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In this study, we estimate the locally defined permeability of the magnetizing particles in a soft magnetic composite (SMC) material whose effective permeability, observed by macroscopic measurements, is known. This procedure is repeated for a wide range of geometries whose electrical insulation layer thicknesses between the magnetizing particles are altered. It is shown that even if the effective permeability of the material is known by some measurement, the permeabilities of the individual particles cannot be immediately determined but remain heavily dependent on the thicknesses of the insulation films. This dependence is quantified. It is shown that when using periodic geometries to imitate SMC materials, one also has to use higher local permeabilities for the magnetizing particles or thinner insulations between the particles compared with the situation where randomized structures are used in order to obtain the effective permeability determined by measurements.

This article presents the implementation of an equivalent stress model to analyze a Galfenol-based magneto-mechanical energy harvesting concept device. The equivalent stress approach can transform any arbitrary stress tensor into a uniaxial stress acting along the direction of the magnetic flux density. Unlike multiaxial magneto-mechanical phenomenological models, it offers simple implementation to predict the permeability change by interpolation from uniaxial measurements. For the first time, the proposed model is implemented in a 3-D finite element solver using COMSOL Multiphysics software to analyze an energy harvesting setup, where the effect of mechanical preload and magnetic bias on the output power can be studied. The model is tested under different compressive loading cases ranging 20-80 MPa, and the simulated results are compared with the measurement results for validation. The results show that the approach can successfully be used as a tool to analyze the behavior of the harvester device and to determine the optimal design parameters. acceptedVersion

This article presents a modified Jiles–Atherton hysteresis model for a weakly anisotropic non-oriented silicon steel sheet. In a toroidal inductor, the magnetic flux density can point toward any direction compared to the sheet orientation, and the hysteresis model should take this into account. We identify the model parameters independently for unidirectional alternating B(H)-characteristics in seven different directions. Then, we construct an anisotropic hysteresis model, where the model parameters can depend on the magnitude and direction of the applied magnetic flux density. We demonstrate that the parameters identified in the rolling and transverse directions of the silicon steel sheet (M400-50A) are sufficient to describe the hysteresis losses in other directions.

The Future Circular Collider (FCC-hh) project is a conceptual study whose goal is to design the successor of the Large Hadron Collider, increasing the collision energy from 14 to 100 TeV. The energy stored in the 16-T superconducting dipole magnets and the length of the sectors composing the 100-km FCC tunnel are considerably larger than those in present accelerators. This means that the energy stored in the FCC-hh dipole circuit is likely to be much higher than that in existing superconducting circuits. In the case of magnet quenches or faults, the circuit needs to be protected, i.e., its energy needs to be rapidly dissipated without inducing excessive voltages in the magnet chain. This article proposes a conceptual design for the FCC-hh dipole circuit, which satisfies the constraint of the maximum allowable voltage-to-ground and fulfills additional requirements related to the FCC-hh operation and tunnel layout. A compromise among the considered requirements leads to a relatively simple circuit layout and a large number of circuits for the entire machine. The behavior of the proposed circuit during the critical fast power abort phase is simulated through a numerical model, which covers the electrical circuit domain and the electrothermal magnet domain. Each FCC-hh dipole magnet is protected by means of the coupling-loss-induced quench (CLIQ) protection system, which also acts at the circuit level. The simulations predict severe voltage oscillations in the FCC-hh dipole circuits that may pose a problem for the quench detection system. The simulations also show that the severity of the oscillations is not due to the presence of CLIQ. This protection system can be integrated into the proposed circuit layout and represents an effective protection system for the entire string of FCC-hh dipole magnets. publishedVersion

Novel nanocomposites for dielectric applications-based polypropylene/poly(ethylene-co-octene) (PP/POE) blends filled with nano silica are developed in the framework of the European &lsquo, GRIDABLE&rsquo, project. A tailor-made low-pressure-plasma reactor was applied in this study for an organic surface modification of silica. Acetylene gas was used as the monomer for plasma polymerization in order to deposit a hydrocarbon layer onto the silica surface. The aim of this modification is to increase the compatibility between silica and the PP/POE blends matrix in order to improve the dispersion of the filler in the polymer matrix and to suppress the space charge accumulation by altering the charge trapping properties of these silica/PP/POE blends composites. The conditions for the deposition of the acetylene plasma-polymer onto the silica surface were optimized by analyzing the modification in terms of weight loss by thermogravimetry (TGA). X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray fluorescence spectroscopy (EDX) measurements confirmed the presence of hydrocarbon compounds on the silica surface after plasma modification. The acetylene plasma modified silica with the highest deposition level was selected to be incorporated into the PP/POE blends matrix. X-ray diffraction (XRD) showed that there is no new crystal phase formation in the PP/POE blends nanocomposites after addition of the acetylene plasma modified silica. Differential scanning calorimetry results (DSC) show two melting peaks and two crystallization peaks of the PP/POE blends nanocomposites corresponding to the PP and POE domains. The improved dispersion of the silica after acetylene plasma modification in the PP/POE blends matrix was shown by means of SEM&ndash, EDX mapping. Thermally stimulated depolarization current (TSDC) measurements confirm that addition of the acetylene plasma modified silica affects the charge trapping density and decreases the amount of injected charges into PP/POE blends nanocomposites. This work shows that acetylene plasma modification of the silica surface is a promising route to tune charge trapping properties of PP/POE blend-based nanocomposites.

Breakdown strength, DC resistivity, permittivity and loss of thermally sprayed alumina coatings were studied at various temperatures and relative humidities. The studied coatings were sprayed by utilizing three different spray techniques: flame, high-velocity oxygen fuel (HVOF) and plasma spraying. Breakdown behavior of HVOF sprayed alumina were studied up to very high temperatures (800 °C). At 20-180°C, no significant trend could be seen in the breakdown strength of HVOF and plasma sprayed alumina coatings. The breakdown strength of alumina coatings decreased gradually from 300 to 800 °C reaching a value which was only 14% of the breakdown strength measured at 20 °C/RH 20%. Increasing humidity (from 20 to 90%) decreased the DC resistivity of the alumina coatings five orders of magnitude. Correspondingly, permittivity and losses increased with the humidity; in most cases with a notable contribution due to DC conduction. The material behavior may be linked to the microstructure of coatings consisting of amorphous and crystalline regions with interfaces in between. Moreover, the alumina coatings exhibited notable amount of highly hygroscopic γ-phase which also affected the moisture sensitivity of the coatings. acceptedVersion

openaire: EC/FP7/339380/EU//ALEM Electrical steel sheets of motors and generators are usually shaped to the final form by punching. The punching and other cutting processes generate large plastic deformations and residual stresses. These are known to deteriorate the magnetic properties of the edge region. However, the characterization of this deterioration in the form of magnetic properties is missing. The main aim of this paper is to propose a method to identify the magnetic properties of the edge region based on experimental results. This approach is demonstrated by using previously presented test results for magnetic properties of rectangular strips. The width of these strips is varied, and thus, the share of the edge region to the whole can be used as a variable. Based on this variation, a simple model is developed and the model parameters fitted to the experimental results. The correspondence between the calculated and experimental results is good.

Biaxial-stretching-induced morphology development and dielectric properties of hydrophobic fumed silica-BOPP nanocomposite films were investigated. The precursor (cast) film morphology, pre-heating as well as biaxial stretching temperature and stretch ratio were found to profoundly affect the silica-BOPP film bulk morphology, surface texture and the formation of particle/agglomerate-induced cavitation upon biaxial stretching. Isothermal high-field conductivity and thermally stimulated depolarization current measurements indicated decreasing high-field/high-temperature conductivity and modification of trap density of states upon incorporation of nanosilica in comparison to neat BOPP. Large-area DC dielectric strength is, however, sensitive to thermal/mechanical stresses upon biaxial stretching, hence necessitating careful optimization of the nanocomposite formulation and processing. publishedVersion

A systematic study of the influence of surface modification of nanosilica with coupling agents having different polarities on the dielectric properties of a PP/POE/silica blend was performed. The main goal of this investigation was to tailor the chemical composition of the silica surface, in order to modify the charge trapping properties of the nanocomposites. For the modification of the silica surface, a “green” approach was utilized: a dry silanization method, which is performed without the need of a solvent. The results of the thermogravimetric analysis (TGA) indicate that the dry process is an effective method to perform silica surface modification using alkoxysilanes. The charge trapping properties were studied by Thermally Stimulated Depolarization Current (TSDC) measurements. The obtained TSDC results show significant differences in charge trapping properties of PP/POE composites filled with differently modified silicas. Polar functional groups attached to the surface of the silica appear to have a strong effect on the charge trapping properties: The trap depth distribution becomes deeper and the trap density decreases to significantly lower levels. All results show that incorporation of surface modified nanosilica into a PP/POE matrix is a promising approach to tailor its dielectric properties. Further development of these composites may lead to benefits for application in high-voltage cable and capacitor applications. Keywords: silica surface modification, dry silanization, charge trapping, PP, POE, TSDC, high voltage cable. publishedVersion

The measurement and interpretation of high-field thermally stimulated depolarization current (TSDC) of capacitor-grade biaxially oriented polypropylene (BOPP) films are investigated in order to facilitate the derivation of trap parameters. Two main relaxations associable with the glass transition and detrapping of space charge from deep traps are observed, and their dependence on the polarization/depolarization conditions are studied. The complex bipolar nature of the space charge relaxation makes the derivation of trap parameters challenging. Lastly, a brief attempt is made to correlate the TSDC features with morphology and other short-term dielectric properties. publishedVersion

The use of practical high temperature superconductors (HTS), REBCO tapes especially, in magnet applications has become possible thanks to the increasing interest of manufacturers. One difficulty has been the nonlinear material properties that are challenging to measure and model. To advance in such, demo systems are needed and they must be thoroughly analyzed. Recently, one of the first HTS dipole magnets was built to study the usability of REBCO Roebel cables in particle accelerator magnets. The prototype magnet Feather-M2 was designed, constructed and tested within EUCARD2 collaboration project at CERN in 2017. In the measurements, the magnet behaved in an unexpected way: the magnet was able to be operated at operation currents above the maximum current that was predicted based on short-sample measurements. Additionally, unexpectedly gradual dependency between magnet's resistive voltage and operation current was observed. In this work, a thermodynamical model is formulated in order to study the behavior of Feather-M2. The model was parametrized and the parameters were solved via inverse problem by finding the best match to experimental results. Thereby insight was gained on the prospects of the utilized thermodynamical model and also on the behavior and operation conditions of the magnet via the inverse problem solutions. To summarize, this paper presents a new methodology for analyzing magnets in operation and applies it to a state-of-the-art magnet. acceptedVersion

Battery Energy Storage Systems (BESSs) play an important role in grid-connected renewable energy systems as they provide great flexibility in the energy production. For photovoltaic (PV) energy applications, the BESS may be connected to the dc-link of the converter system to inject the deficit or to absorb the surplus of generated power. One of the main issues in such systems is the voltage variation at the dc-link, especially under grid imbalance, distorting the current fed to the grid. This paper studies a method to control the BESS to emulate a capacitance, thus mitigating the dc-link voltage fluctuation, for example, under heavy imbalance. The derived model initiates impedance-based analysis at the output of the battery charger. acceptedVersion

The deadtime is an important factor in the design of power-electronic converters in order to prevent shoot-through faults. The deadtime may also cause a voltage error and an undesired damping effect which, in turn, affect the converter stability. As most effects caused by the deadtime are highly nonlinear, conventional modeling techniques to analyze these effects cannot be straightforwardly applied. This paper proposes a novel frequency-domain approach to model the damping effect caused by the deadtime in single-phase half-bridge inverters. Hardware-in-the-loop (HIL) simulations and laboratory measurements are presented and used to demonstrate the effectiveness of the proposed method. acceptedVersion

Various dielectric nanocomposite materials are studied in the frame of the European Commission funded project GRIDABLE. This project has the aim to develop DC cable extruded insulation and medium and low voltage DC capacitor films exhibiting enhanced performance with respect to presently used materials. The nanocomposites intended for cable applications are based on polypropylene (PP) blends filled with surface modified nano-silica particles. The surface modification is carried out via the state-of-the-art solution method using a polar silane as the modifying agent. Thermally Stimulated Depolarization Current (TSDC) measurements were carried out in order to study the charge trapping behavior of the nanocomposite samples. TSDC results indicate that the addition of the treated nano-silica, for most cases, reduces the density of the deep traps significantly. While more detailed studies are necessary, these results imply that the depth and the density of the deep trap states is profoundly influenced by the level of the silica modification i.e. the amount of the grafted silane on the silica surface. acceptedVersion

The functioning and resilience of modern societies have become more and more dependent on critical infrastructures. Severe disturbance to critical infrastructure is likely to reveal chaotic operational conditions, in which infrastructure service providers, emergency services, police, municipalities, and other key stakeholders must act effectively to minimize damages and restore normal operations. This paper aims to better understand this kind of operational environment resulting from, for example, a terrorist attack. It emphasizes mutual interdependencies among key stakeholders in such situations. The empirical contribution is based on observations from a workshop, in which participants representing the critical services and infrastructures in Finland discussed in thematic groups. Two scenarios guided the workshop discussions, nationwide electricity grid disruption and presumably intentionally contaminated water supply in a city. The results indicate that more attention should be paid to the interdependencies between critical infrastructures, as well as to the latent vulnerabilities hidden inside the systems. Furthermore, producing security seems to require continuous interaction and creation of meanings between extremely different actors and logics. This implies a need for changes in thinking, particularly concerning the ability to define problems across conventional administrative structures, geographical boundaries and conferred powers.

The short-sample critical current is only an indicative property for the maximum current a magnet can be continuously operated with. This was especially visible in the experiments of one of the world's first Roebel-cable-based high temperature superconductors dipole magnet prototype built and tested at CERN in 2017 where the thermal runaway developed very slowly in many cases. Consequently, the maximum stable operation current could be overstepped and stable operation could be recovered by lowering the current below the maximum of the stable range again. It is non-trivial to quantitatively predict this behavior from the critical current measurements which are observed under specific cooling conditions and based on an arbitrarily selected electric field criterion for the critical current. To make more rigorous predictions on the maximum stable operation current, one needs to consider in detail the interplay of cooling over the magnet surface and heat generation in the winding. This paper presents a methodology to determine the maximum stable operation current for a given magnet, as well as studies its mathematical background. Insight to this problem comes from the Roebel-cable-based dipole magnet studied at CERN during 2017. acceptedVersion

International audience; Part of the Future Circular Collider (FCC-hh) study is dedicated to the development of the 16 Tesla ${\rm Nb_3Sn}$superconducting dipole magnets. The design of the magnets was enabled by a cooperative effort of national research institutes, universities, and CERN. These actors tackled the problem from different sides, namely, the electromagnetic design, the mechanical design, the design of the quench protection systems, and the circuit design. The article deals with the design of the quench protection systems and provides solid motivations for the selection of the coupling-loss-induced quench (CLIQ) device as the baseline protection system for the FCC-hh main dipole magnets. The article shows that the design domains mentioned above are tightly interconnected and, therefore, the simulation of a quench event involves a complex multiphysics problem. The STEAM cosimulation framework, recently developed at CERN, is applied to address the complexity. The STEAM-SIGMA models are employed to simulate the CLIQ quench protection system applied to the FCC-hh dipole magnets. Dedicated CLIQ configurations are identified to protect the magnets in case of a quench. In addition, the possible implications of the CLIQ protection system on the mechanical design of the magnets are discussed. To this end, the article employs the co-simulation of different software platforms to calculate the mechanical stress during a quench. The results show that CLIQ does not produce additional stress.

The utility-scale wind and solar electricity production is typically connected to the power grid through multiple parallel three-phase inverters. One of the main issues in such grid-connected systems is the harmonic resonance caused by interactions between the grid and inverters. A common method for the analysis of these systems has been the impedance-based stability criterion. However, in systems that have multiple parallel inverters, the system complexity and challenges in obtaining the required impedance measurements may deteriorate the accuracy of the impedance-based approach. This paper discusses the aggregation of parallel inverters and the stability analysis of such grid-connected system. A simple method, based on impedance measurements, is shown for defining the allowable number of paralleled inverters so that the system remains stable. Experimental results are shown from power hardware-in-The-loop setup recently developed at DNV GL Flexible Power Grid Lab. acceptedVersion

Three different 16 T dipole magnet options for the Future Circular Collider (FCC) have been designed within the H2020 EuroCirCol collaboration, namely a cosθ-, a block-, and a common-coil -type of magnet. All magnets were designed using the same constraints related to magnetic field, cable parameters, mechanics, and quench protection. The quench protection analysis during the magnet design was centralized and done for all the options using the same tools and methods. This paper summarizes the final conceptual protection schemes. They are based either on the novel CLIQ-technology (Coupling Loss Induced Quench) or on traditional quench heaters. We compare the performance of the protection systems and show that while both can protect the magnet at nominal operation current, CLIQ is more efficient in reducing peak temperatures than heaters, and the system operation is simpler due to smaller amount of protection units. Therefore, CLIQ has been chosen as the baseline protection option for the FCC dipole magnets. The future development of heaters is considered as a back-up option. acceptedVersion

The process of collecting low-level kinetic energy, which is present in all moving systems, by using energy harvesting principles, is of particular interest in wearable technology, especially in ultra-low power devices for medical applications. In fact, the replacement of batteries with innovative piezoelectric energy harvesting devices can result in mass and size reduction, favoring the miniaturization of wearable devices, as well as drastically increasing their autonomy. The aim of this work is to assess the power requirements of wearable sensors for medical applications, and address the intrinsic problem of piezoelectric kinetic energy harvesting devices that can be used to power them, namely, the narrow area of optimal operation around the eigenfrequencies of a specific device. This is achieved by using complex numerical models comprising modal, harmonic and transient analyses. In order to overcome the random nature of excitations generated by human motion, novel excitation modalities are investigated with the goal of increasing the specific power outputs. A solution embracing an optimized harvester geometry and relying on an excitation mechanism suitable for wearable medical sensors is hence proposed. The electrical circuitry required for efficient energy management is considered as well.

This paper presents space charge accumulation characteristics of cable grade and capacitor grade nanostructured polypropylene (PP) materials, providing initial results regarding the electrical properties of nanofilled PP materials belonging to the European project GRIDABLE. This project has the aim to develop DC cable extruded insulation and MV and LV DC capacitor films having enhanced performance with respect to presently used materials. The paper shows that nanostructuration may be beneficial, especially at higher temperature, to improve the capacitor-grade material performance. On the other hand, this work validates the importance of surface functionalization of nanofillers, leading to lower space charge accumulation on DC cable-grade materials. acceptedVersion

This paper examines the morphology and short-term dielectric performance of commercial capacitor BOPP films, with the purpose of serving as an industrial reference in European project GRIDABLE in which next-generation PP-silica nanocomposite materials are developed for DC capacitor and cable applications. Results from dielectric spectroscopy, thermally stimulated depolarization current (TSDC) and DC conductivity measurements are reported, providing insight into the dielectric loss behavior and charge trapping/transport properties of the studied films. Comprehensive short-term dielectric breakdown strength analysis using small- and large-area approaches indicates high dielectric breakdown performance, even at an elevated temperature of 100 °C. acceptedVersion

Effects of cast film extrusion and biaxial orientation on morphological development and DC dielectric breakdown performance of non-oriented and biaxially oriented polypropylene (BOPP) films were studied. Cast films, based on two capacitor-grade isotactic polypropylene (iPP) homopolymers, were manufactured using three different extruders under different crystallization conditions and subsequently biaxially oriented using a laboratory stretching machine. Analysis of polymorphic composition and microstructural features was carried out in conjunction with large-area multi-breakdown performance assessment under progressive DC voltage ramp conditions. Polymorphic α/β-form crystalline composition and spherulitic/trans-crystalline morphology formed during cast film extrusion were found to directly affect the biaxially oriented film morphology and breakdown characteristics. For the BOPP films, decrease in dielectric strength was traced back to β→α crystal transformation and subsequent microvoid/porosity formation upon biaxial stretching. Topographical features and increasing surface roughness were also found to correlate with lower BOPP film dielectric strength. Dependence of breakdown strength on the biaxial stretch ratio was demonstrated, with higher biaxial stretch ratio resulting in improved BOPP film breakdown performance. As an extreme case, the biaxially oriented films were compared against non-oriented cast films in similar thickness range, highlighting the essential role of biaxial orientation in the high-dielectric-performance of thin films in capacitor applications. Control of polymer design as well as optimization of film processing and morphological development was found to be necessary to avoid formation of structural defects and loss of electrical insulation integrity. acceptedVersion

The non-uniform air gap in an electrical machine caused by rotor eccentricity creates an asymmetrical flux-density distribution in the air gap. This can affect the nominal torque produced by the machine. Eccentricity also produces forces that act on the rotor which may also have an effect on the torque. Thus, it is important to know how the torque of the machine behaves. In this paper, the torque of a cage induction machine is studied when the machine has dynamic eccentricity. The study is performed using the finite element method and a magnetic vector potential formulation. The torque is calculated by the method of energy balance. The harmonic components of the torque are also analyzed. The results show that the machine under eccentricity does not exhibit the same torque as a normal healthy machine. The harmonic components around the first principal slot harmonic is most affected.

A quench protection concept based on coupled secondary coils is studied for inductively transferring energy out of a quenching superconducting dipole and thus limiting the peak hotspot temperature. So-called 'quench absorption coils' are placed in close proximity to the superconducting coils and are connected in series with a diode for the purpose of preventing current transformation during regular operation. During a quench, current is then transformed into the quench absorption coils so that a significant fraction of the stored magnetic energy is dissipated in the these coils. Numerical calculations are performed to determine the impact of such a concept and to evaluate the dimensions of the quench absorption coils needed to obtain significant benefits. A previously constructed 15 T Nb3Sn block coil is taken as a reference layout. Finite-element calculations are used to determine the combined inductive and thermal response of this system and these calculations are validated with a numerical model using an adiabatic approximation. The calculation results indicate that during a quench the presence of the quench absorption coils reduces the energy dissipated in the superconducting coils by 45% and reduces the hotspot temperature by over 100 K. In addition, the peak resistive voltage over the superconducting coils is significantly reduced. This suggests that this concept may prove useful for magnet designs in which the hotspot temperature is a design driver. acceptedVersion

In this study, the DC conductivity from low electric fields up to breakdown fields is studied for several different thermally sprayed ceramic coatings. Although the DC conductivity of bulk alumina ceramic has been observed to follow the space charge limited current conduction mechanism, the studied ceramic coatings do not follow or follow only partly this mechanism. Possible reason for this is their different microstructure since bulk alumina exhibits fully crystalline microstructure while the ceramic coating consists of crystalline and amorphous regions as well as voids, defects and numerous interfaces. A possible conduction mechanism of the ceramic coatings based on the different conductivities of the amorphous and crystalline regions of the coatings is proposed. The microstructural features (e.g. volumetric porosity) are found to affect the breakdown strength for some of the studied coatings. The step-test breakdown strengths of the coatings were lower than the ramp-test ones due to the longer stress durations in step tests giving an indication of effects of electrical stress duration and possible short-term degradation of the coatings. acceptedVersion

PurposeNon-oriented electrical steel presents anisotropic behaviour. Modelling such anisotropic behaviour has become a necessity for accurate design of electrical machines. The main aim of this study is to model the magnetic anisotropy in the non-oriented electrical steel sheet of grade M400-50A using a phenomenological hysteresis model.Design/methodology/approachThe well-known phenomenological vector Jiles–Atherton hysteresis model is modified to correctly model the typical anisotropic behaviour of the non-oriented electrical steel sheet, which is not described correctly by the original vector Jiles–Atherton model. The modification to the vector model is implemented through the anhysteretic magnetization. Instead of the commonly used classical Langevin function, the authors introduced 2D bi-cubic spline to represent the anhysteretic magnetization for modelling the magnetic anisotropy.FindingsThe proposed model is found to yield good agreement with the measurement data. Comparisons are done between the original vector model and the proposed model. Another comparison is also made between the results obtained considering two different modifications to the anhysteretic magnetization.Originality/valueThe paper presents an original method to model the anhysteretic magnetization based on projections of the anhysteretic magnetization in the principal axis, and apply such modification to the vector Jiles–Atherton model to account for the magnetic anisotropy. The replacement of the classical Langevin function with the spline resulted in better fitting. The proposed model could be used in the numerical analysis of magnetic field in an electrical application.

An energy-based approach for anisotropic modeling of stress-dependent magnetization curves in electrical steel sheets is presented. The model is based on an orthotropic extension of an existing isotropic thermodynamic model and coupling to the Jiles-Atherton model of hysteresis. The model fits well to experimental results under uniaxial excitation, and it is numerically tested under rotational fields. acceptedVersion

A large-area electro-thermal ageing test setup was developed and utilized to age several laboratory-scale biaxially oriented polypropylene (BOPP)–hydrophobic silica nanocomposite films. The films were aged in test capacitors with self-healing metallized film electrodes, which enabled the ageing test to continue beyond the first breakdowns. Eight different films were aged for 1000 hours under 100 V/µm DC stress at 75 °C, and large-area DC breakdown measurements, dielectric spectroscopy, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and dielectric spectroscopy were used to detect localized and bulk degradation after the ageing period. The effects of antioxidant contents, different PP grades and compounder screw speed were evaluated. Material characterization indicates no bulk degradation had occurred during ageing, which was associated with moderate temperature stress and inert nitrogen atmosphere. On the other hand, low-field breakdowns (weak points) were observed in all but two of the aged materials, indicating that ageing was dominated by localized degradation which may have introduced new breakdown mechanisms. Weak points were also measured in a similarly aged commercial capacitor-grade BOPP film aged at a lower field, supporting this conclusion. The importance of long-term characterization in material development is demonstrated, and it is shown that long-term properties of the evaluated nanocomposites were at least on the same level compared to neat BOPP films. acceptedVersion

International audience; In this paper, proper orthogonal decomposition method is employed to build a reduced-order model from a high-order nonlinear permanent magnet synchronous machinemodel with multiple inputs. Three parameters are selected as the multiple inputs of the machine. These parameters are terminal current, angle of the terminal current, and rotationangle. To produce the lower-rank system, snapshots or instantaneous system states are projected onto a set of orthonormal basis functions with small dimension. The reducedmodel is then validated by comparing the vector potential, flux density distribution, and torque results of the original model,which indicates the capability of using the proper orthogonal decomposition method in the multi-variable input problems. The developed methodology can be used for fast simulations of the machine.

A large-area high temperature breakdown measurement and an ageing test method are presented. These methods facilitate the development of reliable higher energy density film capacitors by exploiting large measurement areas to provide information on weak point formation and subtle changes in breakdown behavior after electro-thermal or thermal ageing. The test methods were used to characterize two types of highly isotactic biaxially oriented polypropylene capacitor films, which had similar breakdown behavior at room temperature, but different breakdown properties at high temperature and out of which one was more susceptible to electro-thermal DC ageing. acceptedVersion

openaire: EC/FP7/339380/EU//ALEM A simplified multiscale approach and a Helmholtz free energy based approach for modeling the magneto-mechanical behavior of electrical steel sheets are compared. The models are identified from uniaxial magneto-mechanical measurements of two different electrical steel sheets which show different magneto-elastic behavior. Comparison with the available measurement data of the materials shows that both models successfully model the magneto-mechanical behavior of one of the studied materials, whereas for the second material only the Helmholtz free energy based approach is successful.

The abilities of a simplified multiscale and a Helmholtz energy HE models models from the literature to predict the multiaxial stress dependent magnetic hysteresis behavior of electrical steel sheets are analyzed. The identification of the models is performed using only uniaxial magneto-mechanical measurements. Reasonable accuracy between the measurements and the modeled results is obtained. With this paper, the applicability of the HE-based model for predicting the multiaxial magneto-mechanical behavior of electrical steel sheets is verified for the first time. The differences between the studied models and possible modifications to increase the accuracy of them are discussed. Some brief guidelines for the applications are given. acceptedVersion

A 37 kW induction motor is modeled numerically taking into account the deterioration of the magnetic material properties at the cut edges of the core laminations. The magnetization properties and specific loss curves of the damaged edge and the intact material have been estimated from measurements published earlier. The deteriorated edges affect the iron losses of the machine in two distinct ways. First of all, the reduced permeability at the edges causes higher local flux densities elsewhere, which increases the losses. On the other hand, the cutting also has a direct effect on the specific loss density at the edges. By numerical simulations we show that the main effect is the latter one, and that the effect of decreased permeability remains low. The iron losses increase up to 37.6 %. The torque and active power of the machine are not much affected by the cutting, although also the current and resistive losses slightly increase due to the decreased permeability at the edges.

Thermally sprayed insulating ceramic coatings can be utilized in conditions where polymers are inapplicable. The coatings exhibit a special lamellar microstructure with interfaces and some defects (e.g. voids, cracks) in between. The aim of this study was to analyze the relationship between the microstructural features and the dielectric properties of various thermally sprayed ceramics. The structural characterization of the ceramic coatings was made based on following properties: porosity, volumetric gas permeability and the characteristic size of crystalline lamella. High gas permeability of the coatings decreased the breakdown strength but similar effect cannot be seen in the DC resistivity and the permittivity results. Decrease of DC resistivity at high humidity did not correlate to microstructural properties; rather it is speculated to indicate the hydrophilic nature of the coatings. The characteristic crystalline domain sizes showed no clear correlation with the dielectric properties. acceptedVersion

A coupled magneto-mechanical model for hysteresis in an electrical steel sheet is presented. The foundation of the model developed is the classical Sablik-Jiles-Atherton (SJA) model. A comprehensive model for the stress dependent magnetostriction is also proposed and implemented in the SJA model. Improvements in the SJA model as well, are proposed and validated with simultaneous measurements of magnetostriction, magnetic field and flux density. The measurements were performed on a single electrical steel sheet under various levels of stress (-35 MPa to 100 MPa). The proposed model was found to adequately model the permeability change and the local bowing of the BH-loop due to stress. acceptedVersion

openaire: EC/FP7/339380/EU//ALEM Electrical sheets of electrical machines are laminated to reduce eddy current loss. However, a series of punching and pressing processes form random galvanic contacts at the edges of the sheets. These galvanic contacts are random in nature and cause an additional eddy current loss in the laminated cores. In this paper, a stochastic Galerkin finite element method is implemented to consider random interlaminar contacts in the magnetic vector potential formulation. The random interlaminar conductivities at the edges of the electrical sheets are approximated using a conductivity field and propagated through the finite element formulation. The spatial random variation of the conductivity causes the solution to be random, and hence, it is approximated by using a polynomial chaos expansion method. Finally, the additional eddy current losses due to the interlaminar contacts are estimated from a stochastic Galerkin method and compared with a Monte Carlo method. Accuracy and computation time of both models are discussed in the paper.

Future accelerator magnets are pushed to their limits in terms of magnetic field, mechanical strength and from the quench protection point of view. This forces the magnet designers to re-think the quench modelling. One issue that has not so far been largely explored is the mechanical behaviour of the superconducting coils during a quench. This can cause limitations to the design of high field accelerator magnets. This paper focuses on mechanical behavior in the event of a quench of a Nb3Sn 16 T dipole magnet currently developed in the framework of the EuroCirCol project in view of the Future Circular Collider conceptual design study. The thermo-mechanical analysis is performed through finite element modeling. The analysis takes into account the Lorentz force and the thermal stress due to the non-uniform temperature distribution in the winding during a quench. acceptedVersion

In this paper, a set of performance metrics for modern biaxially oriented polypropylene (BOPP) capacitor films is established. The fundamental and applied properties of BOPP films required for application in state-of-the-art DC metallized film capacitors are reviewed, highlighting aspects related to high temperature operation, base PP properties and film processing. Commercial BOPP films—both base films and metallized films based on classic isotactic PP—are studied comprehensively, encompassing structural–morphological characterization and short- to medium-term dielectric characterization. Dielectric spectroscopy results demonstrate the negligible dielectric losses of BOPP, being in the range of 10−4 or less in the expected operation temperature regime. Thermally stimulated depolarization current (TSDC) measurements indicated a modest density of shallow traps (~0.75 eV) and a high density of deep traps (~1.08 eV) in the 5 µm and 10 µm film variants showing differences presumably arising from film processing. Such an electronic structure was found to be connected with ultra-low conductivity (in the range of 10-17–10-16 S/m), high breakdown strength (~700 V/µm) and negligible space charge accumulation up to temperatures of ~70 °C. It is shown that at current design stresses (~200 V/µm at ~60 °C) BOPP is operated close to its fundamental thermal and electrical limitations. Voltage endurance tests at higher fields revealed the onset of high-field degradation and drastically reduced insulation life, and thermal activation of deep traps in the high temperature region (~100 °C) was found to result in reduced dielectric performance. acceptedVersion

This paper presents the scientific background of a new European project, GRIDABLE, which was launched at the beginning of 2017 and has to deliver results in manufacturing and characterization of LV-MV capacitors and MV-HV cables for DC application. The innovation is in the development of nanostructured materials based on polypropylene and silica, and the relevant capacitor and cable manufacturing procedures. The initial results regarding the electrical properties of PP-SiO2 materials, which have brought to the proposal of this project, are presented in this paper, focusing on breakdown strength and space charge measurements performed on nanofilled PP films for capacitors. acceptedVersion