noise by 1-2dB (A).
(2) Placing sound insulation materials inside the reinforced iron of the fuel tank can absorb some of the vibration energy of the tank, thereby achieving the goal of reducing the noise of the body, which can reduce the noise by 2-3dB (A).
4.3 Adding a sound insulation layer to reduce noise
According to the structure of the fuel tank, it can be divided into two types: combination type and high-efficiency type. The combination formula is: several soundproof panels can be made according to the external structure of the fuel tank, and sound-absorbing materials can be placed inside the steel plate. Sound absorbing materials include rock wool, glass fiber, etc. The soundproof wall can reflect back some of the noise emitted by the power transformer body; And when the noise passes through the soundproof wall, it can also be absorbed to reduce the noise. The single piece soundproof panel is fixed to the fuel tank reinforcement iron with bolts, and using this soundproof panel can reduce noise by 10-15dB. Efficient soundproof panels can also be used, placed between two reinforced irons and fixed with thin spring steel plates. The frame shaped additional weight is used to adjust the vibration characteristics of the high-efficiency soundproof board, so that its amplitude is significantly lower than that of the reinforced iron, effectively shielding the noise emitted by the power transformer body and reducing the noise by 10-5dB (A).
4.4 Noise reduction method
Using noise reduction method to reduce noise. Place several noise generators within the lm of the power transformer, so that the noise they emit cancels out the noise emitted by the power transformer. Its principle is to first convert the noise signal of the power transformer into an electrical signal, then amplify and excite the noise generator, so that the noise emitted by each noise has equal amplitude and opposite phase, causing destructive interference to the noise of the power transformer, which can reduce the noise by about 15dB (A).
4.5 Other methods
(1) When designing low no-load noise power transformers, it is advisable to use self cooling as much as possible to remove the superposition of noise from fans and oil pumps. When the capacity is difficult to meet the needs, in order to meet the heat dissipation of large capacity power transformers, low-noise submersible oil pumps and low speed fan coolers should be selected as much as possible. By using a low-speed fan, the capacity of the self cooling power transformer can be increased by 33%, and the noise can reach 69dB (A). Depending on the specific situation, a dual speed fan can also be used. When the load is low and the low-speed fan is turned on, the noise can be correspondingly reduced; When the load is high, turning on the high-speed fan can increase the capacity of the self cooling power transformer by 67%, but at this time, the noise is high, reaching 75dB (A).
(2) When arranging power transformers indoors, consideration should be given to the possible increase in noise caused by the reflection of noise on the wall. This value-added is a function of the ratio of the surface area of the power transformer to the surface area of the power transformer room, and is related to the sound absorption coefficient of the walls and ceilings. The use of slag cotton or similar materials for wall coating treatment can increase the sound absorption coefficient and significantly reduce noise.