Optimal Inductor Design and Material Selection for High Power Density Inverters Used in Aircraft Applications

This paper presents the design and optimization of power inductors for three-phase high-power-density inverters to be used in aircraft applications. The inductor’s geometric parameters, magnetic properties, core material selection, core and copper losses in addition to temperature calculations are taken into account to meet the low losses and high frequency specifications of the considered high power density inverter. A multi-objective optimization algorithm was developed to calculate weight, volume, and losses of the inductor for different current ripples, different switching frequencies and different inductor core materials. The results of a weight-objective optimization are presented showing the optimal efficiency and power density of the inverter for five chosen core materials, namely the silicon steel, ferrite, iron powder, amorphous and nanocrystalline

Design and Optimization of InterCell Transformers for Parallel MultiCell Converters

Les convertisseurs multicellulaires parallèles permettent de traiter des puissances importantes et de profiter d'une certaine standardisation des équipements. Ces dernières années, ces structures ont connu un regain d'intérêt lié notamment à la possibilité de couplage magnétique des inductances. Ce couplage aboutit à un composant magnétique aux propriétés très différentes appelé Transformateur Inter-Cellules (ICT) ; il ne modifie pas le courant de sortie, par contre il réduit l'ondulation de courant dans les bobines et l'ondulation de flux dans certaines parties du noyau. On peut montrer que ce couplage entraîne une réduction des pertes Joules dans les conducteurs et des pertes magnétiques dans le noyau. La réduction de l'ondulation de courant diminue également le courant efficace dans les semiconducteurs ce qui réduit les pertes par conduction, et la différence entre le courant à l'amorçage et au blocage des interrupteurs, ce qui permet la diminution des pertes dans les semiconducteurs lorsque les pertes au blocage sont supérieures aux pertes à l'amorçage. Le dimensionnement d'un ICT n'est pas fondamentalement différent de celui fait pour d'autres composants magnétiques en ce sens qu'il est basé sur le respect de certaines valeurs limites (induction, température) ce qui suppose une évaluation des différentes pertes et l'élaboration d'un modèle thermique. Par contre, la manière d'évaluer ces différentes grandeurs est tout à fait spécifique et n'a que quelques points communs avec les méthodes de calcul des inductances et des transformateurs Dans ce travail de thèse, on montre comment dimensionner ces ICTs en considérant plusieurs topologies et méthodes différentes, correspondant à différents niveaux de sophistication et de complexité. L'explication de ce dimensionnement est divisée en quatre parties : Pertes Cuivre, Pertes Fer, Densité de Flux de Saturation et Aspects Thermiques. L'évaluation des pertes cuivre liées aux composantes alternatives des ICTs constituent un point particulièrement délicat dans la mesure où elles résultent de la combinaison de deux facteurs eux-mêmes difficiles à évaluer ; l'inductance de fuite qui détermine l'amplitude des courants alternatifs mais dépend des flux principalement non canalisés et circulant dans l'air (volume d'étude important, effets 3D…), et la résistance équivalente des bobinages qui en haute fréquence est sujette à des phénomènes complexes comme les effets de peau et de proximité. En se basant sur l'utilisation d'un logiciel simple mais néanmoins robuste et fiable pour calculer précisément les résistances en haute fréquence et les inductances de fuite des ICTs, plusieurs astuces permettant de réduire les pertes cuivre non seulement des ICTs mais aussi des transformateurs et des inductances sont suggérées. Des tableaux simples sont développés pour aider le concepteur de transformateurs à identifier la meilleur configuration de conducteurs dans une fenêtre de bobinage en prenant en compte la forme d'onde du courant, le nombre de tours des enroulements, la fréquence des courants et les paramètres géométriques. Des formules analytiques et des outils de calcul adéquats ont ensuite été utilisés pour développer des routines d'optimisation ayant pour but la réduction de la masse, du volume, des pertes ou du coût des ICTs. Des interpolations multidimensionnelles des valeurs présimulées des résistances et inductances de fuite en haute fréquence sont utilisées afin de réduire le temps d'exécution de la routine d'optimisation. Plusieurs dimensionnements des ICTs ont été comparées vis-à-vis des matériaux du noyau et des conducteurs, du nombre de cellules de commutation et de la fréquence de découpage. Des comparaisons avec des selfs ont également été faites afin de montrer les avantages de ces ICTs. Des aspects de la commande des convertisseurs multi-niveaux triphasés ont également été étudiés vis-à- is du flux circulant dans les ICTs. Des homopolaires, spécifiques pour chaque stratégie MLI et chaque topologie convertisseur/charge, sont créées afin de minimiser le flux dans les ICTs et par conséquent de réduire davantage la masse et la taille de ces composants. Des comparaisons entre différentes méthodes de MLI sont effectuées et vérifiées expérimentalement. ABSTRACT : In recent years, the interest for parallel multicell converters has grown, which is partially due to the possibility of coupling the inductors used to connect the different commutation cells together. Coupling the inductors to form an InterCell Transformer (ICT) does not usually modify the output current, but it reduces the current ripple in the windings and the flux swing in some regions of the core. It can be shown that this brings a reduction of copper and core losses in the magnetic component. The reduction of the phase current ripple also reduces the difference between turn on and turn off current in the switches, which brings a reduction of switching losses for devices generating more losses at turn off than at turn on. The design of an ICT is not that different from any other magnetic component but it is very specific and inherent features must be taken into account. Taking full benefit of the potential advantages of ICTs requires the development of special tools and methods which are the focus of the study. We show how to design ICTs considering several topologies and different methods, from the most precise and time-consuming to the less accurate but more quickly calculated. The explanation of the ICT design is divided in four main parts: Copper Losses, Core Losses, Flux Density Saturation and Thermal Aspects. Further attention is given to high frequency copper losses since complex phenomena such as skin and proximity effects highly influence the ICT design. Based on Finite Element Method simulations, smart practices are suggested to reduce high and low frequency copper losses, not only in ICTs but also in inductors and transformers. Simple tables are developed to help transformer designers to identify the best configuration of conductors inside a given core window, depending on the current waveform and frequency, number of turns and geometrical parameters. Optimization routines to reduce the ICT total mass, volume, losses or cost are developed and multidimensional interpolation of pre-simulated values of AC resistance and leakage inductance is used to speed up the optimization routine. Comparison of ICT designs with regard to core and conductor material, number of cells and switching frequency is performed. Comparison with regular inductors is also made in order to verify the benefits of this kind of magnetic component. Multilevel converter control aspects applied to three- hase systems is also investigated in terms of the ICT flux. Zero sequence signals, specific for a PWM strategy and converter/load topology, are created in order to minimize the flux in ICTs and consequently reduce even further the mass and size of these components. Comparison between several PWM methods are performed and experimentally verified

Characterization of High Power SiC Modules for More Electrical Aircrafts

One of the main challenges associated to the More Electric Aircraft is to significantly increase power density of electrical power systems, such as electromechanical chains applied to actuation systems, without compromising on reliability. Nowadays, the best way of considerably increasing power density of power converters is by the use of disruptive technology such as Wide Bandgap (WBG) semiconductors. This paper explains the advantages of using WBG semiconductors made of Silicon Carbide (SiC) in power converters used in an electromechanical chain when compared to classical silicon-based technology. Characterization of a specific SiC power module is described in details. A non-intrusive method for measuring switching energy is explained and applied to this module in order to obtain turn-on and turn-off energies for different circuit and driver parameters. These results are used in a numerical routine which calculates losses in a three-phase inverter. These losses are compared to the ones calculated with the datasheet of this SiC module as well as with a “best-in-class” silicon-based IGBT adapted to the power level of this converter. Results show the superiority of SiC MOSFETs when compared to IGBTs as well as the highest performance which can be achieved with this component based on the characterization performed this paper

3D Analytical Modelling of Sink Heat Distribution for Fast Optimisation of Power Converters

With the development of embedded systems, it is crucial to reduce weight of equipments. In power converter, heat sink is a heavy part that often can be reduced in volume and weight. There are several models and methods to calculate a heat sink thermal resistance. However the more precise these methods are, the more time consuming they are and thus they can be hardly integrated in a weight optimization routine. Using analytical models to calculate heat sink thermal resistance is a good compromise between execution time and precision of results. They are usually one-dimensional models which are simple but do not take into account heat spreading effects, which is important when power electronic heat sources are small compared to their heat sink. This paper describes a three-dimensional analytical model of forced convection plate fin heat sink, which will be compared with numerical simulation. A maximum difference of 2.5% has been observed between models. This analytical model will be used in an optimization routine to reduce the weight of an existing heat sink in order to show that fast and precise optimization of cooling system is possible with analytical models

Influence of PWM methods in semiconductor losses of 15kVA three-phase SiC inverter for aircraft applications

In near future, modern aircrafts will be more electrical, having more power converters in different applications. Inverters is one of the most common applications, which are used in actuators, cabin pressurization, among others. In order to significantly reduce weight, size and losses of these converters, disruptive technology such as Wide Bandgap components (SiC and GaN transistors) should be used. In this paper, we study the influence of SiC components in a 540V/15kVA inverter for motor driving applications. The approach proposed here starts by the dynamic characterization of SiC modules with the aim of obtaining accurate switching energies in order to precisely calculate semiconductor losses in three-phase inverters. Then, precise conduction and switching loss calculation is developed to verify the influence of PWM methods on losses of this converter. A generic 15kVA inverter setup is built. Experimental results in this setup confirm the characterization and loss calculation and shows the best PWM methods for specific aircraft applications

Trade-off between Losses and EMI Issues in Three-Phase SiC Inverters for More Electrical Aircrafts

Power converters will only be effectively used in future aircrafts if they are compact, efficient and reliable. All these aspects can be improved by the use of disruptive technology such as the so-called Wide Bandgap (WBG) semiconductors made of Silicon Carbide (SiC) or Gallium Nitride (GaN). These components can switch much faster than their silicon counterpart, which can reduce converter losses and also decrease differential mode filter given the increase of switching frequency. However, such a fast commutation increases Electromagnetic Interference (EMI) issues in the converter and load connected to it. This paper shows the approach developed at the French Institute of Technology (IRT) Saint-Exupery, in order to evaluate the trade-offs between losses and EMI issues of three-phase inverters used in future aircraft applications. Given the high voltage DC bus of 540V, SiC MOSFETs are investigated and experimental results show the impact of these components on losses and EMI for different parameters

Optimization of forced-air cooling system for accurate design of power converters

Finding a global optimum of power converters requires models of all its parts (semiconductors, filters and cooling system), rules to describe the interactions between the different parts, an optimality criterion and optimization routines to converge towards the best design. In this paper we implement a model of forced-air cooling system to be used in such a design process. Heat exchange between fins and forced air is described, and fluid mechanics are used to account for the interactions between the fan and the heat sink. Using these descriptions, it is shown how a given performance index can be maximized using optimization routines. The developed model is experimentally verified in the case of a custom heat sink designed with this process. Finally, we investigate the influence of different parameters such as the fan power and characteristic, base plate, heat sink dimensions and materials

Impacts of the use of SIC semiconductors in actuations systems

Driven by customers’ demands to improve aircraft performance on one hand, while ensuring compliance to ACARE (Advisory Council for Aeronautics Research in Europe) environmental requirements for 2020 on the other, the aircraft industry has been pushing toward the concept of More Electric Aircraft (MEA) for the last ten years or so. One of the main challenges associated to the More Electric Aircraft is thus to increase drastically the power density of electrical power systems, such as electromechanical chains applied to actuation systems, without compromising on reliability. This paper explains the advantages of using Wide Bandgap (WBG) semiconductors made of Silicon Carbide (SiC) in the power converters that are used in an electromechanical chain as well as the associated drawbacks when it comes to EMI and partial discharge, which are mainly related to high dv/dt and overvoltage during commutation. It also shows the development of a generic electromechanical chain platform at the Institut de Recherche Technologique (IRT) Saint-Exupéry and all related research. This platform is being designed in order to test different technologies composing an electromechanical chain (SiC transistors, passive filters, cables, innovative motor) and to evaluate the impact of the use of such technologies

Trade-off between Losses and EMI Issues in Three-Phase SiC Inverters for Aircraft Applications

Power converters will only be effectively used in future aircrafts if they are compact, efficient and reliable. All these aspects can be improved by the use of disruptive technology such as the so-called Wide Bandgap (WBG) semiconductors made of Silicon Carbide (SiC) or Gallium Nitride (GaN). These components can switch much faster than their silicon counterpart, which can reduce converter losses and also decrease differential mode filter given the increase of switching frequency. However, such a fast commutation increases Electromagnetic Interference (EMI) issues in the converter and loads connected to it. This paper shows the approach developed at the French Institute of Technology (IRT) Saint-Exupery, in order to evaluate the trade-offs between losses and EMI issues of three-phase inverters used in future aircraft applications. Given the voltage DC bus of 540V, SiC MOSFETs are investigated and experimental results show the impact of these components on losses and EMI for different parameters

Unshielded Cable modeling for Conducted Emissions Issues in Electrical Power Drive Systems

In power electronics applications, high frequency models for cables are necessary to understand EMI issues in pulsewidth modulation drives. This paper shows the approach developed at the French Institute of Technology (IRT) Saint-Exupery, in order to take account of the frequency dependency of unshielded power cables per-unit-length parameters for EMC simulations. Fast, predictive models are compared to different shapes numerical models. The method was applied to unshielded two and three wires cables. Finally, common mode (CM) emissions modeling is proposed to predict the CM noise currents, which are the most disturbing in any variable-speed drive systems. The modeling principle is to consider the complete CM circuit as a chain of quadripolar matrices