MK 2024-01-05

Lightning electromagnetic pulse simulation setup

Fig. 1 shows the 3D lightning electromagnetic pulse (LEMP) simulation model of an aircraft including cable harness. The cable harness is represented by blue cable paths and blue and yellow nodes. Yellow nodes allow cable currents to flow into the 3D object of the LEMP simulation model. The cubic red frames located at the yellow nodes mark cable ports which are available in CST Studio Suite since version 2023. Exterior to the aircraft, blue wires allow to inject lightning current waveforms at port 1 (represented by a red conus). For the TLM solver the open boundary (not shown here) of the simulation domain serves as a return conductor.

Fig. 1: 3D LEMP simulation model of an aircraft including cable harness.

Fig. 2 shows the schematic setup including cable pin loads (resistors and shorts) and probes (P1 - P8) of the LEMP simulation model. At pin 1 an external port 1 is attached where a lightning current waveform is injected which is defined as an excitation of a transient task. The probes allow to monitor transient pin voltages and currents, respectively, which results in Actual Transient Levels (ATLs) and finally in Transient Control Levels (TCLs) by adding a margin [1].

Fig. 2: Schematic model of a LEMP simulation.

Two different workflows are available in CST Studio Suite(R) to perform lightning indirect effects analysis. These are described below as CST transient co-simulation workflow and CST transient task with combine results workflow. The first one is available since many years but the second one became available with CST Studio Suite 2023 when cable ports where introduce in 3D. With that additional feature now it's possible to run 3D-EM-cable coupled simulations in 3D and to perform the lightning indirect analysis as a post processing step in the schematic.

CST transient co-simulation workflow

The well established workflow for lightning indirect effects analysis in CST Studio Suite makes use of CST transient co-simulation. Generally it bidirectionally couples 3D electromagnetic (EM) with multiconductor transmission line (MCTL) simulation. The transient task of the schematic view in Fig. 3 controls the coupled simulation after cable pin loads have been assigned in the schematic. A drawback of this workflow comes with changing cable pin loads and different lightning current waveforms. Once cable pin loads or waveforms are changed the transient task has to updated and with it the time consuming 3D-EM-cable coupled simulation.

CST transient task with combine results workflow

The CST transient task with combine results workflow allows to characterize 3D models of aircrafts with installed cable harnesses in terms of scattering parameters in 3D first. The actual lightning indirect effects analysis is then performed with a transient task making use of combine results in the schematic. This is a pure post-processing step as the scattering parameters have already been calculated in 3D. Different cable pin load configurations and lightning current waveforms can easily be investigated without repeating the full 3D-EM-cable coupled simulation as it is the case for the CST transient co-simulation.

Workflow steps

Step 1: In Fig. 4 the workflow starts with the a 3D full wave simulation of an aircraft with installed cable harness. The TLM solver is used to characterize the problem in terms of scattering parameters. To properly capture diffusion in the aircraft skin the 3D transient simulations have to run long enough. The required time is not a fixed value and has to be evaluated by the user. For the aircraft simulation shown in Fig. 4 40us where enough for each solver run. The model contains 11 ports and as thus it required 11 solver runs to be solved.

Fig. 4: 3D-EM-cable coupled simulations in 3D.

Step 2: The actual lightning indirect effects analysis is performed as a post processing step in the schematic. To accurately include the frequency dependent multi-port scattering parameters of the previous 3D simulation in the transient lightning simulation vector fitting is employed. As shown in Fig. 5 the user can select in between the built in and the IDEM method. In many cases the built in method is already a good choice. In other cases when the built in method does not give satisfactory results IDEM is an option. Often IDEM yields macromodels of lower order and same or higher accuracy than the built in vector fitting.

Fig. 5: Vector Fitting method selection.

Step 3: As mentioned mentioned before the 3D-EM-cable coupled simulation has not to be repeated for different cable pin load configurations and lightning current waveforms sown in Fig. 6, respectively. Only transient tasks of a circuit in the schematic have to updated. An update of the 3D-EM-cable coupled simulation is only necessary in case the an aircraft model with installed cable harness is modified.

Fig. 6: LEMP analysis with multiple standardized current waveforms.

Conclusion

Both previously described workflows have advantages. The CST transient co-simulation workflow is probably the preferred one when aircraft models with a high cable pin count have to be analyzed but also when only one or few LEMP simulations have to be performed.

In cases the user is interested in analyzing many different cable pin load configurations and different waveforms the CST transient task with combine results workflow might be the preferred one as in particular the 3D-EM-cable coupled simulation has not to be repeated for each case. Also long pulse durations of (e.g. > 500us) can be solved in seconds instead of many days for a CST transient co-simulation.

At the end the user now has two workflow for LEMP simulation at hand and has to make the right choice with respect to the case under investigation.

References

[1] "Protection of Aircraft Electrical/Electronic Systems against the Indirect Effects of Lightning, " Advisory Circular AC 20-136B, Federal Aviation Administration, 2011.