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A new probabilistic approach to transient stability evaluation of electrical power systems is proposed. The method is relevant to planning studies, where load level is a major source of uncertainty. The probabilistic nature of both the load demand and the circuit breaker operating time is taken into account for the purpose of obtaining an analytical evaluation of the instability probability. The approach is based upon the analytical computation of the probability distribution function of the critical clearing time for stability in terms of the load demand distribution. It allows the expression of the instability probability as an analytical function of all the relevant system parameters. The method is illustrated with reference to a simple (one machine - infinite bus) system, and the results are compared with those obtained by means of statistical (Monte Carlo) simulation. The time requirement of the two methods is comparable, but the proposed analytical approach offers a remarkable advantage over the Monte Carlo method for a sensitivity analysis with respect to the system parameters.

This article deals with the parameters’ measurement of interarea oscillations, which are low-frequency damped oscillations that affect the high-voltage transmission lines when an unbalance between the power demand and the generated power occurs. The authors propose a method that processes the samples provided by the phasor measurement unit (PMU) and performs the online estimation of the parameters that characterize the interarea oscillations. The method takes advantage of the speed of a heuristic algorithm, such as the particle swarm optimization (PSO), but performs an enhancement on the results to increase the reliability. The tests carried out for the method assessment prove that the proposed method is characterized by a fast response, high accuracy, and excellent reliability, regardless of the type of oscillation that is considered.

È proposta una strategia di controllo per la partecipazione flessibile dei collegamenti HVDC alla regolazione di frequenza basata sulla variazione adattativa della potenza sincronizzante e versatile rispetto alle condizioni di carico del sistema.

This paper deals with the problem of the optimal rating of mineral-oil-immersed transformers in large wind farms. The optimal rating is derived based on the probabilistic analyses of wind power generation through the Ornstein–Uhlenbeck stochastic process and on thermal model of the transformer through the integration of stochastic differential equations. These analyses allow the stochastic characterization of lifetime reduction of the transformer and then its optimal rating through a simple closed form. The numerical application highlights the effectiveness and easy applicability of the proposed methodology. The proposed methodology allows deriving the rating of transformers which better fits the specific peculiarities of wind power generation. Compared to the conventional approaches, the proposed method can better adapt the transformer size to the intermittence and variability of the power generated by wind farms, thus overcoming the often-recognized reduced lifetime.

The paper deals with design and control of a fault tolerant and reconfigurable photovoltaic converter integrating a Battery Energy Storage System as a standby backup energy resource. When a failure occurs, an appropriate control method makes the energy conversion system capable of operating in open-delta configuration in parallel with the grid as well as in islanded mode. In case network voltage is lacking due to heavy anomalies or maintenance reasons, the proposed control system is able to quickly disconnect the inverter from the grid while ensuring the energy continuity to the local load and the emergency fixtures by means of the integrated battery packs. In particular, the paper proposes a fast islanding detection method essential for the correct operation of the control system. This specific technique is based on the Hilbert transform of the voltage of the point of common coupling, and it identifies the utility lack in a period of time equal to half a grid cycle in the best case (i.e., 10 ms), thus resulting in good speed performance fully meeting the standard requirements. A thorough numerical investigation is carried out with reference to a representative case study in order to demonstrate the feasibility and the effectiveness of the proposed control strategy.

This paper focuses on very-short-term photovoltaic power forecasting based solely on power measurement. The approach proposed in the paper is tailored for real time operation of smart grids and microgrids, that deals with energy management and control of the various resources of the grid and, particularly, on those based on renewable energy sources. These resources are characterized by variable power production, thus constituting a challenge for the effectiveness of strategies and tools of management and control. This paper refers to photovoltaic power production and presents a new real time forecasting method which improves the classical persistence model. The proposed method is based on the idea of conveniently weighting information on past data by imposing continuity of the function, of its first derivative and of the second derivative so obtaining three estimates of the function in the forecast interval which are then opportunely weighted to provide the forecast. The effectiveness of the proposed approach is shown in the numerical application based on actual measured data. The accuracy of the proposed approach is also tested through comparisons with two models based on the persistence and auto regressive moving average models.

This paper deals with hybrid energy storage system (HESS) management strategies, optimized for urban road electric vehicle applications. These new control strategies aim to extend battery pack durability by reducing charging/discharging current peaks by means of supercapacitors. The optimization is carried out with reference to the case study of a HESS composed of a high power unit, i.e. supercapacitor module plus a high energy unit, i.e. a battery pack, based on nickel-chloride ZEBRA (ZEolite Battery Research Africa) technology. On-board integration of the two storage devices is obtained through a DC/DC bidirectional power converter, as this configuration is particularly convenient for many kinds of urban vehicle operations. The novelty of this work consists in an analytical methodology, based on non-linear programming and calculus of variations theory, to evaluate management strategies characterized by high effectiveness in reducing battery current transients. The identification and optimization of these strategies are initially performed on the basis of a vehicle model, built in the Matlab-Simulink simulation environment. To this purpose, the experimental characterization of the supercapacitor module is obtained with reference to three different models, whose selection depends on the required fitting performance and computational effort indexes, as evaluated in the paper. The energy management strategies identified show promising results in the simulation environment, followed by experimental activities carried out by means of a dedicated 1:1 scale laboratory test bench. The various experimental results presented in this manuscript highlight that the identified λ-control strategy presents effectiveness values up to 57%, close to the ideal results obtained in the simulation environment. In fact, the methodology proposed in this paper, validated by laboratory experiments, definitely reduces the negative consequences of power peaks on the HESS, indicating the real possibility of using these results in the design and control of urban road vehicles.

This paper describes a novel Energy Management Strategy (EMS) for hybrid energy storage systems, when used to supply urban electric vehicles. A preliminary off-line procedure, based on nonlinear programming, is performed in order to optimize the battery current profile for fixed working cycles. Hence, a suitable control strategy, which is based on a constrained minimization problem, is tailored for real-time applications. This control strategy exploits the off-line solution of a proper isoperimetric problem and aims to dynamically optimize the battery durability by reducing peak charging/discharging current values. The main advantage of the analysed EMS consists in the easy on-board implementation through the use of one single parameter, which can be quickly identified through a simple off-line numerical procedure. The proposed strategy is evaluated in simulation environment, through the use of a Matlab-Simulink model, for the case study of an urban electric vehicle running on a ECE 15 driving cycles. Simulation results have confirmed the good performance of the above strategy in reducing the battery peak charging/discharging current through the proper management of the hybrid energy storage system.

The study presents a significant development of an advanced control strategy for stand-alone three-phase induction generators. The system under study consists of a squirrel-cage induction generator driven mechanically by an internal combustion engine and electrically by a fault-tolerant converter coupled with a LC filter. As a novelty, in this study three low-cost electrical contactors are introduced in order to re-arrange the converter topology after a fault occurrence and a proper re-starting procedure is developed, allowing the converter to operate with only two legs. The control architecture is mainly described with respect to this last operating condition, evidencing its intrinsic robustness. The study focuses on the composite control of the converter and the prime mover in order to improve the dynamic assessment of the whole system. The converter control is designed around the sliding mode technique, due to its attractive performance, especially as far as parameter uncertainties are concerned. In order to validate the theoretical framework, this study presents the first ever made experimental tests of this system with reference to a generation unit of about 10 kVA. The results confirm the high capability of the proposed system in regulating voltage and frequency with very good dynamic features.

This paper describes the feasibility analysis of a methodology based on the Hilbert transform in order to detect the gear rattle phenomenon of automotive transmissions. The technique adopts the Hilbert transform to analyze experimental signals of the relative angular motion of gears starting from high-resolution optical encoder measurements. By this procedure it is possible to evaluate the severity of impacts occurring between the teeth of unloaded gears, by examining the root mean square value of the signal. The Hilbert transform is largely adopted in the detection of gear defects, such as tooth cracks, but it has not yet been used to detect gear rattle. This feasibility study highlights the capability in discriminating between impacts occurring on the two different sides of the tooth. The method puts in evidence the influence of oil lubricant between the teeth in reducing the severity of the impacts, by comparing many experimental tests performed with two different oils for transmission gears. Moreover, in order to evaluate the reliability of the proposed methodology some comparisons are conducted with a metric based on the fast Fourier transform, often adopted in this field, to highlight the goodness and simplicity of the technique based on the Hilbert transform.

In this paper, an analytical approach that deals with the optimal sizing of energy storage systems in direct current networks is proposed. In modern power systems, the widespread use of power electronics, storage devices, and automation is driving power engineers to focus on the use of direct current networks. This new focus requires specific tools for the optimal planning and operation of these networks in order to increase energy efficiency and reduce operating costs. This paper is focused on the improvements in the efficiencies of direct current networks, which are characterized by the presence of loads, units for the generation of renewable power, and storage devices. Based on the calculus of variations, an original matrix formulation which starts with the nodal representation of the direct current network is proposed. Two attractive closed-form solutions are presented for minimizing power losses, i.e., (1) a solution based on the approximation of considering the voltage constant at all the network’s busses and (2) a solution based on the linear approximation of the load flow. In both cases, the goal is to minimize losses over a given time horizon (e.g., the daily cycle). The formulation of the problem allows an analytical solution to be obtained that represents a suitable tool for the purpose of designing storage. In addition, the proposed approach can be applied and extended to the optimal sizing of storage systems. The proposed sizing procedure, which uses an analytical approach, is formulated in a general manner that can be used for various storage technologies. The results of numerical applications clearly have demonstrated both the feasibility and accuracy of the methodology to be used in the proposed design. We also propose an interesting parametric study in order to determine the optimal technology and the optimal size of the storage device.

This study is devoted to a proper modeling and estimation method of battery capacity fading under the widely adopted exponential decay model, in view of an efficient lifetime assessment of battery. The method is based upon recursive online Bayes estimation of capacity fading. It is also shown how a degradation-based modeling of battery reliability can be adopted, leading to a Bernstein lifetime distribution. The performances of the proposed method are successfully evaluated by means of extensive Monte Carlo simulations based upon available literature and experimental data. A brief account is also given of a robustness analysis of the proposed methodology with respect to departures from the assumption of Gaussian noise.

This paper proposes a new methodological approach in the field of studies devoted to proper and accurate selection of a reliability model for battery systems. The study is performed with particular emphasis on the modeling and estimation of the Conditional Reliability Function, conceived as a key analytical tool in predicting the "Remaining useful life" of the battery, which is in turn an important information in order to identify the best maintenance strategy selection, or for inspection optimization, and also spare parts provision. Estimation of the Conditional Reliability Function is developed by means of a method based on the Inverse Gaussian Distribution and its Bayes Estimation. The performances of this estimation are developed and validated by means of extensive simulations and available experimental data. A brief account is reported of robustness analyses of the method with respect to the assumed prior Distribution.

Four-phase transmission lines have been recently proposed and studied as an alternative to the traditional three-phase lines. The identification of the best transformer typology to link the three-phase and four-phase system sections is still an open problem, despite some specific solutions were proposed. However, these solutions were not included in a general framework yet. Resorting to the classic Parton's theory of phase transformation and to a rigorous mathematical approach, this study provides a general framework for the specific problem in study. After having recalled the underlying principles of the Parton's theory of phase transformation, by formulating proper optimisation problems and solving them using a rigorous mathematical procedure, this study derives several solutions for three-to-four phase transformation, some of which were never proposed yet. This way, both traditional solutions (T-connection, Leblanc connection, Scott connection) and more recent proposals are described together with the new solutions within one common framework.

The use of the micro-gas turbine devices is an important solution for distributed power generation and cogeneration. A cogenerating micro gas turbine of 30kW, operating with a recuperated thermal cycle, is examined under the point of view of the transient developments, induced by variations in electrical loads. The result is defined by the fuel supply law and by the response delay of the rotating and stationary components. In the paper, an optimization method in order to reduce the fuel consumption by varying load is presented. The control strategy adopted is able to determine the optimal reference value of the rotational speed and the ratio between the inlet air and fuel mass flow rate by a simple PI controller. Some results have been reported in terms of TIT temperature, global efficiency and outlet recuperator temperature to evaluate the respond time during a transient operation.

This paper deals with the loadability characteristics of overhead transmission lines (OHLs). The target is to provide a general analytical formulation of the loadability characteristics of uncompensated OHLs. This is a completely new approach: no previously published work has adopted an analytical approach to characterize the different regions of the loadability curves. In order to allow a direct comparison with previous works, an application example concerns the 400 kV single circuit OHLs widely used in Western Europe.

This paper deals with the loadability of series-compensated lines. Aim of the paper is to provide loadability curves calculated considering various technical limits, namely the thermal limit, the voltage drop limit, the Joule losses limit, and the steady-state stability limit. These curves are evaluated with respect to single circuit 400-kV overhead transmission lines (OHLs) equipped with standard triple-core aluminium conductor steel-reinforced (ACSR) conductor bundles of $\pmb{3\times 585}\mathbf{mm}^{\pmb{2}}$ cross section. The paper describes the optimization procedure used to maximize the active power transported at the receiving-end of the line. The optimal level of series compensation is obtained as a result of the optimization problem constrained by all the technical limits listed above. The last part of the paper is devoted to an extensive numerical application that has the aim of demonstrating the feasibility and applicability of the nronosed annroach. This study highlights the loadability increase that can be obtained through series capacitors, and might support the decision-making for the investments required by series compensation.

The paper presents an advanced Bayesian technique to carry out availability analyses of photovoltaic inverters in presence of uncertain reliability data. In order to proper take into account the above uncertainty, photovoltaic inverter hazard rate and repair rate are considered as random variables characterized by proper Log-normal and/or Gamma distributions. These assumptions are adequately motivated by well known and established physical and mathematical considerations. In this manner the photovoltaic availability uncertainty can be expressed as a function of the component uncertain data. Afterwards, thanks to the tailored Bayesian technique we demonstrate how, even in presence of scarce data, an efficient updating of the system performance can be accomplished during the photovoltaic inverter operating life. The methodology is really useful in view of an optimal allocation of the components’ reliabilities at the design stage and in order to establish the actual expected payback of the whole photovoltaic system. Extensive numerical simulations confirm the effectiveness and the validity of the developed Bayesian technique by also verifying its inherent robustness and efficiency through a rigorous analysis.

Small signal stability is a crucial aspect of accurately keeping control in modern interconnected power systems, in order to ensure the required security and reliability standards. Such an aspect could represent a serious limiting factor in the search for ever higher power system exploitation levels. Hence, in order to exploit the existing networks as much as possible, ensuring at the same time adequate security levels, the system operators must monitor the electromechanical oscillations continuously, estimating their fundamental parameters (e.g. frequency, damping factor/ratio, amplitude and phase), and determining the actual dynamic stability margins. Nowadays, electromechanical oscillations tracking can be performed in real time thanks to the quick development of wide area measurement/monitoring systems, and the employment of so-called measurement-based methods (a.k.a. mode meters). In this regard, the paper presents a performance comparison among several measurement-based methods currently operating on the Italian Wide Area Monitoring (WAM) platform. The main objective of this investigation is to analyze the behavior of each of them with respect to actual critical/not critical electromechanical oscillations recorded on the European Network Transmission System Operators for Electricity (ENTSO-e) Continental Synchronous European System (CESA). The considered case studies cover a large variety of situations that can be encountered in the ordinary ENTSO-e CESA operation: local and inter-area oscillations; single mode and multi-mode power signals; ambient and ringdown data; and so on. By keeping in mind that usually no best estimator exists, owing to the lack of an optimality definition, the present analysis aims at examining whether some algorithms are generally more suitable than others, with respect to the variety of aforementioned situations. In detail, the measurement-based methods considered in this work are the Tufts–Kumaresan method, Hankel Singular Value Decomposition-VarPRO, Hilbert–Huang Transform, its refined Empirical Mode Decomposition with Fourier-Based Masking Technique (R-EMD), and a recently published Hilbert Transform-based estimation algorithm. Obviously, this set of estimation techniques cannot be considered exhaustive for the research problem under analysis. Nonetheless, it represents a collection of the major, recent and most fashionable methodologies implemented in actual WAM architectures.

In this paper the design and the control of an advanced PV (Photovoltaic) inverter is discussed. The input power stage consists of a dc–dc converter based on coupled-inductors boost topology. This circuital configuration guarantees the high voltage amplification needed in grid-connected PV applications. The second power stage is a three-phase inverter operated in order to exploit the functionality of a two legs configuration. The operation principle of the proposed circuit is analyzed and a proper PV panel model is used. The adopted control strategy is based upon sliding control technique in order to exploit its well-known properties of robustness. Furthermore, a Maximum Power Point Tracking (MPPT) approach based on the extremum seeking scheme has been employed for fast tracking the irradiation changes by performing an adaptive sliding surface. Moreover, a great attention has been devoted to the control of the inverter in order to obtain power factor correction, effective reduction of both unbalances and waveforms distortion. Finally, the numerical results reported in the paper permit to confirm the feasibility of the proposed design and control strategy.

In this paper the hardware implementation of a PV microinverter based on dual coupled-inductors dc-dc converters is presented. Coupled-inductors were sized to ensure a high coupling factor, as well as to reach the desiderate voltage gain. Moreover, wide Band-Gap devices allowed to reduce weight and volume of the overall system. The control technique was optimized by adding a PI regulator to a sliding mode controller. Preliminary experimental tests are shown to demonstrate that the circuit is fully-functional and that the design approach was well-conceived.

This paper investigates the loadability increase that can be obtained in radial transmission lines by means of a synchronous condenser connected at the receiving end of the line. In this configuration, the reactive power reserve at the receiving node can be used (at least in the best part) to control the reactive power flow in the radial line. This way, the voltage drop across the line can be controlled, avoiding to exceed the maximum voltage drop allowed. The result is a considerable increase of the loadability curves in the voltage drop region and, thus, a considerable power capability increase for medium length and long radial transmission lines. In order to accommodate growing load power, the use of a synchronous condenser can be a convenient alternative to the construction of a new line or to the doubling of a single-circuit line.

In the last years a great interest has been paid in the relevant literature to the benefits deriving from the use of direct current for ship power systems. This is in compliance with the most recent concept about all-electric-ship and smart electrical distribution for achieving better performances by exploiting as far as possible the potentiality of integrated power systems. In this context the storage devices play a crucial role in improving the reliability, the efficiency and the dynamic performances of the whole electric system. Moreover, compared to conventional ship propulsion plants, this approach has a significant impact on ship design (particularly when combined to azimuthing thruster or podded thrust units) as regards comfortable (noise and vibrations), flexibility (for general plan), consumption and environment protection. The variety of pulsed and intermittent loads also requires a proper choice of the storage devices and of the techniques for the optimal system design. Focusing on DC ship power systems, in this paper two different methodologies for the design of combined storage systems constituted by batteries and supercapacitors are proposed. Based on some typical loads, some relevant case studies are analyzed and discussed.

This paper investigates the loadability increase of overhead transmission lines (OHLs) equipped with high-temperature low-sag (HTLS) conductors. Compared with traditional ACSR conductors, HTLS conductors allows to get a large increase of the thermal limit, maintaining the same weight and size of the conductors. Of course, the loadability curves increase only in the thermal limit region and, thus, this measure provides a significant advantage only for short lines. However, the advantage provided by HTLS conductors can be extended to longer lines if the voltage drop across the line can be controlled. This can be done injecting reactive power at the receiving end of the line by means of synchronous condensers or static var compensators (SVC). This paper highlights the loadability increase that can be obtained this way in the voltage drop region over a wide range of lengths. Accordingly, HTLS conductors can be better exploited up to much longer line lenghts.

Storage systems are recognized as a viable mean for improving the performances of electrified transportation systems. The recent literature highlighted some significant solutions to design problems thanks to procedures-based upon optimization theory. More specifically, an analytical solution can be deduced based on the formulation of an isoperimetric problem. This formulation exhibits the intrinsic ability to determine the optimal storage size, once the train power absorption has been fixed. Unfortunately, the analytical expression is not very advantageous to the control because it includes terms that require both real-time traction load estimation and its prediction over the whole traction cycle. The originality of this paper lies in the suitable simplification of the control structure, avoiding both traction load estimation and the heavy computations linked to predictive analysis. The storage device control law is reduced to an appropriate constant voltage over the whole traction cycle. The traction load prediction is performed offline, and the robustness of the consequent control law is tested against random variations in significant traction parameters. Some comparisons are made using other control strategies proposed in the technical literature. Finally, numerical and experimental results confirm the validity of the affected assumptions.

This is the second part of an investigation aimed at providing a thorough technical comparison between traditional and alternative overhead power transmission lines based on the loadability characteristics. The previous Part I has outlined and applied the fundamentals of the loadability analysis to three comparable single-circuit lines. In the present Part II, the analysis is broadened to three comparable double-circuit lines: traditional three-phase ac double-circuit lines (assumed as the reference system for the comparison), bipolar HVDC lines and non-conventional combined ac-dc lines. As in Part I, the loadability analysis takes into account thermal limits, voltage drop, power losses and – when feasible – steady-state and transient stability. Part II also performs an overall comparison among the different systems in single-circuit and double-circuit configuration to draw final conclusions.

The paper deals with a novel method to measure inter-area oscillations, i.e., electromechanical oscillations involving groups of generators geographically distant from one another and ranging within the frequency interval from 0.1 Hz up to 1 Hz. The estimation of the parameters characterizing inter-area oscillations is a crucial objective in order to take the necessary actions to avoid the instability of the transmission electrical system. The proposed approach is a signal-based method, which uses samples of electrical signals acquired by the phasor measurement unit (PMU) and processes them to extract the individual oscillations and, for each of them, determine their characteristic parameters such as frequency and damping. The method is based on Hilbert transformations, but it is optimized through further algorithms aiming at (i) improving the ability to separate different oscillatory components, even at frequencies very close to each other, (ii) enhancing the accuracy associated with the damping estimates of each oscillation, and (iii) increasing the robustness to the noise affecting the acquired signal. Results obtained in the presence of signals involving the composition of two oscillations, whose damping and frequency have been varied, are presented. Tests were conducted with signals either synthesized in simulated experiment or generated and acquired with actual laboratory instrumentation.

Nowadays, worldwide environmental issue, associated to reduction of pollutant and greenhouse emissions are gaining considerable attention. Aviation sector contribution to the whole CO2 released accounts to around 2%, but it is expected to grow in the next future due to increase of demand. Probably, combustion engine design and fuel efficiency have already reached their optimum technology level and only a breakthrough as hybrid-electric propulsion could be able to satisfy the new international more demanding requirements. However, an improvement of the technology readiness level of hybrid-electric propulsion is strongly necessary and many operational and safety challenges should be addressed. In the work here reported, a hybrid-electric model was designed and developed for general aviation aircrafts, by means of the Mathworks® Matlab – Simulink 1D/0D simulation environment. Both thermal and electric energy storage units, transmission systems and power management devices were considered and the overall performances were evaluated during cruise phase and a conventional training mission, characterized by several run(lap) “touch-and-go”. Furthermore, an innovative mathematical methodology was implemented for battery pack discharge profile interpolation. Finally, reliability and accuracy of the new proposed model were evaluated through comparison with the commercial code Simcenter AMESim® software and an average bias only equal to 5% was achieved.

Aim of the present paper is assessing the technical-economic comparison between three-conductor bundled (3B) and four-conductor bundled (4B) 380 kV overhead transmission lines (OHLs). In both cases, the widely used standard ACSR 31.5 mm is taken as the base conductor. Increasing the number of conductors with respect to the commonly used 3B improves the transport capacity, which can be properly represented in terms of loadability, i.e. the maximum allowable power that can be transmitted for an assigned value of the line length. The consequent higher power limit can be very useful in reducing transmission congestions and, thus, wind-generated energy curtailment that currently is not negligible in Southern Italy, promoting the integration of renewable energy. Moreover, reducing congestions has positive effect in relieving the differences in terms of zonal prices of the electric energy, bringing a further economic benefit for the system. The paper addresses these items performing a technical-economic comparison between a 3B and a 4B 380 kV OHL, assuming basically the same load diagram for both solutions.

This paper faces with the design of oil filled distribution transformers by a feasible and simple optimization procedure. The original aspect of the proposed methodology rests on the ability in properly integrating both electromagnetic and thermal aspects at the aim of deriving the principal specifications of the transformer. The generality of the procedure allows designers to examine every type of transformer, even if the present paper focuses on the three phase design of wound core transformers type, equipped with winding regulator. The finite element analysis is also employed for verifying the goodness of the results obtained from the proposed technique by properly taking into account the non uniform distribution of ampere-turns due to tap changing. Some useful considerations are made for demonstrating the incidence of the thermal constraints on the transformer geometrical parameters. A numerical application is presented with respect to 630 kVA transformer having losses parameters aligned with the new European standards.