Mitsubishi Heavy Industries Technical Review Vol. 61 No. 1 (March 2024)
1Development of Analytical Design Techniques forGround Lines to Prevent Effects of Inflow of Current
into Composite Structures on Power Sources

TATSUFUMI AOI*1

HIROOMI MIYAHARA*1

AKIO IKEDA*2HISASHI SHIMIZU*2

KAZUKI YAMAWAKI*2

YUKI MANDO*2

The development of products, in which carbon fiber-reinforced plastics with higher
electrical resistivity than metals (hereafter referred to as "composite materials"), has been
progressing in recent years. They used extensively and require the design of groundlines to reduceelectromagnetic effects (current inflow, interference through groundlines between electricaldevices) caused by disturbances such as lightning. Therefore, Mitsubishi Heavy Industries, Ltd.
has developed an analytical evaluation technology to reduce the electromagnetic effects ofgroundlines in composite structures. The developed analytical evaluation technology forgroundline structures divides the target frequency range into two and utilizes two complementary
methods corresponding to them: quasi-static electromagnetic field analysis plus circuit analysis(low frequency range) and electromagnetic field analysis based on the FDTD (Finite DifferenceTime Domain) method (high frequency range). We performed the evaluation of the measures tosecure current paths and against the effects of induced magnetic fields and the evaluation of theeffects of interference between power sources sharing groundlines, and confirmed the validity of
the analytical evaluation technology through test and analysis using a mock-up.

|1. Introduction
Mobile products such as aircraft are becoming more electrically powered and have moresophisticated onboard electronic systems. To reduce the weight of such products, composite
materials have been increasingly used in their structures. Since composite materials have a highresistivity (three orders of magnitude higher than metal), groundlines need to be installed in thestructure as current returning paths. In such cases, analysis technology to evaluate electromagnetic
characteristics is required to prevent lightning currents from affecting the operation of electrified
and sophisticated on-board electronic system equipment. This time, we have developed ananalytical evaluation technology for groundline structures that utilizes commercially availableelectromagnetic field analysis tools. This report describes on the results of the verification of thedeveloped analytical evaluation method by testing and evaluating the effects of electromagnetic
noise and lightning current inflow on the power source system using a mockup with simulated
wiring installed.

|2. Requirements and issues (background)(1)*1Requirements for electrical systems and targets to be handled with design tools
Mobile products such as aircraft are equipped with systems to drive and control them,and the electrification of these systems has been progressing in response to initiatives such as
the reduction of CO2 emissions.
For the electrical equipment systems installed in such products, it is necessary to (i)reduce the weight, (ii) maintain and secure the quality of the power source, (iii) take electricalmeasures against the effects of lightning current inflow, (iv) take measures against the effects ofChief Staff Researcher, Electron･Physics Research Department, Research & Innovation Center, Mitsubishi Heavy Industries,Ltd.

*2Electron･Physics Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.

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ground fault currents, and (v) take measures against the effects of electromagnetic interferencesand external electromagnetic fields (including induced lightning). This report discusses ananalytical evaluation technology which is related to the requirements of (i), (ii), and (iii) above.
The standards related to (ii) maintaining and securing the quality of the power source
include, in the case of aircraft, MIL-STD-704 (MIL standard for aircraft electrical power
characteristics). Regarding the target frequency range, the operating frequency range ofelectrical equipment power sources is direct current (DC) to several hundred Hz, and thestandard requirements range is DC and 10 Hz to 1 MHz. The standards related to the effects of(iii) lightning current and (iv) ground fault current include MIL-STD-464 (electromagneticenvironment requirements for equipment systems) and MIL-STD-461 (lightning immunity test
requirements at the equipment level). Considering the frequency components of lightning andground fault currents, the target frequency range is several Hz to 1 MHz. Therefore, the target
total frequency range is DC to 1 MHz.
In addition, in order to (i) reduce the weight, electrical systems of moving products may
share current returning paths in the power lines, and it is necessary to be able to consider theserequirements in actual system design.
(2) Issues
Conventional metal structures mainly use aluminum or titanium alloys, which have low
resistivity (about 3x10-8 to 6x10-7 Ωm) for the structural frame, so the resistance and reactanceare usually low, at levels of 1mΩ or less, resulting in a small voltage drop generated whencurrent flows through them. Therefore, metal structures are often used as direct current return
paths.
On the other hand, the resistivity of a composite structure is approximately 100 to 1,000times larger, ranging from 1x10-5 to 3x10-5 Ωm, causing a large voltage drop when current
flows through the structure. Therefore, there is a problem that using a composite structure as
current return paths causes significant interference between power sources and is difficult toapply. For this reason, it is necessary to install metal-based groundlines on the structure ascurrent return paths.
Therefore, composite structures and metal groundlines coexist in the system, i.e.,
multiple current paths with significantly different resistivities exist, which make it difficult toeasily identify the current paths. To address this issue, it is necessary to perform the evaluationof (1) securing current paths against large current inflows, such as lightning, and of the effects
of electromagnetic fields. In addition, multiple power sources are connected to one shared
groundline, affecting the operation of one power source on another power source. To address
this issue, it is necessary to (2) develop an analytical evaluation technology that takes into
account the effects of interference between power sources sharing groundlines.

|3. Analysis model
3.1

Analysis Method

It is necessary to simultaneously evaluate the current paths of currents at several Hz to 1
MHz, which correspond to the frequency characteristics of lightning currents, and the interference
at DC to several hundred Hz, which correspond to the operating frequency of electrical equipment
power sources. However, it is difficult to perform the evaluation accurately in a practical
calculation time with a single analysis method, so we considered the complementary use of two
methods: quasi-static electromagnetic field analysis plus circuit analysis (for low frequency range)and electromagnetic field analysis based on the FDTD method (for high frequency range). The low
frequency range method was applied to the frequency range of (1) DC to 100 kHz and the highfrequency range method was applied to the frequency range of (2) 100 kHz to 1 MHz.
(1) Low frequency analysis (DC to 100 kHz)The quasi-static electromagnetic field analysis (electromagnetic field analysis method
applicable where no displacement current flows with respect to time variation) analyzes
electromagnetic fields using the finite element method or the moment method. Based on theanalysis results obtained, electrical circuit parameters (R: resistance, L: inductance, C:capacitance, G: conductance) between the components are output as S-parameters, and theMitsubishi Heavy Industries Technical Review Vol. 61 No. 1 (March 2024)
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current and voltage are calculated by circuit analysis. We used Ansys Q3D Extractor, acommercially available analysis tool.
(2) High frequency analysis (100 kHz to 1 MHz)Electromagnetic fields are analyzed using the FDTD method. The voltage and current of
the target section are output directly from the integral calculation using the outputs of theelectric and magnetic fields. We used Ansys EMC Plus, a commercially available analysis tool.

3.2

Verification items

For the issues described in chapter 2, we performed the following verifications. For (1)securing current paths and the effects on electromagnetic fields, we evaluated the current
distribution through the structure containing composite materials when lightning current flows into
the structure. For (2) the effects of interference between power sources sharing groundlines, we setup two power source systems that share groundlines and supply power to the power source on one
side (disturbing side) to evaluate the voltage and current generated at the terminal of the other side
(disturbed side).

3.3
(1)

Test specimen and test Setup
Test specimen
Figure 1 shows an overall view of the test specimen, which is composed of composite,
resin, and metal elements. While the target mobile products such as aircraft and transportation
systems have structures in the size of several tens of meters, the test specimen was fabricated at
approximately 1/10 scale. The frequency band is up to 1 MHz (300m wavelength), and theeffects of standing waves are negligible for both the actual object and the test specimen, so thisdimension was determined to be not particularly problematic. In addition, two power lines
sharing groundlines were installed in the test specimen. Figure 2 shows section A-A of the testspecimen. The test specimen is a three-dimensional structure with a hexagonal cross-section.
The frame is made of resin, and an aluminum panel is attached to the bottom. The evaluation
was conducted in the case where only composite panels were installed on the surface and in thecase where composite panels with a Cu mesh for lightning protection were installed. The panels
were electrically connected to each other using groundlines composed of a metal conductor
cable with a conductor cross-sectional area of 50 mm2.

Figure 1 Overall view of test specimen

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Figure 2(2)Test specimen (section A-A)

Test setup and test conditions
Figure 3 illustrates the current application test setup and shows the test status. We
applied current from the power source to the test specimen and performed evaluation. For theevaluation of frequency characteristics, a single frequency current (0.1Hz to a maximum of 1MHz) was applied. In addition, for the evaluation related to lightning current waveforms, asimulated lightning current waveform (Component A: 1kA peak) based on the test standards
used in the aircraft field (1) was applied to evaluate the transient characteristics. In considerationof design applicability, five cases were tested with different cable types (50mm2 cable,15mm-wide copper braided cable), with and without Cu mesh, and with different combinationsof current draw in/out (single point/equalized, equalized/equalized, etc.) Figure 4 shows anexample of the applied waveform used in the test.
As the evaluation of the effects of interference between power sources sharing
groundlines, we supplied power to one power source (disturbing side) of the two power lines
sharing groundlines in the test specimen and evaluated the voltage and current generated at theterminal of the power source on the other side (disturbed side) (Figure 1).

Figure 3

Test set up

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Figure 4

3.4(1)

Analysis model
Ansys Q3D Extractor model
Figure 5 shows the model. Cable diameters and fittings were modeled according to thedimensions of the test specimen. For the resistance values of components and contact resistancepoints, the values measured with DC were used and reflected in the conductivity of the model
elements in the relevant areas. We evaluated the frequency characteristics of the impedance
between two points by analysis and compared them with the measured results to confirm thevalidity of the model. The analysis was performed at frequencies of 0.1Hz, 10kHz, and 1MHz.

Figure 5(2)Example of simulated lightning current waveform

Analysis model (Ansys Q3D)

Ansys EMC Plus model
Figure 6 shows the model. The bottom panel and copper mesh were modeled with planeelements, while the rest were modeled with line elements. The conductivities of the elements
and contact resistance points were converted from measured values, taking into account a meshsize of 20mm. The branch currents flowing through each element of the metallic conductor
cables were output. The frequency characteristics at only high frequency (100kHz, 1MHz)
conditions were analyzed and evaluated because analysis in the low frequency range with theFDTD method requires a longer time.
The encircled numbers 1 to 6 in Figure 6 show the longitudinal conductors, while number
7 shows the internal first-row conductor placed longitudinally to the direction of current
application. The results of the analytical evaluation of the current flowing through theseconductors are described later in chapter 4.

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Figure 6

Analysis model (Ansys EMC Plus)|4. Verification result
4.1

Evaluation of measures to secure current paths and against effects of induced
magnetic fields

Figure 7 to Figure 9 show the evaluation results of the analysis accuracy with respect to themeasured current distribution values through the structure when currents of 0.1Hz, 10kHz, and1MHz frequency components flowed into the test specimen. As a result of the evaluation in fivecases where the tests described in chapter 3 were conducted, the analysis error was almost within a±6dB range. Safety-critical equipment on aircraft is often given a design margin of 6dB or more (2).
Hence, it was confirmed that the analytical evaluation could be made with sufficient accuracy
allowing the consideration of excessive design margins to be avoided. In addition, in terms offrequency characteristics, the accuracy of Ansys Q3D Extractor was worse than that of Ansys EMC
Plus at 1MHz as shown in Figure 9. Therefore, it was confirmed that evaluation with an accuracyof within about a ±6dB range is made possible by using Ansys Q3D Extractor in the low frequency
range and Ansys EMC Plus in the high frequency range when applying the analytical design
method.

Figure 7

Evaluation result of single frequency characteristics (0.1Hz)

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 1 (March 2024)
7Figure 8Evaluation result of single frequency characteristics (10kHz)

Figure 9

Evaluation result of single frequency characteristics (1MHz)

The obtained analysis accuracy resulted from improvements made by taking into account thevariation of contact resistance and the refinement in modeling of form-dependent inductance.
Figure 10 shows the analysis results of the frequency characteristics of current flowing throughconductor encircled number 7 (Figure 6) constituting the groundline for the case where the contact
resistance is given uniformly and for the case where the resistance variation is considered by givingthe measured resistance value of each structural element, which indicates an improvement in theanalysis accuracy. Current values closer to the measured values were obtained by considering theresistance variation. This variation in resistance is mainly due to the variation in contact resistanceat the points where the cable conductor is connected. The metal cables in this test specimen arearranged at intervals of about 25cm. When they are designed in the actual product structure size
(meters order), the resistance of the structure itself is larger than that in this test. So, the effect ofresistance variation is relatively small. Thus, it is expected that the effect observed in this test
verification would be small.
Next, Figure 11 compares the test results of current distribution measurements obtained byapplying the simulated lightning current (Component A: 1kA peak) shown in Figure 4 to the test
specimen with the results of analysis using Ansys EMC Plus, which can handle the high frequencyof lightning as confirmed in the previous section. The waveforms matched well, verifying theapplicability of the analytical model to the design, not only in terms of frequency response but also
in terms of transient characteristics. Therefore, the current paths can now be identified by thisanalysis.

Mitsubishi Heavy Industries Technical Review Vol. 61 No. 1 (March 2024)
8Figure 10

Improvement in analysis accuracy (effects of resistance)

Figure 11

Evaluation result of applying simulated lightning current

4.2

Evaluation of effects of interference between power sources sharing
groundlines

In terms of the effects of interference between power sources sharing groundlines, Figure 12
shows the test and Ansys Q3D Extractor analysis results of the current and voltage values
generated in the disturbed side when a sine wave of 50Vpp is applied to the disturbing-side system,with respect to the frequency. The voltage and current increased from the frequency ofapproximately 10kHz, indicating the effects of interference between power sources. Nevertheless,the test and analysis values showed very good agreement, with the analysis accuracy for the current
value of 3.7dB or smaller and the voltage value of 2.4dB or smaller. Although the test verification
was conducted in the frequency range of 0.1Hz to 1MHz, there was almost no change in thecharacteristics below 1kHz, and it is expected that this can be extended to DC. Therefore, thevalidity of the analytical model for the evaluation of the effects of interference between powersources was confirmed in the frequency range from DC to 1MHz.

Figure 12

Evaluation results of power system noise effect and voltage dropMitsubishi Heavy Industries Technical Review Vol. 61 No. 1 (March 2024)9|5. Conclusion
In order to prevent lightning currents from affecting the operation of onboard electronicsystem equipment that has been increasingly electrified and sophisticated, analysis technology toevaluate electromagnetic characteristics is needed for mobile products such as aircraft, in which
composite materials with high electrical resistance have been increasingly used. We havedeveloped an analytical evaluation technology for groundline structures by utilizing twocomplementary methods: quasi-static electromagnetic field analysis plus circuit analysis (for low
frequency range) and electromagnetic field analysis based on the finite difference time domain
method (for high frequency range). We verified the analysis and evaluation technology byconducting the evaluation of the measures to secure current paths and against the effects of induced
magnetic fields and the evaluation test of the effects of interference between power sources sharing
groundlines using a mock-up. Now, the design of groundlines through analysis is possible, leading
to a reduction in the number of prototypes and elemental tests, which is expected to improve theefficiency of the design of electromagnetic field countermeasures.
Moving forward, we plan to apply this technology to product design and establish an
analytical design technology for comprehensive effects of electromagnetic fields on compositestructures.

References(1)(2)

SAE ARP5412, Aircraft Lightning Environment and Related Test Waveforms
MIL-STD-464, ELECTROMAGNETIC ENVIRONMENTAL EFFECTS REQUIREMENTS FOR
SYSTEM