Search

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.

Direct model predictive control (MPC) with reference tracking, also referred to as finite control set MPC (FCS-MPC), has gained significant attention in recent years, mainly from the academic community. Thanks to its applicability to a wide range of power electronic systems, it is considered a promising control method for such systems. However, to simplify the design, researchers frequently make choices that - often unknowingly - reduce the system performance. We discuss and analyze in this paper the factors that affect the closed-loop performance of FCS-MPC. Based on these findings, design guidelines are provided that help to maximize the system performance. To highlight the performance benefits, two case studies will be considered; the first one consists of a two-level converter and an induction machine, whereas the second one adds an LC filter between the converter and the machine. acceptedVersion

We present a novel approach for stochastic finite-element (FE) modeling of the electromagnetic field in ferrites, combining a thin shell model (TSM) for highly permittive grain boundaries with a Voronoi-tessellation-based geometry generation algorithm. The TSM is validated in the case of a periodic grain structure in a linear 2-D time-harmonic case over a frequency range of 10 kHz-1 GHz, and problems related to standard FE discretization are discussed. The TSM is then applied in a stochastic study for simulating the effect of varying grain structure on the effective resistivity, losses, and reactive power densities of a ferrite core., SCOPUS: ar.j, info:eu-repo/semantics/published

In well-designed photovoltaic (PV) power plants, such as in typical utility-scale PV plants, operational conditions are quite stable and homogeneous apart from the fast irradiance transitions caused by cloud shadings. These partial shading events cause fast power fluctuations leading even to stability and quality problems in power networks. Fast non-homogeneous irradiance transitions cause also mismatch losses in PV generators and the occurrence of multiple maximum power points (MPPs), which appear in a wide voltage range of the PV generator. In consequence, the global MPP can fluctuate in a wide voltage region causing possible MPP tracking problems and power losses. This article presents a study of the behaviour of the global MPP voltage of various PV arrays during irradiance transitions caused by clouds. The global MPP voltage, its rate of change and the differences in voltage and available energy between the global MPP and the MPP with the highest voltage of the PV generators have been analysed comprehensively, for the first time, by using the characteristics of around 8000 measured irradiance transitions. The global MPP voltage was found to vary between 28 and 112% of the nominal MPP voltage of the array. acceptedVersion

The operation of more electric aircraft is dependent on the embedded power grid. Therefore, the onboard power-distribution system must be reliable, having a high level of survivability, and promptly respond to any change in aircraft’s operation. Recent studies have presented a number of frequency-response-based tools with which to analyze both single- and multi-converter systems. The methods can be efficiently applied for on-board system analysis, stability assessment and adaptive control design. Most often, wideband measurement techniques have been applied to obtain the frequency response from a specific converter or a subsystem required for the analysis. In the methods, a broadband excitation such as a pseudo-random binary sequence (PRBS) is used as an external injection, and Fourier techniques are applied to extract the spectral information. This paper presents implementation techniques of the wideband methods using power-hardware-in-the-loop measurements based on OPAL-RT real-time simulator. The presented methods make it possible to modify the system characteristics, such as impedance behavior, in real time, thereby providing means for various stability and control design tools for on-board power distribution systems. Experimental measurements are shown from a high-power energy distribution system recently developed at DNV GL, Arnhem, Netherlands. acceptedVersion

The present energy system is facing various challenges in the near future due to changing demand and productions structures. Novel ideas are required to address the control of large number of the flexible resources. The paper aims to introduce integrated business platform of distributed energy resources in Finland. The platform is used to exchange information between laboratories, pilots, and actual facilities of all market participants. The paper describes the bare minimum information exchange requirements of some use cases for the integrated business platform to enable novel business opportunities for monitoring, verification, and application of flexibility services of microgrids and energy communities for the purposes of transmission and distribution grid management. The proposed information exchange platform is based on definition of use cases for the smart grid architecture model. publishedVersion

In the control of electric drives, inaccurate estimation of the motor parameters affects the robustness of the control. This effect is observed particularly when the linear lumped parameter-based models are employed to represent a machine composed of nonlinear magnetic materials. Finite-element (FE) models consider the material nonlinearity accurately. However, implementing these models in a real-time system is challenging due to the computational burden. In this article, we propose a fast-dynamic model, based on a model order reduction method, to control a permanent magnet synchronous machine in a wide range of speed. The stator currents in the rotor frame of reference are given as inputs to a reduced FE model, which computes the nodal values of the magnetic vector potential and thereafter the flux linkages. A discrete-time model is used to control the rotor speed and the stator current components. Experiments on a 2.2 kW interior permanent magnet synchronous machine verify the viability of the proposed model. acceptedVersion

Modern electric distribution systems typically contain several feedback-controlled parallel inverters that together form a complex power distribution system. Consequently, a number of issues related to stability arise due to interactions among multiple inverter subsystems. Recent studies have presented methods where the stability and other dynamic characteristics of a paralleled inverter system can be effectively analyzed using impedance measurements. This paper presents implementation techniques for a comprehensive online stability analysis of grid-connected paralleled inverters using power hardware-in-the-loop measurements based on an OPAL-RT real-time simulator. The analysis is based on simultaneous online measurements of current control loop gains of the inverters and the grid impedance, and aggregated terminal admittance measurements of the inverters. The analysis includes the measurement of the inverters’ aggregated output impedance, inverters’ loop gains, global minor loop gain, and grid impedance. The presented methods make it possible to rapidly evaluate the system on both global and local level in real time, thereby providing means for online stability monitoring or adaptive control of such systems. Experimental measurements are shown from a high-power energy distribution system recently developed at DNV GL, Arnhem, Netherlands. publishedVersion

Photovoltaic (PV) systems are prone to irradiance variation caused by cloud shadows leading to fluctuations in generated power. Since these fluctuations can be harmful to the operation of power grids, there is a need to restrict the largest PV power ramp rates (RR). This article proposes a method to estimate the largest expected PV power RRs. The only inputs of the method are the minimum PV system dimension and the measurements of point irradiance and cloud shadow velocity. Since cloud shadows cause the largest power RRs for well-designed large-scale PV power plants, the relation between the largest RRs in irradiance and power during partial cloud shading events was studied based on irradiance measurements. The largest RRs in PV power are estimated from RRs in the average irradiance across the PV system. The proposed method was validated using measured data of 57 days from two PV systems. It showed superior performance compared to an existing method enveloping the RR in the measured power over 99.99% of the time. The method can be used in design and component sizing of PV power plants. publishedVersion

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

Partly cloudy days possess two characteristics that can significantly increase the photovoltaic (PV) generator power: the operating temperature of the PV panels can cool down during the shade periods, and the irradiance can be enhanced due to the cloud enhancement phenomenon. If an overirradiance event is preceded by a long shade period, the maximum power of a PV generator can occasionally be much higher than the nominal nameplate power. During the overpower events, the inverter is operating in power-limiting mode whereby the operating voltage is increased to decrease the power of the PV generator. We created a simulation model of a 31.9 kW PV generator and used 12 months of irradiance and PV panel temperature measurement data to analyze its operation. We analyzed the PV generator power during the overirradiance events and applied various static power limits to calculate the operating voltage ranges in case of power curtailment. During the observation period, the maximum power produced by the PV generator was 1.42 times its nominal power. The duration of the overpower events was up to several minutes, but the typical duration was only some tens of seconds. The strongest overpower events occur seldom and their duration is only some seconds. Due to the overpower events, the operating voltage may receive high values, especially if the DC-to-AC power ratio is large. publishedVersion

When identifying and comparing forecasting models, there may be a risk that poorly selected criteria could lead to wrong conclusions. Thus, it is important to know how sensitive the results are to the selection of criteria. This contribution aims to study the sensitivity of the identification and comparison results to the choice of criteria. It compares typically applied criteria for tuning and performance assessment of load forecasting methods with estimated costs caused by the forecasting errors. The focus is on short-term forecasting of the loads of energy systems. The estimated costs comprise electricity market costs and network costs. We estimate the electricity market costs by assuming that the forecasting errors cause balancing errors and consequently balancing costs to the market actors. The forecasting errors cause network costs by overloading network components thus increasing losses and reducing the component lifetime or alternatively increase operational margins to avoid those overloads. The lifetime loss of insulators, and thus also the components, is caused by heating according to the law of Arrhenius. We also study consumer costs. The results support the assumption that there is a need to develop and use additional and case-specific performance criteria for electricity load forecasting. publishedVersion

Grid characteristics have a drastic impact on the stability and control performance of grid-connected systems. Because the grid conditions typically vary over time, online measurements are most desirable for the stability assessment, protection design, and control-system optimization of the systems. Previous studies have presented methods based on Fourier techniques and broadband sequences with which the frequency responses of the grid-connected systems, such as the grid impedance or inverter control loops, can be measured. However, online measurements under unbalanced grids with harmonic voltages have not been comprehensively considered. The present paper demonstrates how the previously applied online-measurement methods fail in the presence of unbalanced grid and voltage harmonics due to the spectral leakage caused by Fourier transform. This work also proposes a simple signal-design method to avoid the leakage. Experimental results based on a high-power grid-connected system are shown to demonstrate the effectiveness of the proposed method. acceptedVersion

A large share of renewable energy production is connected to a weak grid with significant grid impedance. The transmission impedance causes unintended flow of reactive power to grid, coupling the grid reactive power to the active power fed from the inverter. The reactive power causes transmission losses, strains the grid with reactive power requirements, and can even compromise system stability. Requirements on reactive power support were recently imposed on new photovoltaic inverters, which are often implemented with proportional power factor control or droop control for local voltage regulation. The present work proposes a method for real-time compensation of the unintended reactive power, which decouples the reactive power from the active power of a photovoltaic inverter. Based on real-time measurement of the grid impedance, the unintended reactive power is estimated and autonomously compensated in the inverter. The method removes the fluctuating reactive power component, while still permitting unrestricted manual control of the reactive power. Unlike conventional methods, the proposed method requires no prior knowledge on grid impedance values or delicate tuning. The method outperforms conventional power factor control even when the conventional method is tuned optimally with known grid inductance. The method is validated with simulations and experiments on three-phase photovoltaic inverter connected to a weak grid. publishedVersion

acceptedVersion

Online measurements of the battery impedance provide valuable information on the battery state-of-charge and state-of-health, which can be utilized for improving the safety and the performance of the associated system. The electrochemical-impedance spectroscopy (EIS) is widely used for battery impedance measurements, but it is not the most applicable solution for online measurements due to its slowness and complexity. These drawbacks can be improved using broadband signals, such as pseudorandom sequences (PRS), which are fast and easily implementable. However, the nonlinear behavior of batteries have a significant effect on the impedance measurements and the selection of the PRS signal. Majority of the PRS signals are applicable for measurements of linear systems, but also signals for nonlinear system identification do exist. Moreover, the reduced accuracy and signal-to-noise ratio of the PRS signals compared to the EIS make the filtering of the results as well as the amplitude design important aspects. This paper demonstrates the use of two PRS signals, the pseudorandom binary sequence (PRBS), and a ternary sequence with better toleration to battery nonlinear effects, with comprehensive amplitude and filtering design for battery impedance measurements. It is shown that the ternary sequence provides accurate measurements and the effects of nonlinear dynamics of the battery impedance are reduced with respect to the PRBS measurements. The results are referenced and validated to practical EIS measurements in various operating conditions for lithium-iron-phosphate (LiFePO 4) cell.

The cloud enhancement (CE) of solar irradiance is a well-known phenomenon, but its effects on PV power plants are not thoroughly understood. Because of scalability, the diameters of PV generators can vary from some meters up to several hundreds of meters. The output power of a PV generator depends mainly on the irradiance to the PV panels. Therefore, if the irradiance is enhanced, the actual output power might exceed the nominal power. We created a method to estimate the average irradiance over typical land areas of PV generators. The average irradiance over the PV generator land area was used to calculate the frequency, duration and average irradiance of the CE events. The analysis was based on actual irradiance measurement data from an array of pyranometers on an area of 1400 m2, which corresponds to the land area of 0.1 MW PV generator. We found out that the focus areas of the CE events have diameters of the order of tens of meters. Therefore, up to 0.1 MW power range, the land area of the PV generator does not greatly affect the maximum average irradiances. In this power range, the average irradiance was measured to be up to 1.5 times the clear-sky irradiance. The CE events affecting utility-scale PV generators were estimated to have maximum average irradiances of 1.4 times the clear-sky irradiance, but the number of these CE events is considerably lower. publishedVersion

Currently, photovoltaic (PV) installations target a maximization of annual energy yield. However, as the grid penetration of PV is increasing, PV electricity generation will need to match better with local load profiles. Especially the seasonal variabilities remain challenging. While wind and PV tend to have complementary seasonal variability, wind turbine installation faces limitations especially in densely populated areas. In this paper, we discuss how this challenge may be addressed with climateand consumption-specific PV module technology. In particular, we demonstrate how the temperature coefficient of a PV system can impact the energy yield throughout the year. In colder climates, higher temperature coefficients allow for a better energy balance, favoring production in colder seasons without a significant reduction of yearly energy yield. Simulations for locations at high latitude, and colder climates, indicate that higher temperature coefficients and improved low-light behavior not only enable a higher energy yield in cold seasons, but also negligible losses in the overall yearly energy yield compared to lower temperature coefficients and slightly better low-light behavior. Simulations show that these results can be obtained using commercial PV modules. More broadly, they indicate how PV module technology may be optimized depending on the location and climate. (C) 2020 Elsevier Ltd. All rights reserved. This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No. 751159. The work in this paper was partially funded by the Kuwait Foundation for the Advancement of Sciences under project number CN18-15 EE-01. Govaerts, J (corresponding author), IMEC, Kapeldreef 75, Leuven, Belgium. Jonathan.Govaerts@imec.be

The grid-connection point of photovoltaic inverters may exhibit inductive characteristics (i.e., a weak grid) due to long transmission cables as well as multiple transformers. A large grid impedance can arouse impedance-based stability problems, sustained resonances and impose power-delivery limits. This paper discusses an impedance-compensated synchronous reference- frame-PLL-angle-correction method. The method presented in this paper compensates the PLL angle according to estimated or measured grid impedance in order to achieve improved performance in weak grids. Due to the lack of explicit small-signal impedance models around the aforementioned topic, this paper presents a complete small-signal transfer function model to analyze the impedance-based stability of the PLL-angle compensation method. The results are validated with real-time hardware-in-the-loop simulations and laboratory experiments. 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.

This paper presents a long-horizon direct model predictive control for a series-connected modular rectifier. The topology combines a diode rectifier and an active-front-end (AFE) converter to achieve a modular dc railway power supply. Two formulations of the optimization problem, i.e., power and current control, are investigated. The current control problem-solved with the sphere decoder for reduced computational effort-is compared with the power control problem-solved with exhaustive enumeration-in terms of current distortions and distribution of the harmonic spectrum. The latter have to meet strict grid standards, such as IEEE 519 and IEC 61000-2-4 standards. As shown, thanks to the long prediction horizon the total demand distortion of the converter current can be reduced, while keeping the device switching frequency low due to the medium voltage target. acceptedVersion

Photovoltaic (PV) systems are prone to deep, steep and frequent irradiance fluctuations, mainly originated from overpassing cloud shadows, which cause fluctuations in PV power production. These irradiance transitions have been modelled by using a mathematical function to study their behaviour in a systematic way. Although the used methods and obtained results seem to be reliable, the simulation model has not been verified in detail. In this paper, the accuracy of the used theoretical model for irradiance transitions has been verified experimentally. The results show that the simulation model is accurate enough to study the irradiance transitions caused by moving clouds and their effects on the operation of PV systems. acceptedVersion

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.

Due to relatively good performance and simple implementation, fixed-step direct maximum power point tracking techniques such as perturb & observe and incremental conductance are the most popular algorithms aimed to maximize the energy yield of photovoltaic energy conversion systems. In order to optimize maximum power point tracking process performance, two design parameters – perturbation frequency and perturbation step size – need to be set a priori, taking into account the properties of both interfacing power converter and photovoltaic generator. While perturbation frequency is limited by the combined energy conversion system settling time, perturbation step size must be high enough to differentiate system response from that caused by irradiation variation. Recent studies have provided explicit design guidelines for single-loop maximum power point tracking structures only, where the algorithm directly sets the interfacing converter duty cycle. It was shown that dynamic resistance of the photovoltaic generator, which is both operation point and environmental conditions dependent, significantly affects the combined energy conversion system settling time. On the other hand, no design guidelines were explicitly given for multi-loop maximum power point tracking structures, where the algorithm sets the reference signal for photovoltaic generator voltage and inner voltage controller performs the regulation task. This paper introduces perturbation frequency and perturbation step size design guidelines for such systems. It is shown that while perturbation step size design is similar to that of single-loop structures, perturbation frequency design is quite different. It is revealed that once the inner voltage loop is properly closed, the influence of photovoltaic generator dynamic resistance on settling time (and thus on perturbation frequency design) is negligible. Experimental results are provided to verify the proposed guidelines validity. acceptedVersion

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

In this paper, a coupled magnetoelastic model for isotropic ferromagnetic materials is presented. The constitutive equations are written on the basis of the total energy in which the right Cauchy-Green strain tensor and the Lagrangian form of the magnetic field strength are used as the basic state variables. It is also applied to ferromagnetic electric steel for which the material parameters are calibrated. publishedVersion

In order to optimize the performance of direct (or perturbative) fixed-step maximum power point tracking algorithms (e.g., perturb and observe and incremental conductance), two design parameters-perturbation frequency and step size-must be selected. The main requirement for perturbation frequency design is ensuring the period between two successive perturbations is longer than settling time of photovoltaic generator power transient. According to existing design guidelines, perturbation frequency should be selected at maximum power point, corresponding to standard test conditions. However, due to finite resolution of digital controllers, maximum power region rather than single maximum power point exists in practice. Therefore, operating point can arbitrarily reside within this region, belonging either to constant-current or constant-voltage I-V curve parts. It is shown that the photovoltaic generator power transient settling process is significantly slower in constant current than maximum power region due to increased value of dynamic resistance. Consequently, perturbation frequency design should be carried out in constant-current region rather than at maximum power point. Short-circuit condition should be selected as worst-case design operation point, where photovoltaic generator dynamic resistance obtains highest value. Then, perturbation frequency design becomes photovoltaic generator independent, influenced only by interfacing converter component values. Experimental results validate presented findings successfully. 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.

Photovoltaic (PV) systems are prone to irradiance fluctuations caused by overpassing cloud shadows which can be very large and steep. Cloud shadows have an average diameter of almost 1 km meaning that even the largest PV power plants are widely affected by them. Fast irradiance transitions can lead to failures in maximum power point tracking and to mismatch power losses due to partial shading of the PV generator. In this study, the effects of irradiance transition characteristics: shading strength, duration and apparent speed and direction of movement on the mismatch losses of PV generators were studied by simulations using a mathematical model of irradiance transitions and an experimentally verified MATLAB Simulink model of a PV module. The studied electrical PV array configurations were series–parallel, total-cross-tied and multi-string. Furthermore, three different physical shapes of the configurations were studied. On the basis of the results, module strings of PV arrays should be placed perpendicularly to the dominant apparent direction of movement of shadow edges and the diameter of the strings should be minimised to decrease the mismatch losses. Another finding of practical importance was that there were only minor differences between the mismatch losses of different electrical PV array configurations. acceptedVersion

As the amount of renewable energy increases rapidly, power systems will face novel challenges to maintain power quality. Power quality issues, which usually arise from the grid connection through power-electronic devices, can be reduced by proper control strategies. Grid-connected devices are conventionally optimized into a single operating point, but real performance is affected by the interfaced grid conditions. As the grid conditions change over time, the control performance varies, as does the quality of the power produced. To tackle this issue, the adaptive grid-voltage feedforward is introduced to the control system. The adaptive method, applying a perturb-and-observe algorithm, improves the produced power quality in real time by taking the control system characteristics and the grid conditions into account. The results show the improved power quality in various grid conditions when comparing the adaptive grid-voltage feedforward and the conventional implementations. acceptedVersion

The Siwakoti-H flying-capacitor inverter (sFCI) is a potential candidate for photovoltaic applications, specifically for the transformerless grid-connected systems. One of the main challenges in the control of a sFCI is to maintain the flying capacitor voltage within prescribed limits while balancing the voltages on the three flying capacitors. This paper proposes an indirect model predictive control strategy for a three-phase sFCI connected to the grid via an LCL-filter. By linearizing the system model, the nonlinearities introduced due to the dynamics of the flying capacitor are neglected. Moreover, by not directly controlling the switches, but rather manipulating the modulating signal, the optimization problem can be formulated as a quadratic program (QP) and solved in a computationally efficient manner. The explicit solution computed by the controller makes the realtime implementation feasible by employing a carrier-based pulse width modulator (CB-PWM). The presented results illustrate the steady-state and dynamic performance of the controller. acceptedVersion

The impedance of a Li-ion battery is an important parameter for the battery's state-of-charge (SOC) and state-of-health (SOH) estimation. Battery impedance is typically modeled by an equivalent-circuit-model (ECM) in which the variations in the specific model parameters can be used for estimating the SOC and the SOH. However, the fact that the battery impedance is highly non-linear complicates the parameterization of the model. The model is traditionally obtained by a complex non-linear least-squares fitting algorithm which comes with high complexity. This paper proposes a novel approach to extract all the ECM parameters of the battery impedance obtained with online-capable pseudo-random-sequence (PRS) measurements. Although the algorithm has low complexity, it still captures the desired variations in the ECM parameters as a function of SOC. The algorithm is validated for the impedance data from a lithium-iron-phosphate cell. acceptedVersion

This paper presents a direct model predictive control (MPC) scheme for a three-phase two-level grid-connected converter with an LCL filter. Despite the absence of a modulator, a fixed switching frequency and a discrete current harmonic spectrum are achieved. With a refined objective function of the optimization problem underlying MPC, grid current distortions comparable to those of carrier-based pulse width modulation (CB-PWM) in idealized conditions are produced. Moreover, the grid current harmonics meet the stringent limits imposed by grid standards. A performance evaluation is conducted to highlight the potential benefits of the proposed method. acceptedVersion

Recently, a computational issue of sphere decoding algorithm (SDA) during transient operation of multistep model predictive control has been addressed in [1] and achieved its real-time implementation in [2] for a medium-voltage electrical drive system. This is achieved by projecting the unconstrained solution onto the convex-hull of the finite control set during transient operation. Therefore, a new initial sphere that guarantees feasibility and includes a significant smaller number of candidate solutions is obtained. This reduces the computation time required to solve the optimization problem. However, the reduction of the computational burden comes at the expense of (mild) suboptimal results [3]. This paper analyses the possibility to obtain a suboptimal solution by the SDA based optimization during transient operation. To deal with this suboptimality issue, this work explores the possibility to enlarge the convex-hull, whose size is by definition tied to the original finite control set. Therefore, in this work, the convex-hull is treated as a SDA initialization parameter during transient operation. As will be demonstrated, enlarging the convex-hull size reduces the possibility to obtain a suboptimal solution during the transient operation retaining, thus, the optimality during the whole converter operation. acceptedVersion

The formation of local energy systems (LES) is at a very early stage but their number is expected to increase rapidly. This paper aims to investigate the characteristics of the stakeholders and how their characteristics influence the activities they are engaged in regarding the LES emergence process. This way, our study lays the foundation for further analysis of the causal patterns in LES emergence. The recognition of the entity characteristics is done through a case study of an industrial microgrid. The case study shows how LES emergence is an interplay of stakeholders from different levels with different guiding institutional logics. Institutional complexity becomes apparent in strategic decisions of the municipality and its utility and their negotiations of the microgrid borders with the local DSO. acceptedVersion

The necessary balance between power consumption and production is at risk due to the diminishing amount of controllable resources in the grid. Demand response has the ability to provide the needed ancillary services for the power system management and create a new stream of revenue for the flexible resource owner. The increasing amount of electric vehicles contain a huge balancing power potential due to their ability to store the electrical energy flexibly from the grid in times of long term parking. Sub-aggregator is an actor who possesses capabilities to monitor and control distributed energy resources. In the case of electric vehicles, Sub-aggregator plans and coordinates the smart charging of vehicles to create additional value for all participants. This paper presents a business model for aggregating distributed energy resources in a cost-efficient manner that is based on minimum investments needed. acceptedVersion

Large scale Demand Response (DR) has different impacts on different market actors. The impacts depend significantly on the market actor and the operational environment. The impacts of DR on electricity retailers (ER) in the North-European electricity market are discussed in this paper. DR is a double-edged sword for an ER. On one hand, DR offers a potential tool to improve the operation in the electricity market, but on the other hand, DR poses also risks to the ER. The impacts of DR on ERs depend on many things like the nature of the controllable demand, the characteristics and the rules of the market place in which DR is being operated and the structure and the nature of the electricity contracts made with the customers. These things are discussed in depth in this paper. A case study is presented related to DR based on day-ahead area prices in Finland. The case study shows that on average, the imbalance costs induced by DR to the retailer can even be positive (positive cash flow), but the risk of significant additional costs remain and might even be increasing. acceptedVersion

We focus on exploring the opportunities related to blockchain in regard to three different types of energy communities i.e., industrial microgrid, apartment building and virtual community, as seen by a group of Finnish energy sector experts. To address the research problem, we first summarize results from literature for blockchain as an enabler for energy communities, discuss the technical aspects of blockchain and summarize the status of ongoing pilots and demonstrations around blockchain. Our results from empirical study among energy industry experts show that there clearly is potential for blockchain as an enabler especially of energy efficient use of resources, trading without intermediaries, load balancing, new business, automated functions and long-range in system valuable resources. Despite this potential, multiple gaps exist and future research on blockchain is still called for. acceptedVersion

This study presents a method to solve the dynamic model of a grid-connected photovoltaic (PV) inverter with battery energy storage. A three-phase grid-connected solar-battery system is studied using small-signal state-space modelling in the synchronously rotating dq-reference frame. The combined effect of a PV generator and a bi-directional dc–dc battery charger (BC) on the inverter dynamic behaviour is analysed. It is shown that the characteristic right-half-plane pole in the PV inverter's dc-link voltage control dynamics shifts to a lower frequency when the BC is connected, which improves the stability of the system and allows a lower bandwidth for the dc-link voltage control. publishedVersion

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

Oversizing the photovoltaic (PV) generator improves the profitability of PV power plants, but a downside is energy losses due to power clipping. If an energy storage system (ESS) is needed, connecting it to the DC side enables the utilization of the clipped energy. We compared the AC and DC bus interconnections of ESS in a typical building block of megawatt-scale PV power plants. The DC bus interconnection increases the energy yield, but also the utilization of the ESS. The operation of the ESS is affected by the production profile of the PV generator, ESS energy capacity and DC-to-AC power ratio. 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

A new model predictive torque and flux controller is proposed, which controls the electromagnetic torque and the rotor (rather than the stator) flux magnitude. Analytical expressions for the weighting factors are derived that ensure that the proposed controller achieves the same closed-loop performance as predictive current control. In particular, the same low current and torque distortions result, without requiring an outer field-oriented control loop. 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.