Coupled Electro-Thermal-Fluidic Simulation of Current Conductors with regards to Proximity Effects and Natural Convection

Diploma Thesis by Moritz Gebhardt at Technische Universität Berlin (June 2008)

Prof. Dr.-Ing. Kerstin Weinberg (1st Assessor)

Prof. Dr. rer. nat. Wolfgang H. Müller (2nd Assessor)

and

Dipl.-Ing. Christian Rümpler (Tutor at Fraunhofer SCAI)

Description

Bus bars are thick strips of copper or aluminum that collect and distribute electric energy within power distribution boards. They carry up to several 1000A of alternating current. In the content of bus bars Joule Heating is critical. High currents cause high losses that may be hazardous to technical equipment and living beings. In Germany the allowed temperature rise on bus bars is regulated in DIN-43671. Despite safety critical aspects a rise in temperature increases the resistance of conductors, which further increases the losses. Cooling is either active, with special cooling equipment or passive due to natural convection.

Like all alternating currents, the interaction of the currents on the bars with their own magnetic fields causes a displacement towards the surface, the so called skin effect. The skin effect makes thicknesses over 8 − 9mm at 60 Hz unreasonable, wherefore bus bars are mostly formed as hollow tubes or lyers/Mpthick plates, often in pairs. An interaction with the magnetic fields of adjacent conductors causes a further displacement of the currents, the proximity effect. The resulting current distribution is far from being homogeneous, which greatly complicates a valid construction.

Especially under the aspect of miniaturization a prediction of temperature rise on busbars is critical, to allow a safe and dependable operation. A simulation of bus bar systems can greatly enhance development.

In this work a coupled finite element/ finite volume model is created, that computes the Joule heating on the conductor and the cooling by surrounding air, with regards to skin and proximity effects. The electromagnetic model is built up using Ansys 11.0. For the finite volume model Fluent 6.3 is used. The coupling process is controlled by MpCCI 3.06, a software developed at Fraunhofer Institute for Algorithms and Scientific Computing.

Diploma-Thesis for Download

Diplomarbeit_MoritzGebhardt.pdf