For everything from cooking to charging our phones, we rely every day on the electrical grid that powers buildings like homes, businesses, and schools. This complex network includes stations generating electric power, high-voltage transmission lines that carry electricity across large distances, distribution lines that deliver power to individual homes and neighbourhoods, and the related hardware used for power flow control and protection.
Among this equipment are power transformers for increasing and decreasing voltage levels in power lines that carry alternating current (see Figure 1). Power transfer with higher voltages results in lower losses and so is more desirable for transporting power long distances. However, such high voltage levels would pose a safety hazard at either end of the lines, so transformers are used to increase voltage levels at the power feed-in point and decrease them close to neighbourhoods and buildings.
But transformers come with noise, often manifested as a faint humming or buzzing that can be heard when walking nearby. Although it is impossible to completely silence them, regulations require adherence to safe sound levels, and good product design can minimise these acoustic effects.
One of the biggest manufacturers of transformers used around the world, ABB (headquartered in Zürich, Switzerland), has used numerical analyses and computational applications in order to predict and minimise the noise levels in their transformers. Through the COMSOL Multiphysics simulation software and its Application Builder, is has run virtual design checks, tested different configurations, and deployed its simulation results through customised user interfaces built around its models.
Silencing sound from several sources
Transformer noise often comes from several sources, such as vibrations in the transformer core or auxiliary fans and pumps used in the cooling system. Each of these sources needs to be addressed differently to reduce noise.
ABB’s transformers comprise a metal core with coils of wire wound around different sections, an enclosure or tank to protect these components, and an insulating oil inside the tank (see Figure 2, top). Passing alternating current through the windings of one coil creates a magnetic flux that induces current in an adjacent coil. The voltage adjustment is achieved through different numbers of coil turns.
Because the core is made of steel, a magnetostrictive material, these magnetic fluxes — which alternate direction — cause mechanical strains. This generates vibrations from the quick growing and shrinking of the metal. These vibrations travel to the tank walls through the oil and the clamping points that hold the inner core in place, creating an audible hum known as core noise (see Figure 2, bottom).
In addition to the core noise, the alternating current in the coil produces Lorentz forces in the individual windings, causing vibrations known as load noise that add to the mechanical energy transferred to the tank.
With these multiple sources of noise and the interconnected electromagnetic, acoustic, and mechanical factors at play, engineers at the ABB Corporate Research Centre (ABB CRC) in Västerås, Sweden needed to understand the inner workings of their transformers in order to optimise their designs for minimal transformer hum.
Figure 2 Top left: CAD model of the active part of a three-phase transformer with windings mounted around the core. Top right: The active part of a power transformer that is placed in a tank filled with oil. Bottom: The energy conversion chain for core noise and load noise generation (magnetostriction in the core and Lorentz forces in windings).
Coupling acoustic, mechanical, and electromagnetic effects all in one
“We chose to work with COMSOL Multiphysics because it allows us to easily couple a number of different physics,” said Mustafa Kavasoglu, scientist at ABB CRC. “Since this project required us to model electromagnetics, acoustics, and mechanics, COMSOL software was the best option out there to solve for these three physics in one single environment.”
Kavasoglu; Dr Anders Daneryd, principal scientist; and Dr Romain Haettel, principal engineer, form the ABB CRC team working with transformer acoustics. Their objective was to create a series of simulations and computational apps to calculate magnetic flux generated in the transformer core and windings (see Figure 3, left), Lorentz forces in the windings (see Figure 3, right), mechanical displacements caused by the magnetostrictive strains, and the resulting pressure levels of acoustic waves propagating through the tank.
They work closely with the Business Unit ABB Transformers, often relying on the experience and expertise of Dr Christoph Ploetner, a recognised professional in the field of power transformers, to ensure that they satisfy business needs and requirements.
One simulation models the noise emanating from the core due to magnetostriction. The team began with an electromagnetic model to predict the magnetic fields induced by the alternating current, and then the magnetostrictive strains in the steel.
Their geometry setup included the steel core, windings, and an outer domain representing the tank. “We obtained the displacement from the magnetostrictive strains, then calculated the resonance for different frequencies using a modal analysis,” said Kavasoglu (see Figure 4). “Resonances are easily excited by the magnetostrictive strains and cause high vibration amplification at these frequencies.”
Figure 3 Simulation results showing the magnetic flux density (left) and Lorentz forces (right) in the transformer coil windings.
Figure 4 Left: COMSOL software results showing levels of magnetic flux in the steel. Right: Results showing the resonance of the core. Deformations are exaggerated for visibility.
Figure 5 Results of the acoustic analysis showing the sound pressure field around the core (left) and around the transformer (right).
They were then able to predict the sound waves moving through the oil and calculate the resulting vibrations of the tank, implying sound radiation into the surrounding environment (see Figure 5).
They also simulated the displacements of the coil windings that cause load noise and determined the surface pressure on the tank walls due to the resulting sound field (see Figure 6).
Including parametric studies that illustrated the complex relationships between design parameters (such as tank thickness and material properties) and the resulting transformer hum made it possible to adjust the geometry and setup of the core, windings, and tank to minimise the noise.
Figure 6 Left: Simulation results showing the displacement of the windings. Deformations are exaggerated for visibility. Right: Results showing the sound pressure levels outside the tank and the displacement of the walls.
Figure 7 Cropped screenshot of the first simulation app created for calculating eigenfrequencies of the transformer core. At left, a tab in the app shows the model inputs; at right, results are shown for the calculated eigenfrequencies. Deformations are exaggerated for visibility.
Spreading simulation capabilities throughout ABB
The CRC team continues to use the COMSOL software to not only improve their understanding and their models, but to extend their knowledge to the rest of ABB’s designers and to the business unit. Using the Application Builder in COMSOL Multiphysics, they have begun creating apps from their multiphysics models, which can be easily customised to suit the needs of each department.
These simulation applications simplify testing and verification for the designers and R&D engineers: “The designers have been using tools based on statistics and empirical models. We are filling the gaps by deploying simulation apps. The Application Builder allowed us to give them access to finite element analysis through a user interface without them needing to learn finite element theory,” Haettel explained.
One application (see Figure 7) calculates the specific eigenfrequencies of the transformer core that can imply noise-related issues due to frequencies that fall within the audible range. This app includes both the physics model developed in the COMSOL software and custom methods written in Java code, programmed within the Application Builder.
“Our designers use standard spreadsheets that work well for the transformers they build frequently. But when new designs or different dimensions are introduced, they may run into problems with this approach, like error outputs showing less accurate data for noise levels. This can become quite costly if additional measures to reduce noise are required on the completed transformer,” Haettel continued.
“Besides the cost aspect, there is the time aspect. The new app will make the designers’ job easier and more efficient by using the precision of an FEA code.”
The custom application adds a level of convenience by letting users check how certain combinations of geometry, material properties, and other design parameters will affect the resulting transformer hum. “We’ve been deliberate about selecting which parameters we provide access to —focusing on the ones that are most important,” Kavasoglu added.
With the wide range of industrial applications for which ABB designs transformers, this flexibility is immensely helpful for their design and virtual testing process. “ABB produces transformers for every industrial need. At the moment we’re focusing on AC large power transformers commonly used by power companies that transmit and distribute electricity throughout cities,” he explained.
“But the work we’re doing can be translated to any type of transformer, and of course if we receive a specific request, we adapt the app to that need. This allows us to easily do additional development work. The Application Builder has made the transfer of knowledge and technology much easier.
“We’ve also been using the COMSOL Server license to distribute our app to other offices for testing, which makes it easy to share it. This worldwide license is great; with a global organisation, we expect users in our other locations around the world to benefit from these apps.” With a local installation of COMSOL Server, simulation specialists can manage and deploy their apps, making them accessible through a client or web browser.
The team is focusing on a second application that will calculate load noise. Once deployed to the business unit, this application will further remove the burden of tedious calculations, allowing designers and sales engineers to run more virtual tests without needing to work with a detailed model, and enable ABB to more quickly and easily produce the world’s best transformers.
Left to Right: Mustafa Kavasoglu, Romain Haettel, and Anders Daneryd of ABB CRC.