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We experimentally study a piezoelectric energy harvester driven by broadband random vibrations. We show that a linear model, consisting of an underdamped Langevin equation for the dynamics of the tip mass, electromechanically coupled with a capacitor and a load resistor, can accurately describe the experimental data. In particular, the theoretical model allows us to define fluctuating currents and to study the stochastic thermodynamics of the system, with focus on the distribution of the extracted work over different time intervals. Our analytical and numerical analysis of the linear model is successfully compared to the experiments, 11 pages, 10 figures

This paper is focused on energy harvesting from railcar suspensions. In particular, the power extracted from a train suspension energy harvester based on a mechanical motion rectifier system is evaluated in presence of a passive AC/DC converter. Through a proper theoretical analysis, experimentally validated, it is shown that the extracted power is strictly dependent on the DC side voltage and that the value of the DC voltage at the output of the diode bridge rectifier maximizing the average extracted power is proportional to the generator speed. The above analysis is essential for the proper design of maximum power point tracking techniques.

In this paper, a novel maximum power point tracking (MPPT) technique, which has been named T4S (a technique based on the proper setting of the sign of the slope of the photovoltaic voltage reference signal), is presented and discussed. It is specifically designed with reference to a single-stage grid-connected PV system. Its performance is numerically compared with that of the well-known and widely used perturb and observe (P&O) MPPT technique. The results of the numerical simulations confirm the validity of the proposed MPPT technique which exhibited a slightly better performance, under stationary and also time-varying irradiance conditions. In addition, the T4S technique is characterized by the following features: it does not require explicit power detection or calculation and, moreover, it allows the tracking of the maximum average power injected into the grid rather than the tracking of the maximum instantaneous power extracted by the PV source.

This paper is focused on resonant Piezoelectric Vibration Energy Harvesters (PVEHs). In all energy harvesting applications, and in particular in PVEH applications, the maximization of the extracted power is mandatory. It is well known that, in order to maximize the harvested power, it is necessary to get the optimal matching between the harvesting device and the load. In particular, in PVEH applications under purely sinusoidal input vibrations, in order to maximize the harvested power, the impedance of the load must be coincident with a well-defined optimal impedance ZOPT. However, in practical applications, the load of PVEHs is not a linear impedance, but it is nearly always represented by a non-linear AC/DC converter. In this paper, the full-bridge based active AC/DC architecture, that is theoretically able to emulate just the optimal impedance ZOPT, is analyzed in detail. In particular, a Two-Dimensional Perturb and Observe (TD-P&O) algorithm is proposed and discussed for the emulation of such an optimal impedance. Simulation results confirming the validity of the proposed TD-P&O algorithm and the increase of the extracted power with respect to the simpler case of a diode bridge rectifier source/load electronic interface are presented and discussed.

The prediction of the extractable power from energy harvesting devices is of utmost interest for system designers. In this paper, this issue is addressed in the case of Piezoelectric Resonant Vibration Energy Harvesters (PRVEHs) driven by amplitude and frequency modulated vibrations. The widespread architecture of PRVEH systems made up of a passive rectifier cascaded by a DC/DC converter is here considered. An analytical expression of the harvested power as a function of the voltage at the output of the passive rectifier is derived for the cases of vibrations with amplitude modulation and frequency modulation. Further, an analytical-numerical formula is presented for PRVEHs fed by non-sinusoidal vibrations exhibiting envelope and frequency with slow variations. Numerical simulations confirm the theoretical results.

In this paper, an equivalent electric model for railway vehicle suspension harvester is proposed. With reference to the maximization of the extracted power, the open issues for such kinds of systems are reviewed. In particular, it is evidenced that the use of passive diode bridge rectifiers unavoidably leads to a reduction of power with respect to both the resistive load case and the theoretical optimal load case. Therefore, suitable average power maximization techniques and power electronics architectures are needed. In this paper, two techniques are proposed. The first technique is based on the adoption of additional passive components and, in the considered test cases, it allows the increase of the power output up to 45%. The second technique is based on the adoption of a properly controlled active power electronic interface and it is much more efficient since, in the considered test cases, it allows the increase of the power output up to 135%.

In this paper, the problem of the Maximum Power Extraction is discussed with reference to Resonant Electromagnetic Vibration Energy Harvesters driven by non-sinusoidal vibrations. It is well known that, in order to maximize the harvested power in case of purely sinusoidal vibrations, the Maximum Power Transfer Theorem must be exploited. Instead, in case of non-sinusoidal vibrations, the Nonlinear Dynamic Maximum Power Transfer Theorem must be exploited. An active power electronics AC/DC architecture is considered and two different control algorithms are proposed and discussed. They are respectively used for the implementation of the Maximum Power Transfer Theorem and of the Nonlinear Dynamic Maximum Power Transfer Theorem. The differences existing between the two proposed controls techniques in terms of performances and implementation complexity are discussed and analyzed with the help of suitable numerical simulations.

In this paper, the Ideal Utilization Factor (IUF) is presented and discussed with reference to Resonant Electromagnetic Vibration Energy Harvesters (REVEHs). The IUF is an important figure of merit that allows the identification suitable additional non-dissipative components to insert between the REVEH coil and the bridge rectifier. Such components allow improving the actual energetic performances of REVEHs in the frequency regions of practical interest.

In this paper it is shown that Resonant Electromagnetic Vibration Energy Harvester (REVEH) systems nearly always need to be equipped with suitable MPPT (Maximum average Power Point Tracking) controllers due to the fact that the characteristics of vibrations (frequency, amplitude etc.) change with time. The particular (but frequent) case of Constant Frequency Time Varying Amplitude (CFTVA) vibrations will be considered. In particular, the guidelines for the optimization of the P&O MPPT technique to adopt in REVEH systems in the case of CFTVA vibrations are provided. It is demonstrated that both the sampling frequency íp&o and the amplitude Ad of the P&O duty cycle perturbation need to fulfill suitable inequalities in order to get an efficient REVEH system.

An essential objective for many photovoltaic (PV) applications is represented by the so‐called maximum power point tracking (MPPT). Under uniform irradiance and ambient temperature conditions, the power versus voltage characteristic of a PV array is characterized by the presence of a unique point, termed maximum power point (MPP), where the power provided by the array itself attains its maximum value. Any other operating point leads to the extraction of a smaller PV energy. Therefore, in order to avoid the waste of PV energy, the working of the array in its MPP is mandatory. Since the position of the MPP continuously changes in time as a function of irradiance and temperature, it is necessary to continuously track it. Such an objective can be obtained by using proper MPPT control techniques. A large number of papers have appeared in the scientific literature during the last 15 years on the issue of MPPT, and as a result it may be difficult to decide the best technique for a given application. Therefore, this survey may hopefully be beneficial to students, researchers, and practitioners in PV systems. The article overviews the representative PV MPPT algorithms, organized into four main groups. The first group contains the so‐called “hill climbing” techniques, the second group contains the “curve fitting” techniques, the third group contains the “dynamical” techniques, and the fourth group includes all the techniques that do not belong to the first three groups.

In this paper an important figure of merit is introduced and discussed with reference to electromagnetic vibration energy harvesters which are loaded by diode bridge rectifiers. It is called Harvester Ideal Utilization Factor. It precisely quantifies how close to the optimum the harvester is loaded. Such a figure of merit can be defined not only for electromagnetic but also for piezoelectric harvesters as long as the assumptions which allow the modeling of such vibration harvesters by means of an equivalent linear electric circuit are valid. © 2016 IEEE.

In this paper, the different aging mechanisms taking place in photovoltaic modules are discussed, and the cause-effect links, which exist among such mechanisms, are evidenced. It is also shown that a closed-loop link exists between aging and mismatching since aging (which is nonuniform by its nature) causes mismatching among cells, whereas mismatching, in turn, mainly due to its thermal effects, leads to nonuniform aging. © 2015 IEEE.

Isothermal calorimetric titrations of aqueous solutions of poly(ethylene glycol) (PEG) with sodium dodecyl sulfate (SDS) are known to exhibit a peculiar trend consisting of endothermic and exothermic effects. This behavior was explained with the formation of two different mixed micellar aggregates, one characterized by hydrophobic interactions and the second by ion−dipole association. Present NMR measurements on 13C, 1H, and 23Na nuclei do not support the formation of a number of PEG−SDS aggregates characterized by interactions of different nature. Our data are rather in accordance with the initial formation, at low surfactant concentration, of a polymer−surfactant aggregate in which the polymeric chain assumes a strained conformation in order to bind a small micellar cluster. The subsequent growing of the aggregate with increasing surfactant concentration allows the polymer to relax to a more expanded, energetically favored, conformation. Further calorimetric titrations with a set of PEG samples of diffe...

In PV applications, under mismatching conditions, it is necessary to adopt a maximum power point tracking (MPPT) technique which is able to regulate not only the voltages of the PV modules of the array but also the DC input voltage of the inverter. Such a technique can be considered a hybrid MPPT (HMPPT) technique since it is neither only distributed on the PV modules of the PV array or only centralized at the input of the inverter. In this paper a new HMPPT technique is presented and discussed. Its main advantages are the high MPPT efficiency and the high speed of tracking which are obtained by means of a fast estimate of the optimal values of PV modules voltages and of the input inverter voltage. The new HMPPT technique is compared with simple HMPPT techniques based on the scan of the power versus voltage inverter input characteristic. The theoretical analysis and the results of numerical simulations are widely discussed. Moreover, a laboratory test system, equipped with PV emulators, has been realized and used in order to experimentally validate the proposed technique. © 2014 Gianluca Aurilio et al.

Ultrawide-band detection of partial discharges (PDs) mainly aims at recording the true shape of a PD current signal. It is thus possible to achieve more useful information than that provided by other detection techniques about sources and causes of both PD current signals and the physical processes taking place. However, external interference, reflections, oscillations and stray elements in the measuring circuit may distort the signal being recorded; hence, the information extracted from it might sometimes be meaningless. In the paper, a digital signal-processing method for reliable analysis of ultrawide-band detected PD current signals is proposed. The method, exploiting the multiresolution approach peculiar to the Wavelet Transform, is capable of setting the detected signal free from most disturbances affecting its true shape, thus ensuring the significance of the measurements carried out on the signal itself. After a brief remark about the main features of the Wavelet Transform and its multiresolution approach, the proposed method is described in detail. Experimental results obtained on actual PD current signals are then given in order to highlight the method’s reliability and effectiveness.

In this paper, the main factors limiting the efficiency of Distributed Maximum Power Point Tracking (DMPPT) in PV applications are analyzed. It is shown why, in a PV system adopting DMPPT and operating under mismatching conditions, it is not always possible to obtain the working of each PV module in its own MPP. Index Terms—PV systems, mismatching, DMPPT.

The present paper is dedicated to the evaluation of the influence played by important design parameters like the interturn insulation thickness and the insulating material permittivity on the voltage distribution among the coils of a stator winding fed by a PWM inverter. The system, composed of a feeder cable and a stator winding is modeled and simulated by using multiconductor transmission line theory. A recently developed time-domain equivalent circuit is used to represent the multiconductor line, with losses possibly dependent on frequency, which allows to correctly describe the dielectric losses and the skin-effect in the conductors. The obtained results can offer a broad guideline to design engineers for a suitable dimensioning of the motor insulation.

The clinical use of doxorubicin (DXR) is limited by cardiotoxicity partially due to interference with intracellular Ca(2+) homeostasis and involving the activation of the sarcoplasmic reticulum (SR) Ca(2+) release channels. It is known that docosahexaenoic acid (DHA) is able to potentiate the sensitivity of cancer cells to DXR. The aim of our study was to further evaluate the effects of DHA on [Ca(2+)](i) overload induced by DXR in adult rat ventricular cardiomyocytes in order to verify if DHA interferes with DXR-induced cardiotoxicity too. [Ca(2+)](i) was measured by microfluorimetry. Our data demonstrated that 100 microM DXR induced a statistically significant [Ca(2+)](i)-increase in cardiomyocytes perfused with CaCl(2) Krebs solution (from 135.7 +/- 15 nM to 560.2 +/- 49 nM, n = 9, p < 0.01) and with Ca(2+)-free Krebs solution (from 89.3 +/- 15 nM to 551.1 +/- 35 nM, n = 9, p < 0.01). Treatment with 10 microM DHA for 20 min significantly suppressed DXR [Ca(2+)](i)- increase in cells perfused with CaCl(2) Krebs solution (142.3 +/- 12 nM, n = 9, p < 0.01) and in Ca(2+)-free procedures (100.4 +/- 12 nM, n = 9, p < 0.01). Caffeine 10 mM significantly increased [Ca(2+)](i) in cardiomyocytes perfused with CaCl(2) Krebs solution (from 135.7 +/- 15 nM to 979.2 +/- 17.8 nM, n = 9, p < 0.01) and with Ca(2+)-free Krebs solution (from 89.3 +/- 15 nM to 891.1 +/- 30 nM, n = 9, p < 0.01). Treatment with 10 microM DHA for 20 min suppressed caffeine [Ca(2+)](i)-increase in cardiomyocytes perfused with CaCl(2) Krebs solution (174.2 +/- 28 nM, n = 9, p < 0.01) and in Ca(2+)-free procedures (161.9 +/- 34 nM, n = 9, p < 0.01). In conclusion, our results suggest that DHA is able to prevent acute modifications of calcium homeostasis induced by DXR probably interfering with SR Ca(2+) release channels.

The numerical evaluation of the electrical stress in the line-end coil of the stator winding of a medium voltage motor fed by a pulsed width modulated (PWM) inverter seems to be indispensable for a rational design of the machine. In order to fulfil such a task, the system, composed of a feeder cable and a stator winding, is modelled and simulated by using multi-conductor transmission line theory. The model can take into account the main phenomena occurring along the lines, i.e. the propagation and the reflection, together with the time dispersion introduced by the losses, eventually dependent on the frequency. The multi-conductor transmission line is solved in the time domain by adopting a technique based on the perturbation theory of the spectrum of symmetric matrices, which sensibly decreases the computational effort with respect to the analysis in the frequency domain. Furthermore, an accurate calculation of the characteristic matrices, which contain the cross-sectional information of the line, is performed by means of a FEM package, so taking into account the effective field distribution in the region of interest. The influence of the accurate evaluation of the capacitance and inductance matrices is considered by comparing the numerical results of the proposed model with those obtained by a simple equivalent circuit, frequently adopted in the literature. In order to validate the proposed model, the simulated results are compared with experimental data.

The numerical evaluation of the electrical stress in the line-end coil of the stator winding of a medium voltage motor fed by a PWM inverter seems to be indispensable for a rational design of the machine. In order to fulfill such a task, many computer models have been developed, in which the system is generally modeled by connecting an ideal ramp generator (the inverter) with a single transmission line (the cable connecting the inverter to the motor) in series with a Multiconductor Transmission Line (MTL) (the motor winding). Whichever technique is used for solving the MTL equations, one of the main problems consists in the determination of the per-unit-length electrical parameters of the winding, so that simplified methods are often used for their calculation. In the present paper the calculation of capacitance (C) and inductance (L) matrices is performed by means of a Finite Element Method (FEM) package, so taking into account the effective field distribution in the region of interest. We will show how the use of the simplified methods can result in a wrong prediction of the stresses in the insulation. In order to validate the proposed model, the simulated results are compared with experimental data.

Stress control tubes made out of composite materials characterized by nonlinear and anisotropic electric properties, are widely adopted for the realization of cable accessories, in order to achieve an efficient stress control. In this paper a numerical model, able to take into account both anisotropy and nonlinearity of stress grading composites, will be used to investigate on the effect of the different properties on the field distribution inside cable terminations.

Usually the electric and magnetic fields associated with lightning have been computed by assuming the lightning current to be contained in a straight vertical channel of negligible cross section above a flat perfectly conducting plane. Such a model, which does not take into account that real lightning is characterized by tortuosity and branching, is not able to justify the fine structure of the fields radiated by lightning discharges whose time-domain behavior exhibits a jagged shape with remarkable spectral content in several bands of practical interest. In this work the effect of channel tortuosity and branching is investigated by adopting a suitable numerical technique. The discharge channel has been regarded as a fractal antenna whose associated EM field has been evaluated by superimposing the contribution of the single line radiators composing the whole channel. Such a field has been compared with that generated by a simple dipole antenna in order to study the influence of the fractal nature of the channel on the generated EM fields. The relationship between the fractal dimension of the discharge channel and the fractal dimension of the generated time domain EM fields has been considered and the influence played on such a relationship by the distance between EM source and observation point has also been studied by analyzing the fields evaluated at far and close distances.

The insulated gate bipolar transistors (IGBT's) are a new class of semiconductor devices which, for their performances in terms of driving, switching behaviour, etc., are nowadays widely adopted in adjustable speed drives and are increasingly replacing traditional components like SCR and GTO thyristors. Large effort is dedicated in the literature in order to investigate on their long term behaviour, which limits their employment in high reliability demanding applications, like railway traction. In the present paper, in order to investigate on the long-term performances of the electrical insulation of such devices, an experimental activity has been carried out on two types of commercial IGBT modules produced by two different manufacturers. The IGBTs have been subjected to typical working cycles occurring in a traction converter and the effect of the application of such stresses has been studied by recording, at different ageing stages, the partial discharge activity occurring in each module. A characterization of the first stages of the ageing process has thus been achieved.

In order to study the field distribution in a suspension cap and pin glass insulator covered with an anti-corona layer a numerical procedure has been developed, which relies on the solution of the Laplace equation and the diffusion equation in a two dimensional domain by the Galerkin method. Numerical results concerning the case of applied sinusoidal power frequency voltage are presented and discussed allowing to gain a better understanding of the influence played by the presence of the non linear conducting coating on the electric field distribution.

With ultrawide-band detection of partial discharge (PD) transients the true shape of a PD current pulse can be recorded. It is so possible to achieve more useful information about both sources and causes of PDs and physical processes taking place. However, the recorded signal can be distorted due to external noise, reflections and oscillations, stray elements in the measuring circuit; as a consequence, the extracted information might be meaningless. In this paper a digital technique for a reliable analysis of such PD pulses is suggested. The technique, based on the Wavelet Transform, is able to set the detected pulse free from most of the disturbances affecting its true shape, thus assuring traceability in the measurements carried out on the pulse itself. After a brief remark on theoretical and implementation features of the Wavelet Transform, the proposed technique is described in detail. Experimental results obtained from the application of the proposed technique to actual PD pulses are also given.

The numerical evaluation of the electric field in a cable termination equipped with stress grading materials, is carried out by adopting a model based on the electro quasi-static approximation of Maxwell equations. In such a model, the different nonlinear electrical properties of the stress grading materials, obtained from experimental investigations, are taken into account. The influence on the electric field distribution of the different characteristics (resistivity vs. applied field) is investigated, in order to draw useful information on proper field control.

A new model for the evaluation of the electric field in a cable termination realized through a nonlinear stress control tube (SCT), is presented in this paper. It is based on the electro-quasistatic approximation of the Maxwell equations: the Laplace equation describes the field in the nonconducting regions whereas a diffusion-like equation gives the field dynamics in the stress control tube. A numerical model is devised by solving the Laplace equation by finite difference and diffusion equations by the Galerkin method. It is shown that even the well-known RC transmission line model can be derived from this general approach. The underlying approximations leading to the circuital model are discussed in detail. The proposed model, in contrast with the circuital one, allows us to take into account properly the nonlinear SCT characteristics and the actual boundary conditions: in this way both spatial and temporal effects of the nonlinearity are considered. The numerical results obtained by considering the general field approach and by using the transmission line model are compared.

An accurate knowledge of the potential and tangential electrical field distribution along the interface insulator-vacuum is required for determining the mechanism leading to the flashover and, practically, for the optimisation of the design of the insulator profile. In this paper a comparison has been carried out between results obtained by numerical methods (Finite Difference Method and Charge Simulation Method) and the basic support they are able to give for the insulator design is also presented. A cylindrical chamfered profile is studied: good accordance is found for the results of the two methods provided the choice of the mesh and fictitious charge distribution is optimised.Some experimental results are also presented which indicate the effectiveness of the adopted insulator profile.

Due to the recent trends in power-systems, steep-fronted switching surges are often encountered in many applications as, for example, in induction motors fed by pulse-width modulated (PWM) inverters. In such a case, the effect of these voltage transients can result in undesirable electrical and thermal stresses in the stator windings, which can lead to the premature failure of the insulating material between the coils and, definitely, to the failure of the whole component. Up to now, the dimensioning of the interturn insulation of such motors is based empirically on previous service experience; a more rational design can be performed if a more exact knowledge of the voltage distribution within the stator winding is reached. In the present paper the first results of a simulation carried out in order to predict the voltage distribution between the coils of a stator winding fed by a PWM inverter are presented. Such distributions depend on a very large number of parameters (insulation dimension, coils size, length of connecting cable, shape of the applied voltage, etc.). In the following the influence played on such distribution by the cable length and applied voltage rise-time is discussed.

The lifetime of resin insulated voltage transformers (RIVT) can be considerably shortened due to partial discharge activity taking place inside the insulation close to the active parts of the component. In that region the presence of voids, due to manufacturing processes, and the enhanced electric field stress lead to an intense partial discharge activity which determines accelerated degradation processes and eventually the ultimate failure of the device. Partial discharge measurements together with an estimation of the electric field distribution in RIVT seem to be indispensable in order to fully understand transformer performances. In this paper, a study of the electric field distribution inside the RIVT, together with results of partial discharge tests are presented. The influence of the various test arrangements and methods on the experimental results are investigated.

Stress control tubes (SCTs), made out of materials characterized by nonlinear electric properties, are widely adopted for the realization of cable terminations, in order to achieve a significant stress control. The influence played on the electric field distribution around the cable termination by SCTs parameters (e.g. length, thickness and position) has been studied only in the case of sinusoidal power frequency applied voltage, while no data can be found in the literature concerning applied standard impulse voltages. The purpose of this paper is to investigate on such interesting aspect by evaluating the potential distributions taking place under impulse voltages in cable terminations characterized by different arrangements of the SCT.