Finding customized solutions for any applications and specifications of transformers – this is the task of Siemens Energy T TR PN. It is difficult to obtain off-the-shelf solutions for furnace and converter transformers as well as transformers for High Voltage Direct Current for ratings greater than 200 MVA. 3D EM Simulation with CST EM STUDIO® (CST EMS) is a vital tool in the optimisation process of transformer installations.
The Challenge of Transformer Design
The demands on power transformer design are diverse – they must conform to national and international standards and fulfill the specifications of the electricity utilities and their contractors. These conditions are to a large extent determined by the network to which they are connected and specific factors at the installation site. The dimensioning of the power transformer must adhere to the necessary specifications. In this process, mechanical, electromagnetic and thermal characteristics must be taken into account. For every manufactured transformer, a works test is carried out to ensure correct operation and safety under operating conditions spanning several years....
The development of insulation systems and components is one of the design goals in transformer design. This involves knowledge of the electrical insulation between windings and the insulation between energised windings, conductors and grounded components. The conception, design, construction and material of a given insulation arrangement are determined by the specification requirements. In power transformers, mineral oil and impregnated fibrous material, also known as transformer board, are generally used. The system consisting of pressboard and oil channels is referred to as an insulation system.
Effective Modeling of Complex Geometries
The User-Interface of CST STUDIO SUITE® is very easy to use and enables the fast construction of models. This model is then meshed and solution matrices set-up for use in the numerical solvers. CST EMS allows the user to lower simulation times by defining symmetry conditions which reduce the simulation domain and required memory. Figure 2 shows a model of the active part of a five-leg power transformer with three wound legs which was constructed entirely in CST EMS. For complex geometries, CST EMS allows the import and modification of a wide range of 3D formats. Insulation arrangements can still, even with some simplification, pose a difficult challenge for numerical simulations since the dimensions of these structures can be of the order of several meters while the thickness of the individual components, such as transformer board barriers, is in the range of a few millimeters.
Figure 2 Simplified model of the active part of a transformer.
Since the barrier strength and oil channel width are decisive factors in the dimensioning of the insulation system, a powerful and robust mesher is important. As well as a tetrahedral-based mesh generator CST EMS also offers a hexahedral-based mesh scheme ensuring that the user can apply the best method for the problem. In addition to global mesh settings, the user also has the opportunity to refine the mesh locally on a component or material level. As an example of a typical simulation, Figure 3 shows an equipotential plot from an electrostatic simulation of the outlet section of a winding arrangement (active part). For this simulation, the original 5 leg active part was reduced to a single leg.
Figure 3 Electric potential around a single-leg winding arrangement.
CST EM STUDIO Establishes Operational Stability
A virtual operational stability test involves the 3D simulation, analysis and further development of insulation systems between various electrode arrangements. Electrostatic simulations are used to provide momentary values of the electric stress which typically arise in the testing of a transformer. The tetrahedral-based electrostatic solver in CST EMS is particularly suited to this simulation since it allows an accurate surface and efficient volume discretization. This helps to determine the potential distribution around electrodes and on insulating surfaces which is critical for establishing the effectiveness and strength of the insulation systems. Figure 4 shows an equipotential plot in an oil-transformer board insulation system.
Figure 4 Equipotential Plot of the Electric Field Stress.
The Right Solver for the Job
CST EMS offers a wide range of numerical solvers for various types of problems. For the calculation of induced electromagnetic losses in the individual elements of the transformer, the magneto-quasistatic solver is used. The sources for such simulations include the windings of the active part and the winding connecting leads.
The active part, which consists of the iron core and the leg sections with concentric windings and their insulation, forms the heart of the transformer. Stray fields lead to electromagnetic losses in the surrounding metallic construction elements. Simulation of this situation would aid in the reduction of the losses by appropriate shielding measures or changes to conductor placement. Analysis of the magnetic field in the core, shown in Figure 5, assists the investigation of the shielding effectiveness of the shielding walls.
Figure 5 Analysis of shielding wall effectiveness : Magnetic energy density.
No New Transformer Development Without CST EM STUDIO
The importance of simulation in the high voltage design, construction and dimensioning process as well as the calculation of electromagnetic losses in the active part is continually increasing. For this simulation the CST EMS Low Frequency time harmonic solver was used. In Figure 6, the electromagnetic losses in the pressboard are shown where the skin effect can be clearly seen. To support accurate modeling of the skin depth, 2nd order elements were used. Alternative techniques such as surface impedance modeling are also available, which can significantly increase the performance of simulations.
Figure 6 Loss Density in the press frame
Thermal considerations also play a role in the design process. The loss distribution from the low frequency simulation can be used as a thermal source in the CST MPHYSICS® STUDIO (CST MPS) thermal solver. This transfer of the losses from the electromagnetic solver to the thermal solver is carried out seamlessly within the same model and opens up a range of analysis possibilities.
CST EMS ist mittlerweile bei Siemens ein fester Bestandteil des Entwicklungsprozesses verschiedenster Leistungstransformatorkomponenten geworden, insbesondere wegen der einfachen Bedienungsoberfläche und der Vielfalt der Löser. Dank der sehr engen Zusammenarbeit mit dem CST EMS Entwicklungsteam wurden Wünsche, Anregungen und Ideen immer offen aufgegriffen und zügig umgesetzt. Dies bildet die Basis für unsere Entscheidung auch weiterhin verstärkt die von CST angebotenen Lösungen anzuwenden.