Summary
Transformer noise is the sum of mechanical noise and air noise caused by irregular, intermittent, continuous or random generated by the transformer body and cooling system during the design, selection, layout, installation and use of the body structure. The noise generated by transformers affects residential areas, commercial centers, light stations, airports, factories and mines, enterprises, hospitals, schools and other places.
cause
Specifically, the transformer noise has three sound sources, one is the core, the other is the winding, and the third is the cooler, which is the sum of the noise caused by the no-load, load and cooling systems. The cause of the noise generated by the iron core is that under the action of the alternating magnetic field of the iron core silicon steel sheet, a small change, that is, magnetostriction, which causes the core to periodically vibrate with the change of the excitation frequency, the core magnetostrictive deformation and the winding, the oil tank and the magnetic shield Caused by internal electromagnetic force.
The reason why the winding generates vibration is that electromagnetic force is generated in the current winding, and the leakage magnetic field can also cause vibration of the structural member. The cause of electromagnetic noise is that the magnetic field induces the core lamination to generate noise along the longitudinal vibration. The amplitude of the vibration is related to the magnetic flux density of the core lamination and the magnetic properties of the core material, but has little relationship with the load current. The electromagnetic force (and vibration amplitude) is proportional to the square of the current, and the transmitted sound power is proportional to the square of the vibration amplitude.
Testing standard
Article 61 of the Law of the People's Republic of China on the Prevention and Control of Environmental Noise Pollution stipulates that units and individuals that are endangered by environmental noise pollution have the right to require the injurer to eliminate the hazard; if the loss is caused, the loss shall be compensated according to law.
The National "Code for Design of Residential Buildings" stipulates that it is not appropriate to arrange boilers, transformers and other equipments with noise and vibration sources in residential buildings. If it is required to be arranged under conditions, it shall comply with the current regulations for building fire protection, building sound insulation and related regulations. The "Code for Design of Sound insulation for civil buildings" stipulates that when conditions permit, it is easy to set the noise source underground, but it should not be adjacent to the theme building or under the main building. If it cannot be avoided, reliable vibration isolation and sound insulation measures must be taken.
In 2008, the mandatory standard “GB 3096-2008 Acoustic Environmental Quality Standard†issued by the Ministry of Environmental Protection of China stipulates that the acoustic environment is divided into the following five types according to the functional characteristics of the area and the environmental quality requirements:
The ambient noise equivalent sound level limits specified in Table 1 shall apply to all types of acoustic environment functional areas.
Table 1 Environmental noise equivalent sound level limits for various acoustic environmental functional areas
"Technology" classic analysis of transformer noise problems!
The burst noise at night in all kinds of acoustic environment functional areas shall not exceed 15 dB(A) for the maximum sound level exceeding the environmental noise limit.
In addition, the Ministry of Environmental Protection issued the "GB22337-2008 Social Living Environment Noise Emission Standard" implemented by the Ministry of Environmental Protection on October 1, 2008, which imposes stricter regulations on indoor noise emission limits. The provisions for structural propagation noise are as follows:
When the noise source of social life is located in a noise-sensitive building, when the noise propagates through the building structure to the noise-sensitive building, the indoor equivalent sound level of the noise-sensitive building shall not exceed the specified limit.
For unsteady noise (such as elevator noise, pump noise) during noise measurements, the maximum sound level must not exceed the limit by 10 dB(A).
In addition to the two standards for noise emission by the Ministry of Environmental Protection, the Ministry of Construction's mandatory standard “GB 50368-2005 Residential Building Code†implemented on March 1, 2006 has the following provisions for residential noise and sound insulation:
Residential buildings should adopt noise prevention measures in terms of floor plan and building structure. In the bedroom and living room, the noise level is 50dB (A sound level) during the day when the window is closed, and the noise level is 40dB (A sound level) at night. The elevator should not be placed next to the bedroom or living room. When conditions are required to be placed in close proximity, effective sound insulation and vibration damping measures must be taken. Pipeline wells, pump houses, transformer rooms, and fan rooms should adopt effective sound insulation measures. Pumps, transformers, and fans should adopt vibration reduction measures.
In addition, Section 3.1.1 of the GBJ 118-1988 Code for Design of Sound insulation for Civil Buildings issued by the Ministry of Environmental Protection and Environmental Protection of China stipulates that the allowable noise level of the bedroom, study and living room in the house should meet the limits of the specification.
Since there are so many national standards to regulate transformer noise, why do we often have transformer noise disputes between owners and developers around our lives? A large part of this reason should be attributed to imperfections in building acceptance. We can find out from the construction acceptance record that the acceptance of many buildings is only the construction part, and the remarks of some acceptance certificates clearly indicate that the water supply and power supply are not included in the inspection. It is precisely because of some irregularities and imperfections in the construction and acceptance of construction that the transformer noise dispute after the delivery of the house will be caused.
In the British Standard BS 661 (acoustic terminology), the subjectivity of noise has been emphasized with standard definitions. That is, for the receiver of noise, noise is annoying. Therefore, it is easy to understand why people feel music and noisy at the dance is a kind of enjoyment, and when people want to fall asleep, even the same voice will feel a kind of interference and trouble. Transformer noise is not only continuous, but most of it belongs to medium audio. The hearing damage to human beings is minimal, and there is no inherent harmfulness. This means that the noise caused by transformer noise is probably related to the apparent volume of the transformer. The best way to solve this problem is to determine the apparent loudness of the various types and sizes of transformers.
Solution
The noise of the transformer in operation usually refers to the noise of the transformer's body noise and the noise of the cooling device. Therefore, in order to reduce the noise of the transformer, effective technical measures should also be taken from these two aspects. At present, the domestic transformer noise detection organization has the Building Physics Laboratory of Tsinghua University.
On the one hand, the bulk noise of the transformer can be controlled by reducing the vibration of the core and reducing the divergence of the noise; the noise can be attenuated in the propagation path by means of vibration reduction, sound insulation and sound absorption.
On the other hand, controlling the noise of the cooling system so that its noise is close to or lower than the body noise level can also effectively reduce the transformer noise.
one
Reduce transformer body noise
1. Technical measures taken by the core
The high-quality silicon steel sheet with high magnetostriction ε is used to improve the integrity of the crystal orientation. The special coating increases the tensile strength and reduces the magnetostriction ε. When the magnetic flux density is 1.5T, the magnetostriction ε of the high grain oriented silicon steel sheet is only 60% of that of a general silicon steel sheet. Therefore, under the same magnetic density, the magnetostriction ε of the high-quality silicon steel sheet is small, and the generated vibration is also relatively small, and the noise can be reduced by 2 to 4 dB (A).
Reducing the magnetic flux density of the core B The rated working magnetic density B of the core is usually dependent on the required values ​​of noise and no-load loss. The test results show that the rated magnetic density B is in the range of 1.5~1.8T, and the magnetic core noise is reduced by 2~3dB(A) for every 0.1T of magnetic density reduction. The amount of noise change ΔLpa caused by the change in magnetic density can be determined by the following formula:
In the formula:
B1, B2 - working magnetic density (T) before and after the change
GFe2, GFe1 - core weight corresponding to B1, B2 (kg)
It should be noted that the reduction in magnetic density not only leads to an increase in the volume and weight of the transformer, but also deteriorates the economic index and increases the surface area of ​​the noise emission, resulting in an increase in the sound power level of the transformer.
Core structure with full oblique staggered joints In the traditional stud and yoke staggered joint structure, the magnetic lines of force cross the nearby silicon steel sheet at the joint, causing eddy currents and magnetic saturation, resulting in increased noise and no-load losses. . The use of full oblique staggered seams ensures that the stem and the iron yoke overlap, reducing the flux distortion and ensuring the overall mechanical strength of the core. Practice has proved that when the magnetic density is 1.7T, the core can reduce the noise by 3~5dB(A) by using the full oblique staggered joint noise.
Increasing the area of ​​the iron yoke to reduce the magnetic flux density in the iron yoke Since the noise generated by the transformer stem can be effectively attenuated by the coil and the screen, the bulk noise is mostly derived from the vibration of the iron yoke. In the design of the transformer, it should be ensured that the ratio of the width of the yoke to the stem of each stage should be exactly the same as the ratio of their cross-sectional areas. In this way, it is possible to prevent the noise from increasing due to the leakage flux that generates the surface of the vertical silicon steel sheet when the magnetic flux enters the iron yoke from the stem.
Tests to increase the core joint have shown that when the transformer core is changed from a two-stage joint to a three-stage joint, the noise can be reduced by 3~6dB(A). This is because in the two-stage seam, the corresponding two seam gaps only span one laminate, and the three-stage seam spans the two laminates, and the magnetic density at the end of each layer across the laminate is reduced, thus resulting in The noise is reduced.
Controlling the core clamping force The data shows that the transformer noise is the lowest when the core clamping force is 0.08~0.12Mpa. In the manufacturing process of the core, the clamping force can be reasonably controlled by the torque wrench; at the same time, the insulating rod can be placed between the core columns to make the force of the stem binding uniform, and the magnetostriction ε is prevented from increasing due to the uneven force of the core. Using the above measures, the body noise can be reduced by 3~6dB(A).
Magnetostrictive ε is extremely sensitive to stress using advanced processing techniques. Under the same magnetic density condition, the silicon steel sheet with larger stress has a sharp increase with the increase of stress compared with the silicon steel sheet with less stress. Therefore, advanced and reasonable processing measures such as: automatic horizontal and vertical shear lines are used to control the stacking height of silicon steel sheets, and iron yokes are not stacked, and the paperboard used for oil passage and clamp insulation is pre-compacted. Such measures can reduce the stress increase of the silicon steel sheet, thereby reducing the transformer noise.
The damping rubber is placed between the iron foot and the bottom of the box. As described above, the magnetostrictive vibration of the core is transmitted to the oil tank through the two steps of the foot and the insulating oil. A damping rubber is placed between the core foot and the bottom of the tank to make the rigid contact between the body and the tank elastic. Thereby, the transmission of part of the vibration is blocked, and the body noise is reduced.
2. Technical measures taken by the fuel tank
Increasing the strength of the wall and reducing the amplitude of the wall to reduce the amplitude of the wall must increase the overall rigidity of the tank. To this end, the thickness of the box wall can be appropriately increased or the reinforcing ribs can be reasonably arranged to control the spacing of the ribs. At the same time, with a reasonable welding process, the welding deformation of the box wall is reduced, and the residual stress in the manufacturing process is reduced. In this way, the strength of the wall can be increased, the amplitude of the wall can be reduced, and noise can be reduced.
Increase the tank damping to provide a rubber plate on the inner wall of the tank. For transformers with magnetic shielding, the rubber sheet can be placed between the wall and the magnetic shield. The ordinary industrial steel mesh is welded between the reinforcing ribs, and the 2-3 mm thick damping material is applied on the net, so that the heat dissipation of the box wall is not affected, the vibration of the box wall is reduced, and the noise is reduced.
A damper is arranged between the bottom of the tank and the foundation to provide a damper between the bottom of the tank and the foundation to avoid the rigid connection between the bottom of the tank and the foundation, so that the vibration is attenuated by the damper to achieve the purpose of reducing noise. Rubber dampers and spring rubber dampers are commonly used.
two
Add sound insulation layer to reduce noise
The fuel tank structure is divided into two types: combined type and high efficiency type. The combination type is: the outer structure of the oil tank can be made into several pieces, and the sound absorbing material is placed in the steel plate. Sound absorbing materials are: rock wool, fiberglass and the like. The sound insulation wall can reflect part of the noise emitted by the transformer body; when the noise passes through the sound insulation wall, it can also be absorbed to reduce the noise.
The single-piece baffle is bolted to the tank to reinforce the iron. This baffle can reduce noise by 10-15dB(A). High-efficiency baffles can also be used, placed between two reinforced irons and fixed by thin spring steel plates. The frame-shaped additional weight adjusts the vibration characteristics of the high-efficiency baffle, so that its amplitude is significantly lower than the enhanced iron amplitude, effectively shielding the transformer body from noise, and reducing the noise by 5-10dB(A).
three
Active offset silencer
Noise reduction is used to reduce noise. That is, a number of noise sounders are placed inside the transformer lm so that they emit noise and the noise emitted by the transformer cancels each other out. The principle is to first convert the transformer noise signal into an electrical signal, and then amplify the excitation noise sounder, so that each noise emits noise, the amplitude is equal, and the phase is opposite, so that the transformer noise is destructively interfered, and the noise can be reduced by about 15 dB(A).
four
Reduce cooling system noise
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