Seeking out electrical arcing regions in satellite systems
25 October 2017
Electrical arcing discharge in orbiting satellites can cause system failure, but is hard to predict. This piece from COMSOL explains how engineers at the Russian IHCE have adopted multiphysics simulation software to identify the critical regions where failures originate and to protect onboard equipment.
In 1995, Boeing Satellite Systems introduced a new family of communication satellite buses – the bodies that contain power, control and propulsion systems. They used a high-voltage bus connected to a 100V stabilised power source, instead of the standard 27V. This introduced an increase in operating voltage, which decreased operating currents and lowered the corresponding ohmic losses in the conductors. However, it also introduced a potentially catastrophic failure mode to the satellites’ electronic systems: electrical arcing (Figure 1).
As Vasily Kozhevnikov, researcher at the Institute of High Current Electronics (IHCE) in Tomsk, Russia, explains: “The transition to the new standard of operating voltages has led to the problem of an electric arc ignition between the elements of the electronic circuit boards.
In order to keep the mass of the satellite as small as possible, the space inside the circuit housing is not filled with an insulator or built to hold a vacuum. But that allows electric arc discharge, or discharge cascade, that can potentially spread over a large volume of onboard equipment.”
“The ignition,” he adds, “of an electric arc inside the onboard satellite system always leads to partial or complete failure. In most cases, it causes the termination of satellite use – which obviously results in serious cost and operational consequences.”
This research closely relates to the physics of a gas discharge under extreme conditions, where electrical equipment does not always perform as conventional physics would dictate.
For example, electrical discharges sometimes occur below a threshold known as ‘Paschen’s minimal values’, where the voltage should not normally be sufficient to start a discharge, or electric arc, between two electrodes.
“We think this research will also have potential use for the diagnostic of electronics operated under a wide range of external parameters, such as pressure, ionisation levels, and so on. It’s therefore widely applicable beyond the space industry and space science,” said Kozhevnikov.
As electronic systems are used in increasingly extreme environments, electrical arcing is not just an issue faced by the civil space industry: it affects any electronic application designed for long autonomous operation, with improved fault tolerance requirements. A solution to this problem, therefore, extends beyond satellites – and to terrestrial systems and underwater equipment as well.
Identifying the critical region
To prevent the destruction of an onboard electronic device by a spontaneous electric arc, the so-called ‘critical region’ must be identified, which is the area where self-sustained discharge ignition occurs. Once this potentially problematic area has been found, engineers need to conduct further investigations into what may trigger an electrical arc discharge.
Experimental studies fail to stand up to the challenge of identifying these electronic hotspots, because they cannot reproduce the full range of operating parameters that exist in space orbit.
The only remaining investigative option, namely simulation, also faces significant challenges. For one, a typical onboard electronic device consists of multiple printed circuit boards, distributed over a large area, placed inside a metal casing (Figure 2).
Kozhevnikov says, “The only way to identify possible self-sustained discharge regions is the numerical simulation of the discharge, but this is practically impossible for such large-scale problems, due to the substantial associated computational costs. The discharge problem is both multiphysics and multiscale.”
Catching geometric inaccuracies
The Tomsk-based research team worked hard on finding a computational approach that would prove both accurate and practical. The researchers proposed a ‘decomposition’ methodology, implemented with computational tools, to tackle this problem. Instead of performing a complete direct current discharge simulation for the entire electronic device, they created a custom simulation application that would autonomously partition and analyse the device to identify the most probable critical regions. To this end, they used the COMSOL Multiphysics software, and its Application Builder tool, to create a multiphysics model that supports the entire simulation process.
An important modelling step was preprocessing, which was carried out to apply the proper boundary conditions and import the detailed geometry of the real on-board electronic system.
With the Application Builder, the team performed preprocessing using a custom 3D macromodel method. They also implemented their own import engine with automatic correction of object boundaries. The method consisted of both import and automatic correction of object boundaries, Kozhevnikov explains (Figure 3); without such correction, these errors could have become serious obstacles in the simulation.
Breaking down the plasma physics problem
After preprocessing, the modelling methodology consisted of three stages: preliminary electrostatic analysis of potential critical regions in a 3D model; extraction of field-enhancement areas, and the definition of critical regions with associated 2D models; and DC-discharge simulation of critical regions, to further investigate parameters of interest.
The team initially used COMSOL Multiphysics software because of its unique ability to both implement all the features of the two-moment direct current discharge theoretical model and alter the necessary parameters. The simulation analysed the electron density distribution and identified the critical region (Figure 4). Kozhevnikov explains: “COMSOL Multiphysics software finely meets the requirements of our project – namely, an analysis of the operating pressure range. This is much faster and more convenient than a particle-in-cell (PIC) simulation for medium and high pressures.
“PIC simulations are simply not feasible for such problems, due to extensive computational costs. The simulation of simplified configurations (such as gas diodes) is possible, but depending on the problem, can take five to twenty times longer for medium pressures than a COMSOL simulation. The average computation time in COMSOL for this configuration is less than two hours.”
The custom application that the team built, shown in Figure 5, hides the complexity of the physics involved in the model setup. This exposes the application user only to parameters relevant to the analysis at hand and allows for the inclusion of custom commands and algorithms.
Kozhevnikov states: “Strictly speaking, COMSOL made it possible to perform our research without the creation of our own computational code, which would have been extremely complicated in light of this problem. We expect the software to be most promising for our future investigations concerned with gas discharges.” Other arguments in favour of choosing COMSOL were its wide choice of pre- and post-processing tools, including CAD import features and the Application Builder.
Orbital and interdisciplinary implications
There is scope to integrate such simulations with real-world investigations, Kozhevnikov expands. “If it is possible to perform fully non-destructive testing in the future, a COMSOL simulation will narrow the region of interest for experimental testing by excluding non-essential parts.
Some work towards non-destructive testing development was performed by our colleagues from the Laboratory of Vacuum Electronics at the Institute of High Current Electronics, within the framework of the project we collaborate on.
“Within the spacecraft industry, the automated software system’s adaptability should guarantee its continued use. Standards in spacecraft industry change from time to time, so it is difficult to account for all the consequences of such changes.
We have solved the problem of arcing diagnostics; nevertheless, we expect that the voltage increase will also require serious redesign of certain on-board electronics to fit new operating conditions. Simply speaking, if the operation conditions of some devices significantly differ from ‘normal conditions’, then you need to rebuild the architecture in a certain way.
Our application provides recommendations for the redesign of printed circuit boards in order to make them more arc-resistant, but it could also be useful in designing fault-tolerant electronic systems.”
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