Magnetics Business & Technology - Summer 2013 - (Page 8)

CASE STUDY Designing Current Transformers with Simulation ABB is implementing a valuable way to conduct transient simulations of transformers. By integrating thermalelectromagnetic finite element analysis (FEA) with an electrical circuit model, engineers have an advantage to simulate and develop complex electric and magnetic devices. Every so often processes need to be updated and improved. The design of current transformers, which are used to measure the electrical current in power distribution and control systems, is no exception. Many factors can cause deviations from a transformer’s expected performance: material properties, design constraints, and different loading conditions can all have a strong impact. In such situations, finite element analysis (FEA) makes all the difference. Figure 1. Rolf Disselnkötter, Senior Principal scientist specializing in Industrial Sensor Technology at the ABB Corporate Research Center in Ladenburg, Germany. As senior principal scientist of ABB Corporate Research Center Rolf Disselnkötter describes, “Finite element analysis is a powerful means of investigating the way in which external electrical sources and loads interact with a magnetic subsystem like a transformer core.” There are several elements to consider in the analysis of a transformer, and according to Disselnkötter, “FEA illustrates the transient behavior of these cores and the generated flux density distributions. It also elucidates the self-heating and effects of temperature-dependent material properties that need to be taken into account.” Disselnkötter, alongside engineers from the University of Dresden, has been using Comsol Multiphysics to develop various modeling techniques. Because the models already combine 3D geometry, magnetic non-linearity, and transient analysis, coupling different physics is very challenging. Modeling Design Details Disselnkötter explains, “We are interested in how geometrical design variations, material properties, primary current distribution, temperature, and the electric circuitry will impact the accuracy of the electric current measurement. In order to allow for 8 Magnetics Business & Technology • Summer 2013 easy modifications and subsequent optimization procedures, we use parameter-based 3D model geometries.” In order to plan for various situations, the team introduced a potential problem into the design. They added small air gaps to the transformer’s core to learn about the effects these would have. It was expected that the air gaps would induce demagnetizing fields and change the interactions between the transformer’s windings. The test model includes a ferromagnetic core that connects through coils surrounding the primary winding. The model allows transient finite element analysis of transformers that are integrated with models of external circuitry. Apart from the deliberate air gaps, this is a typical transformer. Its primary winding is made up of one turn (a bulk bus bar) and the secondary winding consists of multiple turns which are arranged on two coil bobbins. The magnetic system is described by Ampère's circuital law and by Faraday's law of induction. For the core material, a nonlinear relationship between the flux density B and the magnetic field H of the type H = f(|B|)•B/|B| is assumed. Because of this, ABB needs a time-dependent simulation to model the electrical signals precisely. The air gaps lead to an asymmetrical geometry and cause an imperfect coupling between the primary and secondary windings. Integrating FEA with a Circuit Model According to Disselnkötter, “We built the circuitry from the predefined components provided with Comsol Multiphysics rather than importing it as a SPICE netlist. The coupling with the magnetic model was then implemented with equations for the currents and voltages on the two sides of the transformer.” The Electrical Circuit interface available in Comsol Multiphysics provided Disselnkötter with the equations for modeling his electrical circuits and solving for the voltages, currents and charges associated with circuit elements interacting with the FE model. At the primary side, the model is coupled to a sinusoidal current source, and at the secondary side it is coupled to an external load resistor. Figure 2. Primary and secondary circuits coupled to the FE model of the transformer.

Table of Contents for the Digital Edition of Magnetics Business & Technology - Summer 2013

Magnetics Business & Technology - Summer 2013
Editor's Choice
Designing Current Transformers with Simulation
Magnets • Materials • Measurement
Application • Component Developments
Research & Development
New Linear Motor Designs Improve Speed and Positioning
Industry News
Marketplace/Advertising Index
Spontaneous Thoughts: Back to Normal

Magnetics Business & Technology - Summer 2013