Electrothermal simulation based on the example of a high-voltage switchgear
Before its release, high-voltage switchgear must undergo a temperature rise test where certain temperature limit values defined by the standard and by Siemens must not be exceeded. Failure to pass this test can result in complex and costly redesigns. Electrothermal simulation on a computer helps to prevent this situation and saves on expensive tests during the product development phase.
Power loss per conducting path, also called ohmic loss, is key to the temperature rise in gas-insulated switchgear. This variable is determined by the conductivity of the conductor material, the conductor cross section, and the proximity effects and inductive currents in the conductor – specifically, as a function of the current and frequency in the CST EM Studio standalone simulation environment. In addition to current displacement due to skin and proximity effects, the inductive currents in the conductor and housing as well as the material and geometry are also taken into account.
Coupled electromagnetic and fluid dynamic simulation
The results of the electrodynamic simulation are then transferred from CST EM Studio to Ansys CFX with the aid of an internally generated Java-based macro. This computational fluid dynamics environment determines the conductor and housing temperatures while taking into account turbulent convection, radiation, buoyancy, and thermal conductivity. When the results are transferred, the component power loss is converted to a power-loss density and applied mesh-independently to the individual components in CFX. This eliminates the time-consuming chore of generating, transferring, and interpolating large results files – with results for each individual network node – onto the CFX mesh. Interpolation errors due to poor mesh resolution are also avoided. With this method, local power loss hotspots are ignored. Nevertheless, this loss can be taken into account because the conductors generate both electrically and thermally, and because each component is assigned its own power loss.
The coupling of CST and CFX is unidirectional, meaning that the temperature findings from Ansys CFX are not transferred back to CST EM Studio where they could be adapted to the temperature-dependent conductivity of the materials and a new, adapted power loss calculation and subsequent flow simulation could be conducted. The temperatures can already be approximated in advance or derived from previous test results. This is why the simulation provides sufficiently precise results even without bidirectional coupling and saves additional simulation time.
Fluid dynamics simulation
In combination with additional macros, the complete setup for CFX is also generated in the form of a CCL file and the correctness of the transferred results is verified. Once the CCL file is loaded, only smaller adjustments must be entered, such as non-standard material assignments and information on gravitational direction, starting temperatures, and emissivity values.
The CFX flow simulation can then be started immediately. Due to the reduced computing effort, this is a static simulation. Temperature and pressure rises in the system are not included in the calculation but only the equilibrium state when the system has settled. Thus, the pressure – like the conductivity – must also be determined prior to the simulation based on the expected temperature and specified as a boundary condition.