Predesign Considerations for the DC Link Voltage Level of the CENTRELINE Fuselage Fan Drive Unit

Electric propulsion (EP) systems offer considerably more degrees of freedom (DOFs) within the design process of aircraft compared to conventional aircraft engines. This requires large, computationally expensive design space explorations (DSE) with coupled models of the single components to incorporate interdependencies during optimization. The purpose of this paper is to exemplarily study these interdependencies of system key performance parameters (KPIs), e.g., system mass and efficiency, for a varying DC link voltage level of the power transmission system considering the example of the propulsion system of the CENTRELINE project, including an electric motor, a DC/AC inverter, and the DC power transmission cables. Each component is described by a physically derived, analytical model linking specific subdomains, e.g., electromagnetics, structural mechanics and thermal analysis, which are used for a coupled system model. This approach strongly enhances model accuracy and simultaneously keeps the computational effort at a low level. The results of the DSE reveal that the system KPIs improve for higher DC link voltage despite slightly inferior performance of motor and inverter as the mass of the DC power transmission cable has a major share for a an aircraft of the size as in the CENTRELINE project. Modeling of further components and implementation of optimization strategies will be part of future work.


Introduction
While noise levels have been reduced drastically, absolute emissions from aviation, especially carbon dioxide (CO 2 ) and nitrogen oxide (NO x ), have been continuously on the rise throughout the last decades, accounting for 2-3% of global greenhouse gas emissions. The large improvements in fuel efficiency are thwarted by the steady increase in passenger numbers [1,2]. The National Aeronautics and Space Administration (NASA) N+3 fundamental technology research for subsonic fixed-wing aircraft [3,4], the Civil Aviation Organization (ICAO) [5,6] as well as the Aviation Transport Action Group (ATAG) [2] and the European Commission's Flightpath 2050 [7] set aggressive target levels for greenhouse gas emissions in the future-up to 75% less fuel burn and 90% NO x emissions by 2050.
To reach those ambitious objectives, a significant improvement in terms of fuel consumption is necessary. These objects can be seen as one of the main drivers for the development of novel

CENTRELINE Project Overview
CENTRELINE is a project funded by the European Union (EU), serving as a demonstrator for propulsion-aircraft integration with the goal of validating results of former studies on boundary layer ingestion (BLI) such as the FP7 project DisPURSAL [24]. The major change in aircraft design is the implementation of the so-called fuselage fan (FF) drive unit which will be installed in the fuselage aft-end [18]. This device ingests and re-energizes the boundary layer of the fuselage to compensate viscous drag in the fuselage boundary layer flow [18,25].
While DisPURSAL [24] focussed on a conventionally driven fuselage propulsor by a third gas turbine, CENTRELINE will evaluate the possible advantages of a fully turbo-electric drivetrain arrangement to overcome shortcomings of a gas turbine in the aft section such as flow losses associated with the air intake duct, vibration issues and high maintenance costs [26]. The electric powertrain consists of two power generation sets, an electric power transmission system as well as a single fuselage propulsor located in the rear of the aircraft, as shown in Figure 1. Each generation is composed of a wing-podded, advanced geared turbofan (GTF), an electric generator that converts mechanical to electric power, and an AC/DC rectifier. The electric generator is mechanically coupled to an additional free power turbine located in the low-pressure turbine (LPT) section of the GTF even though this is in the hot section, as this has the minimal influence on engine stability. The rectifier will be placed in the nacelle of the GTF to evaluate the potential of air-cooled power electronics [23]. A DC power transmission system is chosen, consisting of eight parallel lanes to transfer the power as well as protection and switching devices to control it. In the rear, a single propulsion unit including eight parallel DC/AC inverters, one electric motor and the fuselage fan itself is mounted. The inverter converts electric DC to AC power, while the motor converts it to mechanical power to drive the fuselage fan directly without any additional gearbox [27].
Several limitations with regards to installation space of inverter and electric motor (depicted in Figure 2), derived by aerodynamic calculations, must be considered during the sizing process.  [27] and modified).

Problem Setup
Brelje and Martins [9] as well as Gesell et al. [12] stated that EP is not applicable as drop-in replacement of existing aircraft propulsion unit but has to be optimized for aircraft and mission requirements. As outlined in Section 1, the proposed approach of sizing components of the electric drivetrain with physically derived, analytical models systematically improves the level of model details in order to raise awareness of importance of a better understanding of EP systems for aircraft. The average run-time per system design point is within the small digits of seconds on an Intel R Core TM i7-6600U CPU @ 2.60 GHz.
The approach was applied to the FF drive unit of the concept aircraft, considering only a reduced system with the electric motor, the DC/AC inverter and the cabling, while other components are neglected. This study aimed to investigate the influence of a change in DC link voltage level of component as well as system KPIs, especially mass and efficiency. Figure 3 shows the block diagram of the setup. The diagram only shows the parameters which are relevant for the scope of this study, in detail: • general requirements (shaft power P EM,sha f t , rotational speed n EM,sha f t and diameter-to-lengthratio D rot /L rot of the electric machine as well as the cable length L Cab ) which must be fulfilled by the component sizing process; General requirements as well as global and local variation parameter define a unique input vector x, which results in a unique output vector KPI. The necessity of a fully coupled system sizing model is elucidated using the example of the DC link voltage level. Depending on it, the winding layout as well as insulation aspects in the motor vary, which in term influences the selection of semiconductor models during the inverter sizing process. Furthermore, the selection of the DC link voltage level has significant impact on the sizing of the cabling as it is directly proportional to the current. Those effects can only be made visible if the interdependencies between components are considered correctly.

Requirements, Constraints and Design Target
The requirements for the electric motor as well as the semiconductor technology of the inverter and the length of the transmission cable stem from the aircraft design process. The electric motor has eight parallel winding systems each fed by a DC/AC inverter and connected to a DC transmission cable, transferring an eighth of the total power, respectively. The range of the DC link voltage of the transmission system is based on a pre-assessment of available semiconductor blocking voltage levels [27]. As the semiconductor module selection is based on the chosen inverter topology and the DC link voltage, five different topologies including two-, three-, and five-level topologies, are considered. Additionally, several internal parameters for each component are varied within reasonable ranges. Table 1 summarizes the most important global design requirements as well as global and local variation parameters. Besides, several constraints, especially concerning the available installation space for motor and inverter, must be considered, as shown in Figure 2. Based on the given input, a large design space exploration is performed. The design target is to identify Pareto-optimal design points with respect to the specified KPIs, i.e., system mass m sys and efficiency η sys , for the given input sets x according to Fang and Qin [28], such that where m sys = ∑ i m i and η sys = ∏ i η i . Pareto-optimality is chosen as criterion to enable an evaluation of designs with concurring KPIs. Within the next section, the sizing process of each component is explained in more detail.

Electric Machine
Several machine types have been studied extensively for electric aircraft [29]. Surface-mounted, permanent magnet synchronous machines (PMSM) have been identified as the most promising technology. PMSM have several DOFs, e.g., the number of poles and slots, the choice of materials for magnets and the laminated core of rotor and stator, the diameter-length-ratio of the rotor D rot /L rot or the stator electric current density J e f f , which represent the local variation parameters.
The iterative design process of the electric machine includes five disciplines and starts with a defined set of requirements that the final design must fulfil. In the first step, the geometrical layout of the machine is determined. Afterwards, the electromagnetic design includes sizing the magnetic circuit under no-load and load conditions with a non-linear lumped parameter model (e.g., [30,31]), adjusting phase current to reach the required shaft torque T EM,sha f t as well as the configuration of the winding layout. The electromagnetic calculation is done with Simcenter SPEED 13.06 [31]. The structural mechanics section covers the sizing of the retention sleeve for the magnets for different load cases as well as designing rotor shaft and stator housing. The thickness calculation is based on an analytical press-fit model which evaluates the tangential stresses with respect to the radial overclosure of the fit [32,33]. A further important part of machine design is insulation coordination based on IEC 60664 [34] to ensure secure operating voltage levels. To conclude the design process, a thermal analysis of the motor is performed to ensure that waste heat can be dissipated, which especially dimensions the cooling channels. The allowable conductor current density depends on the heat transfer into the cooling medium, which in turn is a function of the thickness of the insulation hindering heat flux and thus decreases allowable conductor current density. Considering the DC link voltage during the motor design thus facilitates a more realistic design.
Passive masses of the machine including rotor shaft, bearings, housing, and terminal box are hard to calculate as they strongly depend on the actual design as well as installation issues. According to a benchmark of several lightweight motors [35], a factor of two was used to account for the passive masses. This is a fair assumption as the power and rotational speed of the machine is fixed and thus the dimensioning torque for all designs identical.
After those design steps, the result is checked against the requirements. If they are not fulfilled the design process enters a further loop, otherwise the design is valid and the specifications are extracted, e.g., to serve as input for a further component such as the DC/AC inverter.

DC/AC Inverter
DC/AC inverter designs can be realized in different topologies, differing in the height of the single-step output voltage and the total harmonic distortion (THD) [27,36,37]. This study only considers two-level (2L-), three-level (T-type) neutral point clamped (3L(T)NCP-) and fly-cap (3LFC-), and five-level stacked multicell (5LSMC-) DC/AC voltage source inverters. The topologies are explained in detail by the work of Krug [36,37], on which most parts of the design process are based on.
First, the voltage configuration of switches and diodes for the given operation point are determined [36]. A scalable semiconductor characteristic is established from a database with commercially available semiconductor modules and their properties which is then used to perform a coupled calculation of electric and thermal performance. Based on the operation point, the losses of the semiconductor modules are calculated analytically [36,38,39]. The semiconductor modules as well as the heat sink, i.e., a cooling plate, are sized such that the chip temperature increase due to the losses at the calculated operating point is below a certain limit and that the waste heat can be removed via the cooling plate at steady-state conditions. As silicon-carbide MOSFETs (SiC) chips are used, a maximum chip temperature of 150 • C is assumed.
Furthermore, the DC link capacity is calculated based on the expected current ripple calculated analytically using a method provided by Krug [36]. Again, a database of commercially available capacitors is used. The sizing process also includes a mass estimation of the gate driver units and a simple housing but misses a calculation of electromagnetic interference (EMI) filters on AC and DC side. To account for the additional mass of the filters, a factor of two is applied.

DC Power Transmission Cable
The power transmission cables are routed from the generators through pylon and wing to the fuselage center to the fuselage aft, where the motor is located. The total length of the cables adds up to 86.0 m. Due to the long distance, aluminum cables are chosen instead of copper cables because of their higher ratio of electrical conductivity to specific weight. A transmission system with eight parallel DC lanes is used to mitigate the risk of a complete power loss in the event of a cable failure [27].
The geometrical design of the DC transmission cable only consists of a circular conductor material, i.e., copper or aluminum, and a concentric layer of insulation material, as electromagnetic field effects and thus shielding can be neglected. The cable sizing process is based on a coupled model of electrical and thermal analysis as well as insulation coordination. The electrical domain covers the calculation of electrical parameters such as voltage levels, currents and resistances as well as the ohmic losses in the conductor considering the conductor material. Insulation coordination determines the necessary thickness of insulation to avoid arcing according to IEC 60664 [34]. A thermal analysis determines the size of the conductor by iteratively solving a one-dimensional, cylindrically symmetrical heat equation [40]. The size of the conductor is chosen such that the losses can be dissipated via natural convection and radiation at given ambient temperature T amb , and the maximum admissible conductor temperature T cond at the insulation interface is not exceeded.

Results and Discussion of Design Space Exploration
The results of the design space exploration are discussed for the single components as well as for the combined system.

Electric Machine
First, the results of the design space exploration for the electric motor are evaluated. Figure 4 shows the gross mass m EM (active and passive masses) of the electric machine against its electric efficiency η EM at full load for three different DC link voltages V DC , ranging from 1500 V to 3000 V. Each marker represents a valid motor design according to the design process described in Section 3.3.1; however, the graph only shows a fraction of the calculated design for better readability.
The Pareto fronts for each voltage level with respect to minimum mass and maximum efficiency (i.e., minimum losses) are depicted as line plots. As can be seen, the higher is the voltage level, the heavier are the machines for identical efficiency levels. This is a result of the increasing thickness of slot, phase, and turn insulation due to the increased voltage level, which in turn leads to a larger necessary slot area. This effect is intensified by the larger cooling channels required due to inferior heat transfer through the thicker insulation and not compensated by lower current levels.
The influence of increased DC link voltage level V DC on the machine efficiency is minor, as especially the copper losses depend on the electric current density of the wire which was unchanged in this study. The slight decrease of efficiency is due to the increased core losses as the absolute loss is proportional to the iron mass at roughly constant specific iron losses. The best machines reach power densities up to 11.0 kW kg −1 , torque densities up to 50.0 N m kg −1 and efficiencies up to 98.0%. The peripheral speed of the rotor is moderately high (v rot ≈ 90 m s −1 ) due to the large size (D rot ≈ 0.4 m), wherefore the high power density can be attributed mainly to a high electromagnetic and thermal utilization of materials. This also reflects in a high current density in the stator (J e f f = 17.5 A mm −2 ), a value which is challenging but not impossible with state-of-the-art cooling techniques [41].

DC/AC Inverter
As the design of the inverter is based on the use of databases of commercially available semiconductor modules with discrete blocking voltages and capacitors and utilizes different topologies, a different visualization method is chosen. Masses and efficiencies for different topologies are plotted against varying DC link voltage. Figure 5 shows mass respectively efficiency of several inverter topologies against varying DC link voltage. In Figure 5a, the discrete steps at certain voltages can be seen clearly, which are determined by the allowable utilization accounting for derating at high flight altitudes [27] and the blocking voltage of the semiconductors as well as the number of inverter levels. The maximum efficiency within the different sections which can be reached increases with higher DC link voltages V DC for the different topologies except for the 2L inverter due to its inferior loss characteristic at high voltages.
Within the sections, the mass of the inverters decreases slightly due to the lower currents as well as the decreasing current ripple on DC link capacitors and the fly caps. The volatility in mass results from the discrete choice of capacitors out of a database. The absolute higher mass for further increased DC link voltage can be explained by the specifically higher mass of the semiconductor modules with higher blocking voltages (see Figure 5b).
The most lightweight inverters reach power densities up to P Inv,AC /m Inv ≈ 55 kV A kg −1 (gravimetric) and P Inv,AC /V Inv ≈ 50 kV A dm −3 (volumetric) at efficiencies of up to η Inv = 99.0% for maximum chip temperature T chip = 150 • C at a fluid inlet temperature T amb = 75 • C for a specific resistance R th,ca = 0.012 K W −1 of the heat sink surface to ambient (in this case the liquid). Those values are in good accordance with a literature review [42].  Figure 6 shows the DC link voltage vs. the cable mass for the eight DC lane configuration specified in Section 3.2 for aluminum as well as copper conductors. While aluminum has only 60% of the electrical conductivity compared to copper, its density is roughly a third, resulting in 50% of the cable mass per ampere. The mass of the cable reduces significantly with increasing voltage as the current decreases and with that the Ohmic losses, which are the sizing parameter for the conductor cross-section. With increasing DC link voltage level V DC , the cable mass decreases roughly indirectly. For unscreened, stranded aluminum cables with silicone rubber insulation comparable to those in [43], the absolute mass of the insulation only increases slightly for higher voltage levels as the current and thus the conductor cross-section decreases significantly. The relative mass share of the insulation compared to the total cable mass rises from roughly 15% for 1500 V to 35% for 3000 V.

DC Transmission Cable
A high DC link voltage is thus favorable, while a point of minimum cable mass might exist for even higher voltages when insulation starts to dominate the total cable mass. Figure 7 shows the results of the CENTRELINE DSE study. Figure 7a outlines the results with respect to the total system mass m Sys and system efficiency η Sys of the FF drive unit including the electric motor as well as eight DC/AC inverters and DC power transmission lanes. Total system mass m Sys in kg Efficiency of system η  Two effects superpose each other. First, the general trend implies that, with higher DC link voltage level V DC , lower system masses and higher system efficiencies are possible. As both motor and inverter tend to be heavier the mass of the cable must have a major share and its decrease in mass overcompensates the growth of the others. The second effect is more subtle and is explained for the two voltage levels 2100 V and 2200 V. The first effect would imply that a voltage level of 2200 V is more suitable on system level; however, this is clearly not the case. While the difference in motor and cable mass for the two voltages is negligible, it is not in terms of inverter efficiency and mass (see Figure 5) as V DC = 2100 V represents one of the above-mentioned section boundaries where the selected semiconductor blocking voltage and thus the module changes. This results in a quantifiable effect on system level.

System Results
To clarify this, Figure 7b shows the mass breakdown of the single components for the Pareto optimal points at a similar system efficiency η Sys = 94.0% for the four different voltage levels. Motor and inverter masses only feature a minor rise in absolute terms, while cable masses reduce significantly but still have a significant to dominating share (roughly 30-65% of the total mass depending on the voltage level).
It can be concluded that a high voltage level is beneficial in terms of system mass, however not for the electric machine and some inverter topologies, without considering further implications such as available semiconductor modules for even higher voltages or increasing complexity. Possible solutions might be the insertion of DC/DC converters after rectifier and before inverter to reduce the voltage level of motor and inverter and increase the one of the DC transmission system or the use of superconducting technologies [11]. This must be evaluated qualitatively.

Conclusions
The possibilities of coupled, analytical models for sizing EP systems have been demonstrated for the CENTRELINE project. A sensitivity study on the impact of the DC link voltage level revealed opposing effects on the component properties while on the system level the trade was clearly dominated by the DC power transmission cable. The underlying effect of discrete steps for the blocking voltages of semiconductor modules on the system performance has been made visible. Two potential technical solutions to decrease the influence of the DC power transmission system were given.
Further work will focus on the modeling of other EP components, especially heat exchangers to account for additional masses due to the thermal management needed for an electric aircraft, as well as refining existing models, e.g., passive masses of electric machines and EMI filters of inverters. Optimization strategies will be implemented.
A complete integration of the presented approach by mirroring back the results of the electric propulsion sizing process into the preliminary aircraft design loop would unlock high potential to improve quality of results as well as to optimize them.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Abbreviations
The following abbreviations are used in this manuscript: