Focus on Thermal Flow

A new IAV tool makes it possible to select the optimum cables for the high-voltage vehicle electric system

High amperages and some confined packages: During operation, high-voltage cables in hybrid and electric vehicles can get very hot if their cross sections are too small. On the other hand, over-dimensioned cables incur unnecessary costs and increase vehicle weight. IAV’s THS (Thermal Harness Simulation) tool computes the buildup of heat during subsequent operation and helps to select the optimum cable cross section.

Around 15 meters of high-voltage cable are installed in a hybrid or electric vehicle, and in commercial vehicles there can easily be as much as 100 meters of it. Therefore, the cable selected has a major influence on cost and weight – at the same time, though, consideration must be given the build-up of heat. Here, it is true to say: The higher the current, the larger the cable cross section needs to be. So far, manufacturers have in most cases based the cross section on a calculation with constant current flow that proceeds from the worst case and therefore tends to lead to overdimensioned cables. However, using IAV’s THS simulation tool, the developers can run realistic cycles, permitting an optimized selection.

THS is based on MATLAB/Simulink and works by segmenting the high-voltage line: A segment, for example, could be the underfloor area which is relatively well ventilated. Dissipating heat becomes more difficult in the vicinity of hot vehicle components, such as the exhaust system or inside the engine compartment. For this reason, THS lets the developers divide the high-voltage cable into several segments each of their own particular length and, for each section, define contact areas as well as its own ambient temperature – statically or variably. A library also provides various high-voltage lines and contacts from which the cabling model is easy to generate.

At least one size smaller than in the worst-case scenario

During simulation, THS emulates the radial and axial thermal flow in cables. “We compute the radial flow of heat through the conductors and insulation to the outside and how it is then dissipated into the surroundings at its given temperature”, explains Volker Hensel, head of the High-Voltage Vehicle Electric Systems department at IAV. “And we can also compute how heat flows axially between the various segments, and allow for the input of heat from the connected components.” THS takes the ambient temperatures and current as the basis for determining the build-up of heat in the cables which can be used to define the optimum cable cross section. In the way of test cycles, IAV’s experts use either theoretical profiles or measured values their customers already have. By using THS, they have usually been able to select the necessary cable cross section in at least one size smaller than for the worst-case scenario.

THS has been in use at IAV since 2015 and is being advanced all the time. Soon the tool is also to take account of conductor protection – i.e. the changed dissipation of heat through additional mechanical protective measures, such as cable ducts. “So far there have not been any computation models for this”, Hensel says. “This is why we are currently in the process of ascertaining the requisite characteristic values, such as for aramid hoses, and factoring them into our simulation. This will then give us a real USP.”