Siemens Energy Optimises Power Transformers With the Aid of 3D EM Simulation

8/12/2019 Siemens Energy Optimises Power Transformers With the Aid of 3D EM Simulation

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Siemens Energy optimises power transformers with the aid of 3D EM Simulation

2014 CST AG - http://www.cst.com Page 1 of 6

Siemens Energy optimises power transformers with

the aid of 3D EM Simulation

Finding customised 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.

Figure 1: Typical 40 MVA Siemens Transformer

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 safetyunder operating conditions spanning several years.

Insulation Design

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.


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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.

Figure 2: Simplified model of the active part of a transformer

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.

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.


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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 discretisation. 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


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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-quasi-static 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.


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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.

Siemens Energy Sector

Siemens Energy Sector is the worlds leading supplier of a wide range of products, solutions and services for power generation,

transmission and distribution as well as for the production, conversion and transport of the primary fuels oil and gas. We are

the only supplier worldwide with comprehensive knowhow encompassing the entire energy conversion chain and, in particular,

plant-to-grid connections and other types of interfaces. We focus primarily on the requirements of energy utilities and industrial

companies particularly those in the oil and gas industry.

Our Energy Sector has some 85,000 employees worldwide. In fiscal 2009, it generated external revenue of 25.4 billion and

profit of 3.3 billion.

Author

Ronny Fritsche,

Siemens AG Sector Energy T TR PN

Katzwanger Str. 150, 90461 Nrnberg

mail: [email protected]


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Tel: 0911 434 2432, Fax: 0911 434 2315

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Siemens Energy Optimises Power Transformers With the Aid of 3D EM Simulation