<<This article is based on the thesis research conducted by an Engineering Intern within TMC Transformers' Electrical Engineering Department.>>
The challenge of fast transients
As global energy demand continues to rise, the dry-type transformers remain one of the most important and critical elements in medium voltage power grids. Utilizing cast resin and VPI technologies, they have become the industry standard due to their inherent fire safety, environmental compatibility, and low maintenance.
However, while they are highly reliable under normal conditions, dry-type transformers today face increasingly severe electrical stress. The widespread adoption of eco-friendly Vacuum Circuit Breakers (VCBs)—essential for replacing SF6 gas switches—generates extremely fast, high-frequency transients due to their rapid current-chopping mechanisms. Combined with natural threats like lightning strikes, these fast transients subject transformers to dangerous, steep-fronted overvoltages. To prove they can survive these harsh operational realities and comply with international IEC and IEEE standards, transformers must pass strict dielectric tests, most notably the standard Lightning Impulse (LI) test.
Passing these tests is challenging for dry-type transformers. Unlike traditional oil-filled units, solid resin insulation cannot self-heal; a single internal breakdown irreversibly destroys the winding.
In the past, the only way to validate a new custom design for the impulse test was to build an expensive physical prototype and test it in a laboratory. Today, to avoid these prohibitive costs and long waiting times, engineers need advanced software tools. By simulating exactly how the transient voltages and electric fields distribute inside the machine during a lightning strike, designers can identify weak points and optimize the insulation clearances long before the manufacturing process even begins.
The COMSOL electromagnetic FEM solution
To solve this industrial challenge, the R&D department at TMC Transformers developed an advanced Finite Element Method (FEM) model using COMSOL Multiphysics. Simulating a fast transient like a lightning impulse requires solving complex magnetic and electric phenomena simultaneously. Instead of relying on simplified circuit models, TMC predicts this transient behaviour by combining a Frequency-Domain approach with an Inverse Fast Fourier Transform.
To make this advanced physics model fast and practical for daily engineering, the simulation uses a fully parameterized 2D geometry. This precise layout includes all the critical details: the exact insulation thickness of the conductors, the cast epoxy resin, the grounded electrostatic shields, and the plastic barriers in the air gaps.
Setting up the materials is straightforward. The model assigns electric permittivity to the dielectrics and specific conductivities to the conductors. It also uses a smart mathematical formula to simulate the magnetic core. This elegantly captures the high-frequency "flux expulsion" effect: during the very first microseconds of a lightning strike, the core behaves magnetically like a solid block of air, accurately forcing the magnetic flux into the leakage channels.
Designing with the COMSOL application
The final goal of this complex numerical modelling is to translate complex electromagnetic analyses into practical engineering insights, supporting TMC Transformers throughout the design and development process.
By leveraging advanced simulation capabilities, TMC engineers can investigate the electrical behaviour of custom transformer designs under transient conditions, gaining valuable insights into voltage distribution and electric field stresses within the insulation system.
By identifying the exact maximum dielectric stress—typically found in the air gaps and at the sharp corners of the first winding discs—designers can directly compare these peaks against the physical limits of their materials.
This allows the engineering team to rapidly validate complex geometries, optimize electrical clearances, and fix any weak points early in the design phase, guaranteeing a highly reliable product for the modern energy grid.
A complete virtual lab in COMSOL
The standard Lightning Impulse is a crucial test, but this FEM model can do much more. Because it works in the frequency domain, the simulation is not limited to the standard 1.2/50 µs wave. Engineers can test almost any voltage shape. Whether they need to simulate a chopped lightning wave, fast transients from VCBs, or a custom surge requested by a client, the application shows exactly how the internal insulation will handle the stress.
Moreover, this frequency-based approach allows the team to study how the winding behaves at different frequencies. Designers can easily find internal resonances and calculate voltage transfer functions. These detailed frequency plots can be attached to the technical documentation, providing a great added value for customers with special or highly sensitive applications.
This unique flexibility turns the COMSOL application into a complete virtual laboratory. By predicting how the transformer will react to any electrical disturbance, TMC Transformers can deliver safe, highly reliable, and fully customized dry-type transformers, ready for the energy grid of the future.