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. Power transformers are used for increasing and decreasing voltage levels in power lines that carry alternating current. 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. Although it is impossible to completely silence them, regulations require adherence to safe sound levels, and good product design can minimise these acoustic effects. ABB has used numerical analyses and computational applications to predict and minimise the noise levels in transformers. Through the COMSOL Multiphysics® simulation software and its Application Builder, the company has run virtual design checks, tested different configurations and deployed their simulation results through customised user interfaces built around their models.
Silencing sound from several sources
Transformer noise often comes from several sources. ABB’s transformers comprise a metal core with coils of wire wound around different sections, an enclosure or tank to protect these components, and insulating oil inside the tank. 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.
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 Center (ABB CRC) in Västerås, Sweden, needed to understand the inner workings of their transformers in order to optimise their designs for minimal transformer hum.
Coupling acoustic, mechanical & electromagnetic effects
“We chose to work with COMSOL Multiphysics because it allows us to easily couple a number of different physics,” said Mustafa Kavasoglu, Scientist, 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, ABB CRC team working with transformer acoustics had their objective to create a series of simulations and computational apps to calculate magnetic flux generated in the transformer core and windings, Lorentz forces in the windings, 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 set up 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. Resonances are easily excited by the magnetostrictive strains and cause high vibration amplification at these frequencies,” said Kavasoglu.
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.
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.
Spreading simulation capabilities
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 TECHNOLOGYCOMSOL 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,” explained Haettel. One application 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.
“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,” added Kavasoglu. With the wide range of industrial applications for which ABB designs transformers, this flexibility is 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.
Further, as per Kavasoglu, the work that is being done can be translated to any type of transformer, and on a specific request, the app can be adapted 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.