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Why are power transformers so noisy?

As shown in this video of a 120 MVA power transformer.

An example of a noisy power transformer

This noise, known as transformer hum, is so loud that mandatory personnel must wear protective gear at power substations. Close to residential, commercial and medical facilities, the noise is unbearable. It's like using a hair dryer or a vacuum cleaner 24/7! Tables 1 and 2 compare transformer hum to known ambient sound pressure levels in decibels (dB). We can see that the quietest transformer makes a similar amount of noise to a refrigerator. As the rated power increases, so does the noise level, until it is equivalent to that of a vacuum cleaner.

Table 1: Ambient sound pressure levels (left). Table 2: Transformer sound pressure levels (right)

Consequently, installation of transformers close to people without protective equipment must meet strict noise emission requirements. To avoid over-engineering, the transformer hum does not need to be significantly lower than the sound levels of other local equipment, such as cooling fans. Classification, design, assembly and installation affect the sound level.

What is Cold-Rolled Grain-Orientated Electrical Silicon Steel (CRGO/GOSS/GOES)?

Adding silicon to steel increases the electrical resistivity of iron, reducing the eddy current losses. For transformers, unidirectional permeability is desired. This is achieved by orientating the crystals and grains in the material. As a result, the flux permeates easily in the orientated direction with the application of an external field. The material is thinned using cold rolling to align the crystals. This has two advantages over hot rolling: Firstly, it aligns the crystals in the direction of the rolling, and secondly, it makes it possible to make the material thinner. Thinner layers of steel sheets separated by an insulating layer further reduces eddy currents.

What causes transformer humming?

To understand the sources of transformer hum, we must first look at the structure of a transformer. Figure 1shows a three-phase transformer. Phases A, B, and C (or U, V, and W) are wrapped around one of the legs, corresponding to the red, blue, and green bodies in the image below. The main magnetic flux path is well defined and unidirectional. We apply this advantage to maximize transformer power density using unidirectional (anisotropic) electrical steels. They are cold-rolled grain-oriented electrical silicon steels (CRGO/GOSS/GOES). These are electrical steels designed to efficiently permeate flow in one direction only with even less loss. At the corners, we see that the flow must change direction, forcing it through the unoptimized direction. To handle this, the core is divided into upper and lower yokes, and the three legs use specially designed joints. The transformer joint design,Figure 1 , it's a worthy engineering challenge. Affects losses, noise and assembly.

Figure 1: Basic structure of a three-phase power transformer

What is magnetostriction?

The price we pay for using grain oriented steels is higher noise levels. In general, when a ferromagnetic material is exposed to a magnetic field, it undergoes mechanical stresses that it relieves by changing shape. This shape change due to forces induced by the magnetic field is magnetostriction. The strain is either positive (increases), as in the case of iron, or negative (shortens), as in nickel.

Figure 2: The deformation of a transformer member due to a sinusoidally applied magnetic field Ha

In Figure 2 , as the applied field increases, the limb (sleeve or leg) undergoes tension and lengthens, which is maximum according to the peak of the field. As the field decreases, the elongation also decreases. In the negative half cycle, the deformation process is repeated, although the field is inverted (disregarding hysteresis memory effects). The magnetostriction, therefore, occurs with twice the network frequency, that is, for each electrical cycle, there are two cycles of magnetostriction. Other permeable transformer elements such as shields, clamps, and tanks also experience magnetostriction due to exposure to leakage (stray) flux. Now we can consider the effects of magnetostrictive forces on the new version 2212 of Simcenter 3D Low Freq EM.

Although it happens in all ferromagnetic devices, why is it associated with transformers? Simply because of the core material. Grain-oriented electrical steels permeate more unidirectional flow for the same applied field as non-oriented (isotropic) steels. Isotropic steels are used in rotating electrical machines. Thus, grain-oriented electrical steels experience higher levels of magnetostriction and sound pressure.

Is this the only source of transformer noise?

In the animation, you can see that the transformer windings are an important part of the transformer. They also experience a force known as the Lorentz or J x B , where J is the surface electric current density and B is the magnetic induction. If you place a conductor between the poles of a horseshoe magnet and place it over two parallel conductors where it can roll freely, complete the circuit using a battery and a switch. Immediately you flip the switch, the conductor will roll in one direction until it is out of range of the magnetic field. In the same way the loose thread is displaced in the images below. He experiences the Lorentz force caused by current flow in the presence of a magnetic field.

Transformer windings also experience this, as they conduct current in the presence of core, leakage, and winding (self and mutual) fields. Although screwed in, some deformation of the winding structure still occurs, repeating at twice the grid frequency. In addition to noise, Lorentz forces are often the reason for structural winding failure. As is often the case when a system short-circuits and the current increases several times the rated current.

Structural elements of the conductive transformer, such as shields, bolts, and fastening systems, are also subject to Lorentz forces. They are exposed to leaks and stray fields, which induce eddy currents, resulting in Lorentz forces. Reducing leakage and eddy fields is an ever-present challenge in transformer design.

With our newfound knowledge, let's revisit the transformer joint seen on the right in Figure 1 . It is important to observe the air gaps. They interrupt the flow of magnetic flux by establishing an attractive force between the rungs, as when a gap separates two magnets. This is compounded by magnetostrictive effects and increased flux leakage. The attractive or repulsive force is known as the magnetic force or Maxwell's force. The complexity of transformer joint design is addressed by looking at the EMAG and force contributions.

How is transformer noise analyzed?

The 1300 kVar reactor seen in figure 3 was simulated in the new version of Simcenter 3D 2212. In the low-frequency EM (EMAG) environment, the magnetostrictive characteristic curve was defined from deformation and magnetic induction. After setting up the EMAG problem, the forces of interest based on the bodies they act on, in this case the core, were requested and extracted simultaneously during resolution. They were then exported to the Simcenter 3D Acoustics solver. Secondary physics-specific geometric and mesh models were derived and associated with the same primary CAD.

Consequently, any CAD changes are automatically inherited, saving time that would otherwise be spent manually accommodating any new geometric changes.

The field results in figure 3 are the mangetic inductions, the magnetostriction forces and the corresponding acoustic pressure, all in Simcenter 3D.

Figure 3: Magnetic inductions , magnetostrictive nodal forces and resulting acoustic pressure of a 1300 kVar reactor courtesy of Baobian Electric

Up to this point, we have only examined EMAG noise sources, which apply to naturally cooled transformers. For forced cooling using fans, it is important to consider both EMAG sources and mechanical sources (eg fans) in the noise analysis, as illustrated in figure 4 .

Figure 4: A multiphysics power transformer design approach in Simcenter 3D, where different physical attributes are evaluated to verify their performance

How does simulation help?

Power transformer development is a multi-objective process (cost, time, quality) that equipment manufacturers must get right the first time. This is because in the energy and utility industries, the physical prototype is the product. In these industries, physical iterations are really expensive. Manufacturers must therefore understand transformer behavior as soon as possible. The new version of Simcenter 3D 2212 provides this physics-based view into transformer behavior. This is EMAG, structural and acoustic engineering challenges on a CAD-centric multiphysics platform with traceable workflows. That is, the different physical attributes (EMAG, structural and NVH) are verified, guaranteeing the overall performance of the product. This avoids over-engineering based on a single physics. Maintaining geometric links to a primary reference CAD model propagates geometric changes automatically, keeping simulation results (CAE) up-to-date and ensuring their relevance in product development decisions. Traceability standardizes processes, speeding up workflows, reducing the chances of unnecessary duplication of tasks, and improving collaboration across multiple teams.


If you are concerned about transformer hum in your design, CAEXPERTS can help you improve the design of your transformers, reactors and electric motors with advanced digital engineering workflows. We invite you to learn more about our services and request a free diagnostic consultation at the link below.

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