All the right switches with 3D CAD

ABB has developed an intriguing new way of undertaking thermal modelling of its switchgear designs. It breaks down a 3D model into its component parts and automatically applies the thermal equivalent of electrical laws to each part – solving the problem as if it were an electric circuit. Such an approach requires much less computing power than trying to mesh a complex fabrication and then analyse it, using a finite element based method. According to Robert Platek, an associated scientist at the ABB Corporate Research Center in Krakow, Poland, finite element methods such as Abaqus, finite difference methods such as Maya and finite volume methods such as Fluent require a great deal of computation when applied to complex fabrications such as switchgear. “The only alternative is to break up the object in to a number of sub volumes, either nodes or library blocks,” he says. This method has been around since the early days of computing and involves creating and analysing a thermal circuit that is directly analogous to an electric circuit. Under these rules, temperature is equivalent to voltage, heat flow is equivalent to current, resistance and conductance assume thermal roles, thermal mass is equivalent to capacitance and Ohm’s Law becomes heat flow = temperature difference/thermal resistance. Heat generation by ohmic loss, skin effect and solar radiation replaces batteries and power supply sources, but allowance has to be made for the fact that heat transfer can be achieved by convection and radiation paths, as well as through conductors. Turning each solid element of the model into a thermal component by conventional methods is time consuming, but the breakthrough achieved by ABB is to automate the process in a package it has called ‘TNetWorks’. This functions as a plug-in to SolidWorks, where the main working screen includes a graphics window, pull-down Thermal Network menu, and a list of parts with type, material, connections and check points. The analytical process begins by selecting components, either by picking geometry or clicking list items. The clever part is that, when this is done, the software automatically recognises simple shapes and, says Platek, maps them into TNetWork elements. “Cylinders are parts with radii, plates are parts with six faces and enclosures are parts that contain other parts,” he says. This is followed by the selection of an appropriate flow model: turbulent for vertical parts, laminar for horizontal parts or no convection, where parts are contained in a vacuum. After this is initial set-up, where current, frequency, ambient temperature and air pressure are specified, followed by the selection of materials: both the materials of which the components are made and the fluid within the enclosure. Most of the component properties are allocated automatically, but there are always some that have to be defined manually. Interactions between components are then found automatically, based on geometry. If mechanical contact exists, heat is presumed to be by conduction; if they are in an enclosure, it will be via convection and radiation. Also, the user can create interactions manually between components. Check points are then set up where temperature results are to be calculated that can later be viewed. The input file is generated, edited, if necessary, and then solved using LTSpice, an electrical circuit solver, with the results displayed. Experimental verification of predicted temperature rises for a sulphur hexafluoride-insulated set of switchgear handling 2000A at 27.3ºC showed an agreement to within less than 5ºC. ABB has no plans at present to offer the application commercially outside the company. SolidWorks Pointers * It is possible to break down a complex model into its component parts, in order to perform thermal modelling with modest computing load * Analysis is performed by generating an equivalent electrical circuit and applying an electrical solver to the problem * Time can be saved by including automatic recognition of part geometries