Transformer noise is the sum of mechanical and airborne noise generated by the transformer itself and its cooling system during its structural design, selection, installation, and use. This noise is irregular, intermittent, continuous, or random. Transformer noise significantly impacts residential areas, commercial centers, light rail stations, airports, factories, mines, enterprises, hospitals, schools, and other locations.
Causes
Specifically, transformer noise has three sources: the core, the windings, and the cooler. These are the sum of noise generated under no-load, load, and cooling system conditions. The core generates noise because the silicon steel sheets forming the core undergo minute changes under the influence of the alternating magnetic field—a process known as magnetostriction. This magnetostriction causes the core to vibrate periodically with changes in the excitation frequency. The noise is caused by the magnetostrictive deformation of the core, as well as electromagnetic forces within the windings, tank, and magnetic shielding.
The vibration of the winding is caused by the electromagnetic force generated in the current-carrying winding; leakage magnetic fields can also cause structural components to vibrate. Electromagnetic noise is generated by the longitudinal vibration of the core laminations induced by the magnetic field. The amplitude of this vibration is related to the magnetic flux density in the core laminations and the magnetic properties of the core material, but has little to do with the load current. The electromagnetic force (and vibration amplitude) are proportional to the square of the current, while the emitted sound power is proportional to the square of the vibration amplitude.
Testing standards
Article 61 of the Environmental Noise Pollution Prevention and Control Law of the People's Republic of China stipulates that units and individuals suffering from environmental noise pollution have the right to demand that the perpetrator eliminate the harm; if losses are caused, compensation shall be paid in accordance with the law.
The national "Residential Building Design Code" stipulates that boilers, transformers, and other equipment rooms with noise and vibration sources should not be located in residential buildings. If such rooms must be located due to limitations, they should comply with current building fire protection, building sound insulation, and other relevant regulations. The "Code for Sound Insulation Design of Civil Buildings" stipulates that, when conditions permit, noise sources should be located underground, but not adjacent to or under the main building. If this cannot be avoided, reliable vibration isolation and sound insulation measures must be taken.
The mandatory standard GB 3096-2008, "Environmental Quality Standard for Noise," issued by my country's Ministry of Environmental Protection in 2008, stipulates that the acoustic environment is classified into the following five types according to the functional characteristics and environmental quality requirements of the area:
The equivalent sound level limits for environmental noise specified in Table 1 apply to various acoustic environment functional zones.
Table 1. Equivalent sound level limits for various acoustic functional zones
A classic analysis of transformer noise issues! (Technical aspect)
For sudden noises occurring at night in various acoustic environment functional areas, the maximum sound level shall not exceed the environmental noise limit by more than 15 dB(A).
In addition, the Ministry of Environmental Protection's "GB22337-2008 Emission Standard for Environmental Noise in Social Life," implemented on October 1, 2008, imposed stricter regulations on indoor noise emission limits, including the following provisions for structure-transmitted noise:
When the source of noise emission in social life is located inside a noise-sensitive building, and the noise is transmitted through the building structure to the interior of the noise-sensitive building, the equivalent sound level inside the noise-sensitive building shall not exceed the prescribed limit.
For cases where non-steady-state noise (such as elevator noise or water pump noise) occurs during noise measurement, the maximum sound level must not exceed the limit by more than 10 dB(A).
In addition to the two noise emission standards issued by the Ministry of Environmental Protection, the mandatory standard GB 50368-2005 Residential Building Code, which came into effect on March 1, 2006, stipulates the following regarding residential noise and sound insulation:
Residential buildings should incorporate noise reduction measures in their floor plan and building structure. The permissible noise level in bedrooms and living rooms is 50 dB (A-weighted sound level) during the day and 40 dB (A-weighted sound level) at night when windows are closed. Elevators should not be located adjacent to bedrooms or living rooms. If proximity is unavoidable due to space constraints, effective sound insulation and vibration reduction measures must be implemented. Effective sound insulation measures should be implemented for pipe shafts, pump rooms, transformer rooms, and fan rooms; vibration reduction measures should be implemented for pumps, transformers, and fans.
In addition, Article 3.1.1 of the "GBJ 118-1988 Code for Sound Insulation Design of Civil Buildings" issued by the Ministry of Housing and Urban-Rural Development and Environmental Protection of China stipulates that the permissible noise level of bedrooms, studies and living rooms in residential buildings should meet the limits of the code.
Given the numerous national standards regulating transformer noise, why do disputes between homeowners and developers regarding transformer noise still frequently occur in our communities? A significant reason lies in inadequate building inspection processes. Building inspection records reveal that many certificates only cover the construction phase, with some explicitly stating in the remarks section that water and electricity supply are not included in the inspection. It is precisely these irregularities and deficiencies in construction and inspection that lead to transformer noise disputes after property handover.
British Standard BS 661 (Terminology Related to Acoustics) has already emphasized the subjectivity of noise in its standard definition. That is, noise is unpleasant to the receiver. Therefore, it's easy to understand why people find music and noise enjoyable at a dance, while the same sound is a disturbance and annoyance when trying to fall asleep. Transformer noise is not only continuous but also mostly in the mid-range frequency range, posing minimal harm to human hearing and lacking inherent harmfulness. This means that the level of annoyance caused by transformer noise is likely related to the apparent loudness of the transformer. The best way to address this issue is to determine the apparent loudness emitted by various types and specifications of transformers.
Solution
The noise of a transformer in operation typically refers to the combined noise from the transformer itself and the cooling system. Therefore, to reduce transformer noise, effective technical measures should be taken from both aspects. Currently, the Tsinghua University Building Physics Laboratory is one of the domestic institutions specializing in transformer noise testing.
On the one hand, the noise of the transformer itself can be controlled by reducing the vibration of the iron core and reducing the noise dispersion capability; on the other hand, the noise can be attenuated in the propagation path by vibration reduction, sound insulation, sound absorption and other measures.
On the other hand, controlling the noise of the cooling system to make it close to or lower than the noise level of the transformer itself can also effectively reduce transformer noise.
one
Reduce transformer body noise
1. Technical measures adopted for iron core production
High-quality silicon steel sheets with low magnetostriction (ε) are selected. These sheets improve the integrity of the crystal orientation, and a special coating increases their tensile strength, thus reducing their magnetostriction (ε). At a magnetic flux density of 1.5T, the magnetostriction (ε) of high-grain-oriented silicon steel sheets is only 60% of that of ordinary silicon steel sheets. Therefore, under the same magnetic flux density, the lower magnetostriction (ε) of high-quality silicon steel sheets results in less vibration and a noise reduction of 2–4 dB(A).
Reducing the magnetic flux density B of the iron core: The rated operating magnetic flux density B of the iron core typically depends on the required values for noise and no-load loss. Test results show that within the rated magnetic flux density range of 1.5~1.8T, for every 0.1T reduction in magnetic flux density, the noise of the iron core can be reduced by 2~3 dB(A). The change in noise ΔLpa caused by the change in magnetic flux density can be determined by the following formula:
In the formula:
B1 and B2 represent the working magnetic flux density (T) before and after the change, respectively.
GFe2, GFe1 — Corresponding core weights (kg) of B1 and B2
It should be noted that a decrease in magnetic flux density not only leads to an increase in the size and weight of the transformer, resulting in a worse economic performance, but also increases the surface area for noise emission, thereby increasing the transformer's sound power level.
In traditional core-yoke interleaved joint structures, the magnetic field lines transversely across the nearby silicon steel sheets at the joints, generating eddy currents and magnetic saturation, leading to increased noise and no-load losses. The fully oblique interleaved joint structure ensures proper overlap between the core and yoke, reducing magnetic flux distortion and maintaining the overall mechanical strength of the core. Practical experience shows that with a magnetic flux density of 1.7T, the noise level can be reduced by 3-5 dB(A) when using a fully oblique interleaved joint core.
Increasing the yoke area reduces the magnetic flux density within the yoke. Since the noise generated by the transformer core can be effectively attenuated through the coils and shielding, most of the main body noise originates from the vibration of the yoke. In transformer design, the aspect ratio of the yoke laminations to the core laminations should be exactly the same as the ratio of their cross-sectional areas. This prevents increased noise caused by leakage flux perpendicular to the silicon steel lamination surface when magnetic flux enters the yoke from the core laminations.
Experiments have shown that increasing the number of core joints can reduce noise by 3-6 dB(A) when the transformer core is changed from two-stage to three-stage joints. This is because in a two-stage joint, the gap between the two corresponding joints only spans one lamination layer, while a three-stage joint spans two lamination layers. The magnetic flux density at the ends of each lamination layer is reduced, thus resulting in lower noise.
Data shows that transformer noise is lowest when the core clamping force is between 0.08 and 0.12 MPa. During core manufacturing, the clamping force can be appropriately controlled using a torque wrench; additionally, insulating rods can be placed between the core columns to ensure even binding force and prevent increased magnetostriction (ε) due to uneven core stress. Using these measures can reduce the transformer noise by 3–6 dB(A).
Advanced processing techniques are employed because magnetostriction (ε) is extremely sensitive to stress. Under the same magnetic flux density, the ε of a silicon steel sheet with higher stress increases dramatically with increasing stress compared to a silicon steel sheet with lower stress. Therefore, adopting advanced and reasonable processing measures, such as using automated horizontal and vertical shearing lines, controlling the stacking height of silicon steel sheets, avoiding overlapping the yoke, and pre-compressing the cardboard used for oil passages and clamp insulation, can reduce the increase in stress in the silicon steel sheets, thereby reducing transformer noise.
As mentioned earlier, the magnetostrictive vibration of the core is transmitted to the oil tank through two pathways: the core pads and the insulating oil. Placing vibration-damping rubber between the core pads and the tank bottom changes the rigid contact between the transformer body and the oil tank into an elastic contact. This blocks the transmission of some vibrations and reduces the noise of the transformer itself.
2. Technical measures adopted for the fuel tank
To reduce tank wall vibration, the overall rigidity of the fuel tank must be increased. This can be achieved by appropriately increasing the tank wall thickness or strategically arranging reinforcing ribs and controlling the rib spacing. Simultaneously, employing appropriate welding techniques can minimize welding deformation and reduce residual stress from the manufacturing process. This will improve tank wall strength, reduce tank wall vibration, and lower noise levels.
Increased tank damping can be achieved by installing rubber plates on the inner wall of the tank. For transformers with magnetic shielding, the rubber plates can be placed between the tank wall and the magnetic shielding. Ordinary industrial steel mesh can be welded between the reinforcing ribs, and a 2-3mm thick damping material can be applied to the mesh. This approach does not affect heat dissipation from the tank wall while reducing vibration and noise.
Vibration dampers are installed between the bottom of the oil tank and the foundation to avoid a rigid connection between the tank bottom and the foundation. This allows vibrations to be attenuated through the dampers, thereby reducing noise. Rubber vibration dampers and spring rubber vibration dampers are commonly used.
two
Add a sound insulation layer to reduce noise
The oil tank structure is divided into two types: modular and high-efficiency. The modular type involves making several soundproof panels on the external structure of the oil tank, with sound-absorbing materials placed inside the steel plates. Sound-absorbing materials include rock wool and fiberglass. The soundproof wall reflects noise emitted by the transformer body back; when noise passes through the soundproof wall, some of it is also absorbed, thus reducing noise levels.
Individual sound insulation panels are bolted to the reinforcing iron of the oil tank, which can reduce noise by 10-15 dB(A). Alternatively, high-efficiency sound insulation panels can be placed between two reinforcing irons and secured with thin spring steel plates. Adding weights to the frame adjusts the vibration characteristics of the high-efficiency sound insulation panel, making its amplitude significantly lower than that of the reinforcing iron, effectively shielding the noise emitted by the transformer itself and reducing noise by 5-10 dB(A).
three
Active cancellation noise reduction method
Noise reduction is achieved through a noise cancellation method. This involves placing several noise generators within 1 meter of the transformer, causing their noise to cancel out the transformer's noise. The principle is to first convert the transformer's noise signal into an electrical signal, then amplify and excite the noise generators, ensuring that the emitted noises have equal amplitude and opposite phase. This causes destructive interference to the transformer's noise, reducing it by approximately 15 dB(A).
Four
Reduce cooling system noise
Under the premise of meeting design requirements, in the design of low-noise transformers, self-cooled finned radiators should be selected instead of air-cooled radiators or forced oil circulation air coolers. This fundamentally eliminates the noise source of the cooler and can effectively reduce noise by 8~15dB(A).
When selecting a cooling device, a low-noise cooling system should be chosen. Replacing a high-flow, high-noise fan with multiple new, low-noise fans of moderate flow rate has the following advantages: First, the fans are evenly distributed, providing uniform cooling; second, if one set of fans fails, the remaining fans can still operate normally, improving the reliability of the cooling system; third, with the total cooling airflow remaining unchanged, the motor power is only 70% to 75% of that of the high-flow fan, and the noise is reduced by 2 to 3 dB(A).
Noise from the transformer body, when using vibration damping devices, causes vibration of the cooling system through the tank walls and oil. The following measures can effectively control this vibration: First, use vibration dampers between the oil tank and the radiator. Vibration dampers can be made of corrosion-resistant vibration-damping rubber or stainless steel. Test results show that vibration dampers can typically reduce the vibration noise of self-cooling finned radiators by 5-8 dB(A). Second, for side-blowing or bottom-blowing finned radiators, to avoid the fan exacerbating the vibration of the cooling system, the fan bracket should not be directly fixed to the radiator, but rather fixed to the tank wall, and vibration-damping rubber pads should be installed. Third, for transformers where the cooling system and the transformer body are installed separately, the fan should be fixed on a dedicated foundation.
five
Other methods
When designing low-noise-load transformers, self-cooling should be used whenever possible to eliminate the combined noise from fans and oil pumps. When the capacity is insufficient to meet the requirements, low-noise submersible oil pumps and low-speed fan coolers should be selected to ensure heat dissipation for large-capacity transformers. Using low-speed fans can increase the capacity of self-cooled transformers by 33% and achieve a noise level of 69 dB(A). In specific situations, dual-speed fans can also be used. When the load is low, operating the low-speed fan will reduce noise accordingly; when the load is high, operating the high-speed fan can increase the capacity of self-cooled transformers by 67%, but the noise level will be higher, reaching 75 dB(A).
When placing transformers indoors, the potential increase in noise due to reflections from noisy walls should be considered. This increase is a function of the ratio of the transformer's surface area to the transformer room's surface area and is related to the sound absorption coefficients of the walls and ceiling. Coating the walls with slag wool or similar materials can increase the sound absorption coefficient, significantly reducing noise.
