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A probabilistic method for assessing the lightning hazard of structures and a simplification of this method for practical purposes is proposed and discussed. The risk of structural damage is evaluated as a function of the efficiency of the lightning protection system, the probability of damage due to a single flash, and the frequency with which lightning flashes strike the structure. Simplifications of this method are introduced; the damage probabilities are replaced by a weighting coefficient corresponding to different properties of the structure but without respect to economical factors, which should be considered on the background of local particularities. These considerations are illustrated by practical examples. >

Fires and explosions triggered by lightning strikes are among the most frequent Natech scenarios affecting the chemical and process industry. Although the lightning hazard is well known, well accepted quantitative procedures to assess the contribution of accidents caused by lightning to industrial risk are still lacking. In the present study, a quantitative methodology for the assessment of the expected frequency of lightning capture by process equipment is presented. A specific model, based on Monte Carlo simulations, was developed to assess the capture frequency of lightning for equipment with a given geometry. The model allows the assessment of lay-out effects and the reduction of the capture probability due to the presence of other structures or equipment items. The results of the Monte Carlo simulations were also used to develop a simplified cell method allowing a straightforward assessment of the lightning impact probability in a quantitative risk assessment framework. The developed approach allows an in-depth analysis of the hazard due to lightning impact by identifying equipment items with the highest expected frequency of lightning impacts in a given lay-out. The model thus supplies useful data to approach the assessment of the quantitative contribution of lightning-triggered accidents to industrial risk., JRC.G.5-Security technology assessment

The paper investigates the applicability of some closed form expressions for the ground impedance and ground admittance of buried horizontal wires (bare and insulated) for lightning or switching transient analyses based on transmission line (TL) theory. In view of the frequency contents that typically characterize such transients, the behavior of the ground impedance and admittance is studied for a wide frequency range up to 10 MHz. Low frequency approximation of the ground impedance is not always appropriate for transient analysis. Sensitivity analyses show that, unlike overhead wires, the ground impedance for buried wires is little sensitive to the ground conductivity. On the other hand, the ground admittance varies strongly with the ground conductivity. The paper also discusses the results of transient analysis of buried cables performed by means of electromagnetic transient programs (EMTP) that neglect the ground admittance. The limits of such an approximation are discussed in order to evaluate the applicability of EMTP-like programs to the transient analysis of buried conductors. Transient pulse propagation in time domain based on finite difference time domain (FDTD) method of solution of TL equations is also discussed for a future inclusion of non-linear phenomena, like soil ionization and arcing/breakdown mechanisms, in the soil. The analysis presented could be useful in estimating surge propagation characteristics of buried wires for appropriate insulation coordination and transient protection. © 2007 IEEE.

The papers addresses the problem of locating surge protective devices in a medium voltage distribution network by means of an accurate computer model for the calculation of lightning induced overvoltages and a Monte Carlo statistical procedure. The indirect lightning performance of small network cells of different configuration are evaluated and used as a benchmark for the validation of the surge arrester locations provided by a genetic algorithm procedure. The obtained results appear useful for the lightning protection design of networks with complex topology.

This chapter aims at illustrating the power system dynamic behavior during restoration scenarios in transmission networks, with particular reference to the analysis of thermal power plant black-start capabilities. This chapter is structured into three parts. Section 14.2 briefly reviews the requirements and the structure of power system restoration processes. The unbundling and liberalization of electrical power systems justifies the reassessment of power system restoration strategies, also by increasing the number of black-start resources, traditionally only represented by hydro stations and gas turbines (GTs). Hence, Section 14.3 deals with the analysis of the black-start capabilities of thermal stations equipped with both steam turbines (STs) and GTs. Moreover, accurate modeling of the dynamic interaction between the power plant and the restoring electrical power system, throughout the use of modern computer simulation tools, appears to be of considerable help to study the early stage of the restoration plans and to design specific control systems to support the feasibility and reliability of black-start maneuvers. Therefore, Section 14.3 also includes the analysis of the typical transients occurring during load restoration maneuvers and islanding formation for two representative types of power plants, namely a once-through boiler steam group repowered by a gas turbine and a combined combined-cycle unit. The models of the two relevant simulators and their identification by means of experimental data are described in Section 14.4.

∗∗ Non-member ∗∗ ∗∗ ∗∗∗ The assessment of the lightning performance of a compact configuration of MV overhead lines that is expected to be adopted in the near future by an electricity distribution company in a Brazilian region characterized by high keraunic levels is presented. The compact configuration is characterized by the presence of an unenergized wire that has the main function to sustain the spacers of the phase conductors. As the wire could be periodically grounded, the paper analyses its effectiveness in reducing the overvoltages due to indirect lightning. The results are compared with those relevant to a conventional configuration of MV overhead lines now adopted by the same distribution company. The effects of the presence of surge arresters along the line are also analysed.

Various parameters of lightning discharges will be reviewed. Both common negative and less frequent but potentially more destructive positive and bipolar flashes will be covered. Direct lightning current measurements on instrumented towers will be discussed. Properties of first and subsequent strokes will be compared. The average number of strokes per flash and multiple channel terminations on ground will be considered. Parameters of subsequent strokes in natural lightning will be compared to their counterparts in rocket-triggered lightning. New insights into the lightning processes gained from triggered-lightning experiments will be discussed.

One of the challenging tasks related to the realtime control of Active Distribution Networks (ADNs) is represented by the development of fast (i.e. sub-second) state estimation (SE) processes. As known, the problem of SE of power networks links the measurements performed in the network with a set of non-linear equations representing the links between the network node voltage phasors (i.e. the system states) and measured quantities. The calculation of these voltages is accomplished by the solution of a minimization problem by using, for instance, Weighted Least Squares (WLS) or Kalman filter (KF) methods. The availability of phasor measurement units (PMUs), characterized by high accuracy and able to directly measure node voltage phasors, allows, in principle, a simplification of the SE problem. Within this framework, the paper has two aims. The first is to propose a procedure based on the use of the Iterated KF (IKF) aiming at making achievable, in a straightforward manner, the SE of ADNs integrating PMU measurements. The second goal is to present a sensitivity analysis of the performances of WLS vs IKF methods as a function of the measurements and process covariance matrices.

A joint research project between AES Sul, the Federal University of Itajuba and the University of Bologna aims at defining the required protections against lightning-originated overvoltages in a resonant grounded distribution system in order to achieve the adequate lightning performance level with minimum cost. The paper aims at illustrating the effectiveness of the presence of a surge arrester or an arcing-horn gap and its protective zone by means of simulations obtained with a computer code for the calculation of lightning induced voltages that takes also into account the steady-state voltage. The results of a statistical analysis for a single multiconductor line are eventually presented and discussed.

Two approaches for the numerical solution of the full-wave Maxwell’s equations, based upon the finite element method and the method of moments, are here used for the computation of lightning induced voltages on overhead distribution power lines. The methods are well suited to consider both induced and conductive coupling mechanisms between the lightning return stroke channel, the overhead line conductors and the grounding system of the line. It is noted that this last effect is traditionally disregarded in the computation of lightning induced overvoltages. It is shown that the conductive coupling might result in the reduction of the lightning overvoltages induced on overhead lines. This finding justifies the inclusion of the conductive coupling in the available approximate formulae used for the calculation of lightning induced overvoltages.

Le celle a combustibile (Fuel Cells - FCs) a bassa temperatura sono potenzialmente utilizzabili per applicazioni cogenerative, sia nel settore residenziale sia nel terziario. L’energia elettrica prodotta può essere utilizzata o accumulata ed è possibile recuperare la potenza termica fornita durante il funzionamento di tali sistemi. Dal 2003 è condotto un programma di ricerca in collaborazione fra i dipartimenti DIEM e DIE dell’Università di Bologna sulle FCs e sulle applicazioni cogenerative delle stesse. Sulla base dei risultati ottenuti anche mediante la realizzazione di un sistema prototipale della taglia di alcuni kW si vuole ora proseguire tale studio mediante lo sviluppo del progetto di ricerca “stazione di prova per sistemi a cella a combustibile” presso il Tecnopolo di Ravenna, nell’ambito del CIRI Energia e Ambiente. La finalità del progetto è la caratterizzazione completa di un sistema energetico complesso basato su FCs e accumuli. Nel presente articolo si illustrano le caratteristiche del banco prova che si intende sviluppare per il progetto.

An experimentally inferred charge-voltage (Q-V) diagram used to macroscopically model the corona phenomenon is adopted for the calculation of the lightning-induced transients in presence of corona. The paper aim at presenting the calculation procedure as well as the analysis of the corona Q-V diagram on the lightning-induced surges.

n this chapter we attempt presenting a more modern approach to the problem, which makes use not only of simple analytical formulas, but also of advanced models and computer codes. Due to the complexity of the problem, it is indeed almost impossible to achieve sound results without the use of adequate modeling and relevant software. In general, the community dealing with lightning protection is prepared to use EMTP-like computer codes mostly for the calculation of the lightning-response of high voltage transmission lines, while for distribution networks, due to the inherent larger complexity of both topology and configuration, the use of advanced computer codes has not been very popular so far. This is probably due to the fact that, besides the mentioned complexity of the system to be studied, also the phenomena involved are in general more complex for distribution lines than for transmission lines. Think for instance of the electromagnetic coupling between the lightning electromagnetic pulse (LEMP) radiated by a nearby return stroke and the conductors of an overhead distribution network. It is reasonable to imagine that in the future the use of “coupling-transient” codes will progressively enhance its importance with larger benefits for the power quality and reliability of power distribution networks. After all the use of sophisticated computer codes for the calculation of the power flow and of the stability of power network is nowadays representing a common practice nobody can do without.

The paper describes the performances of a phasor measurement unit (PMU) prototype based on a synchrophasor estimation algorithm conceived for the monitoring of active distribution networks, as well as its experimental application during some intentional islanding and reconnection tests of an urban medium voltage power network. With respect to typical applications in transmission networks, the use of PMUs in distribution networks requires very low values of total vector error (TVE), which involves particular low values of phase errors of the synchrophasor estimates. These requirements are met by a specifically developed PMU, the characteristics and experimental characterization of which are illustrated in the paper. Three of these PMUs have been then used to monitor experimental tests carried out to assess the capability of a urban distribution network to operate autonomously when fed by a local 80 MW combined-cycle power plant. The information provided by the installed PMUs significantly facilitate the operator maneuvers and appear to be useful for the development of an improved control and management system of the active distribution network.

The paper aims at illustrating a procedure for the lightning risk assessment of industrial oil process plants. The procedure is based on the use of the Monte Carlo method and applies to each event both a lightning attachment criterion and a damage calculation model. In particular, for the case of oil tanks, the model calculates the expected annual number of damages associated to the metal melting due to direct lightning events. The paper also presents and discusses an application example of the proposed procedure.

A detailed analysis of past accidents evidenced that lightning strikes on storage or process vessels containing flammable materials may cause severe accidents at refineries, storage sites, processing sites and other industrial facilities where hazardous materials are stored or processed. Although the hazard due to lightning is well known and is usually considered in the safety analysis of chemical and process plants, well accepted quantitative procedures to assess the contribution of accidents triggered by lightning to industrial risk are still lacking. In particular, the approaches to the assessment of lightning strike probability, damage caused by lightning strike and intensity of loss of containment (LOC) caused by lightning are mostly based on expert judgment and not on up-to-date models that take into account the relevant progress made in the last years in the analysis of the lightning phenomenon. In the present paper an innovative procedure for the quantitative assessment of the industrial risk due to LOC of hazardous substances triggered by lightning is introduced. The study was focused on the development of models for lightning impact probability and for damage probability. In particular, a Monte Carlo approach was developed to calculate the probability of lightning impact on the basis of the geometry of the structure and of lightning intensity and frequency data.

An extended analysis of past accident data indicated that lightning events may cause significant damage to process equipment handling dangerous substances, often leading to an extended loss of containment followed by fires, explosions or toxic releases. Data reported in the literature show that lightning caused 150 out of 480 tank fires reported in industrial accident databases between 1951 and 2003. The industrial accident databases indicate that lightning impacting on chemical plants is among the most important causes of the technological accidents triggered by natural events that are usually referred to as NaTech accidents. Considering specifically storage and processing activities, lightning events account for 61% of the accidents initiated by natural events. Moreover, due to the increasing frequency and intensity of severe storms, the frequency of such accidents may grow in the future. Nevertheless, a limited work has been dedicated to the development of quantified assessment procedures for the industrial risk generated by lightning phenomena in sites were relevant quantities of hazardous substances are present. The present study focused on the assessment of the lightning hazard and on the development of a detailed methodology for the quantitative assessment of the industrial risk due to loss of containment events triggered by lightning in the chemical and process industry. The procedure takes advantage of the progress made in recent years in the assessment of the expected frequency due to lightning. The methodology was developed within a common framework for the quantitative assessment of risk due to external hazard factors in chemical and process plants.

As known the lightning incidence of a vertical structure is related to the physical processes involved in the final stage of the progression of the downward lightning leader to the structure and upward streamers, in case downward flashes are taken into account. Within this context, the paper presents and discusses a numerical implementation based on the Finite Element Method (FEM) of the Leader Progression Model (LPM): LPM-FEM. The obtained results are compared with those of the original implementation of the LPM based on the charge simulation method and the relevant differences analyzed and discussed.

The centralized control of active distribution networks with large penetration of dispersed resources requires algorithms capable of providing network state estimation. Within this context, the measurement of bus voltage synchrophasors provides useful data that increase the observability of the network. The peculiar characteristics of distribution networks, such as feeder impedances, power flows and harmonic distortion, are different from those of transmission networks. Therefore, in order to obtain reasonably low uncertainty levels, Remote Terminal Units (RTU) with specific characteristics need to be developed. The paper presents an algorithm developed for the synchrophasor measurement in active distribution networks together with its implementation into a specific hardware based on a real-time microcontroller. The paper also presents the experimental characterization of the developed RTU by making reference to both single-tone and distorted signals. © 2009 IEEE.

Monitoring units for the measurement of high frequencyvoltage transients have been in operation at three different busses of an Italian medium voltage (MV) distribution feeder, mainly composed by overhead lines, in March 2007 – August 2008. The feeder is located in a region characterized by a high ground flash density value (4 flashes/km2/yr); many of the recorded voltage transients may be correlated with the lightning events detected for the same region by the Lightning Location System (LLS) CESISIRF. The paper presents some experimental results obtained using the monitoring units and their comparison with computer results obtained using a LIOV-EMTP model of the considered MV feeder. A procedure aimed at achieving the best fit between measurements and calculations, which takes into account the uncertainties associated with LLS data, is also presented.

The evaluation of electromagnetic transients in overhead power lines due to nearby lightning return strokes requires accurate models for the calculation of both the incident lightning electromagnetic pulse ( LEMP) and the effects of coupling of this field to the line conductors. Considering also the complexity of distribution networks in terms of their topology and the presence of power system components and protection devices, the implementation of the LEMP-to-transmission-line coupling models into software tools used to represent the transient behavior of the entire network is of crucial importance. This paper reviews the most significant results obtained by the authors concerning the calculation of lightning-induced voltages. First, the theoretical basis of advanced models for the calculation of LEMP-originated transients in overhead power lines is illustrated; then, the relevant experimental validation using: 1) reduced-scale setups with LEMP and nuclear electromagnetic pulse ( NEMP) simulators and 2) full-scale setups illuminated by artificially initiated lightning are reported. Finally, the paper presents comparisons between simulations and new experimental data consisting of measured natural lightning-induced voltages on a real distribution network in northern Italy, correlated with data from lightning location systems.

The paper deals with the evaluation of the indirect-lightning performance of overhead distribution networks. The novelty of this contribution is that it takes into account the inherent complexity of distribution networks. These networks are indeed characterized by a plurality of lines (main feeder and laterals) and also by the presence of typical power components (e.g., transformers and surge arresters); they consequently differ considerably from the straight line configuration generally adopted in this type of studies. To accomplish such an evaluation we have extended the general procedure already presented in a previous paper based on the use of the LIOV code along with the Monte Carlo method. The extended procedure combines the advantage of the LIOV-EMTP computer code-that allows the calculation of lightning-induced voltages in complex distribution networks-with a heuristic technique specifically developed and integrated in the Monte Carlo routine in order to reduce the computational effort. The application of the proposed procedure to a set of distribution overhead networks characterized by different topologies, but all of the same length, shows that, in general, the usual single straight-line approach may result into a misestimation of the indirect-lightning performance. The paper also analyzes and discusses the influence of both the line terminations, and the position of the line poles. © 2009 IEEE.

Current from nearby rocket-triggered lightning that flowed through the soil and into an unenergized test power distribution line was studied based on experimental data acquired in 2003 at the International Center for Lightning Research and Testing in Florida. The 15-pole, three-phase line was 812 m long, was equipped with four arrester stations, at poles 2, 6, 10, and 14, and was terminated in its characteristic impedance at poles I and 15. The neutral conductor of the line was grounded at each arrester station and at both line terminations. Measurements suggest that a significant fraction of the lightning current injected into the earth a distance of 11 in from pole 15 entered the line through the grounding system of pole 15. The peak value of the microsecond-scale return stroke current entering the line through the pole 15 line ground was 7% of the peak value of the return stroke current injected into the earth. The peak value of the millisecond-scale triggered lightning initial stage current and the millisecond-scale return-stroke and initial-stage charge transfer to the line through the pole 15 line ground was between 12% and 19% of the lightning peak current/charge transfer, indicating that the percentage values for the injected peak currents are dependent on the current waveshape: for microsecond-scale return stroke currents, possibly due to electromagnetic coupling effects, a smaller fraction of the current peak enters the line compared to millisecond-scale initial stage currents. In the latter case, any influence of electromagnetic coupling to the line on ground currents is expected to be negligible.

The paper presents the analysis of black-startup and islanding capabilities of an 80 MW combined cycle power plant. The analysis is carried out by means of a computer dynamic simulator suitably developed in the EMTP-RV simulation environment in order to represent both the power plant and the local network with the relevant loads. The paper describes the main characteristics of the implemented models and presents the analysis of transients due to both energization and islanding manoeuvres. The former simulations aim at verifying the feasibility of the energization of the path from a turbo gas unit, with autonomous black-start capabilities, to the local distribution network loads. The latter simulations allow the analysis of different islanding strategies for rather large levels of power exported to the external transmission network.

The paper presents a fault location procedure for distribution networks based on the wavelet analysis of the fault-generated traveling waves. In particular, the proposed procedure implements the continuous wavelet analysis applied to the voltage waveforms recorded during the fault in correspondence of a network bus. In order to improve the wavelet analysis, an algorithm is proposed to build specific mother wavelets inferred from the fault-originated transient waveforms. The performance of the proposed algorithm are analyzed for the case of the IEEE 34-bus test distribution network and compared with those achieved by using the more traditional Morlet mother wavelet.

In the international research context set-up by the European Technological Platform SmartGrids, the Italian electrical energy distribution utility (ENEL Distribuzione) and the Italian electrical energy production and research utility (ENEL Produzione) launched a large national research project. The program provides the development of a test facility for the integration and test of distributed energy resources management for smart grids with of innovative solutions. The paper describes the objectives of the project and the characteristics of the test facility with reference to schemes, components, support modelling tools, control and communication systems. (4 pages)

—Engineering return stroke models can be categorized either as current generation (traveling current source type) models or current propagation (transmission line type) models. The current generation models are described among other parameters by a corona current distributed along the channel. Recent studies show that there is equivalence between the models of current generation and current propagation types. Due to this equivalence, any engineering return stroke model of current propagation type can be described in terms of an equivalent corona current per unit channel length. The measurements conducted within 10–500 m from triggered lightning flashes show that the electric field of subsequent return strokes at these distances flattens within 15 μs or so. In this paper, the constraints imposed by this feature on the temporal and spatial variation of the equivalent corona current are investigated. The results showthat in order for the close fields to flatten within 15 μs or so, the equivalent corona current, should be bipolar and the corona current wave shape at late times should be identical to that of the longitudinal current time derivative. This is in contrast to most of the engineering models of current generation type, in which the corona current is assumed to be unipolar.

The evaluation of electromagnetic field coupling to transmission lines is an important problem in electromagnetic compatibility. The use of the transmission line (TL) approximation theory has permitted the solution of a large range of problems (e. g. lightning and EMP interaction with power lines). However, the continual increase in operating frequency of products and higher-frequency sources of disturbances (such as UWB systems) makes TL basic assumptions no longer acceptable for a certain number of applications. In the last decade or so, the generalization of classical TL theory to take into account high frequency effects has emerged as an important topic of study in electromagnetic compatibility. This effort resulted in the elaboration of the so-called "generalized" or "full-wave" TL theory, which incorporates high frequency radiation effects, while keeping the relative simplicity of TL equations.

For the accurate statistical assessment of the lightning performance of distribution lines one needs to take into account the presence of protection devices such as surge arresters and shielding wire/neutral groundings. Indeed, the presence of those devices can significantly modify the indirect lightning response of the overhead line and, therefore, the relevant lightning performance. In particular, this paper aims at investigating the impact of the presence of surge arresters on the indirect lightning performance of overhead distribution lines having realistic configuration. Specifically, reference is made to typical Italian distribution lines. The investigation is performed with the help of a statistical procedure developed by the authors – based on the Monte Carlo method and of the use of an accurate modelling/calculation tool for the induced surges – that has been recently compared with one proposed in the IEEE 1410 Std for the same purpose, and shown to be more complete and suitable for the problem of interest. The paper also aims at investigating the influence of the presence of surge arresters on the indirect lightning surges transferred through power transformers. Such an additional investigation is performed by means of a suitable high-frequency power transformer mode integrated with the lightning-induced computation tool.

The aim of this paper is to present the results of the performance study of medium voltage overhead distribution lines against lightning discharges, in the way to define methodologies to reduce the system failures. The results are obtained within the partnership among the High Voltage Laboratory of the Federal University of Itajubá, AES Sul Utility Company and the University of Bologna. The resultant performance is presented in terms of expected faults for 100 km of line for a density of discharges to the ground (GDF) of 1 discharge/km2/year. Commentaries on the relative performance and comparisons of different construction configurations of overhead lines are presented. ©2007 IEEE.

One of the main cause of damages for wind turbine power plants is certainly constituted by lightning. Direct and indirect events can indeed produce damages and malfunctions of electrical and mechanical components. Concerning mechanical components, blades and bearings are the most involved parts. In particular, lightning-damages produced at bearings positioned at the mechanical interface between rotating parts of the wind turbine, can result in high costs, considering the difficulties involved in the replacement of such components. The paper presents a numerical and an analytical model of wind turbine main shaft bearings aimed at calculate their electrical impedance along with their experimental validation. These models provide useful criteria for the design of wind turbines lightning protection systems (LPS). In this respect, the paper presents also the development of a specific LPS for the protection of wind turbine bearings and the relevant effectiveness analysis obtained by means of experimental tests. ©2007 IEEE.

Papier complet : http://www.ipst.org/techpapers/2007/ipst_2007/papers_IPST2007/Session5/100.pdf; International audience; When performing insulation coordination studies, grounding electrodes of substations are frequently represented as as lumped resistances, in some cases even when extended grounding grids are dealt with. This paper presents an analysis of the approximations deriving from such a practice when studying fast transient phenomena, for several cases in term of grid geometry and soil electric resistivity. The influences of two models are compared for the grounding system: a very simple model that consists only of a resistor, and a model based on the more rigorous application of Maxwell's equations. The limits of applicability of these models are investigated and discussed by means of a comparative study. We conclude that depending on the type of engineering problem that one has to tackle, the adoption of one model instead of the other can lead to significant differences. For insulation coordination, lightning fast front overvoltages in the substation could be still computed in a first approximation using a simple resistor to model the grounding grid. Regarding EMC studies, the simplest model can lead to a certain underestimation of the potential rise of the grid, which means that, in general, the application of the Maxwell's equations-based model is recommended.

The aim of this paper is to present an experimental validation of a frequency- domain approach to the solution of the lightning electromagnetic field-to- buried cable coupling equations. The coupling to the inner conductor is evaluated using the concept of cable transfer impedance. The theoretical model and relevant computer code are tested using experimental data on lightning-induced currents in buried cables carried out in 2002 and 2003 at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida where currents induced by triggered lightning events were measured at the ends of a buried coaxial cable, both in the shield and in the inner cable conductors. Reasonably good agreement has been found between numerical simulations and recorded waveforms. In particular, the early-time response of the cable and the peak value of the induced currents were generally well reproduced by the simulations. © 2006 Elsevier B.V. All rights reserved.

In this paper, we discuss the Transmission Line (TL) theory and its application to the problem of external electromagnetic field coupling to transmission lines. After a short discussion on the underlying assumptions of the TL theory, we start with the derivation of field-to-transmission line coupling equations for the case of a single wire line above a perfectly conducting ground. We also describe three seemingly different but completely equivalent approaches that have been proposed to describe the coupling of electromagnetic field coupling to transmission lines. The derived equations are extended to deal with the presence of losses and multiple conductors. The time-domain representation of field-to-transmission line coupling equations which allows a straightforward treatment of non linear phenomena as well as the variation in the line topology is also described. Finally, solution methods in frequency domain and time domain are presented.

The paper deals with a microgrid test facility, settled at the CESI RICERCA laboratories, equipped with various distributed energy resources (DERs) and connected to a LV network. The operation of such a microgrid is supervised by a central automatic microgrid management system (MMS). The paper first reports and discusses some experimental results carried out for the assessment of DERs performance and for the implementation of specific MMS functions. Then, it describes the functions of the energy resources scheduler (ERS) implemented in the MMS. The scheduler periodically updates the set points of DERs regulators in order to achieve economic, reliability and power quality objectives, starting form the load and renewable production forecasts and from the results of the system state estimation. The ERS is composed by two main parts, namely: a day-ahead economic scheduler of active power set points during the following day for the minimization of the overall costs, and an intra-day scheduler that every 15 minutes settles the set points of the DERs regulators to optimize the voltage profile at the grid buses, taking into account both technical constraints and the day-ahead economic schedules. © 2007 IEEE.

The number of power installations lying underground has been increasing in the last few years, and such devices are very sensitive to the effect of the lightning electromagnetic fields, due to the massive presence of power electronics. As a consequence, the scientific community has devoted much effort in the direction of a more accurate modeling of underground lightning fields and their coupling to cables. The exact expressions of the underground lightning fields have been derived by Sommerfeld decades ago. However, their numerical evaluation has always been a hard task because of the presence of slowly converging improper integrals. In the past, some approximate formulas have been derived, which have been included in field-to-transmission line coupling models to estimate the effect of lightning on buried cables. In this paper, an efficient algorithm for the evaluation of the Sommerfeld expression for underground fields is presented, and its mathematical features are discussed. The numerical treatment of the Sommerfeld integrals is based on a proper subdivision of the integration domain, the application of the Romberg technique, and the definition of a suitable upper bound for the error due to the integral truncation. The remarkable efficiency in terms of CPU time of the developed algorithm makes it possible to use it directly in field-to-buried cable coupling simulation codes. Finally, the developed algorithm is used to test the validity of the Cooray's simplified formula for the computation of underground horizontal electric field. It is shown that predictions of the Cooray's formula are in good agreement with exact solutions for large values of ground conductivity (0.01 S/m). However, for poor conductivities (0.001 S/m or so) Cooray's expression yields less satisfactory results, especially for the late time response.

Study Committee C4 was one of the new study committees created as a result of the restructuring of CIGRE in 2002. It has a relatively broad mandate, and is responsible for facilitating and promoting progress in the development of methods and tools for analysis of power system performance related to the following five fields: - Power Quality - Electromagnetic Compatibility - Insulation Coordination - Lightning - Power System Security SC C4 is essentially a merger of significant parts of three former study committees: SC 33 «Insulation Coor dination»; SC 36 «Power System Electromagnetic Com patibility”; Comp atibility”; and SC 38 «Power System Analysis and Techn iques”. It brings together all activities related to the power system “technical” performance under one study committee, thus providing for the coordination and integration of these activities. In addition, it leads to “broadbased” expertise essential for effectively dealing with complex technical issues of present-day power systems. In carrying out studies and other work necessary to meet its objectives, SC C4 takes into account a number of external factors: technological evolution of the electri cal power industry; environmental impact considerations; impact of socio-economic, sociopolitical and ownership changes on power systems; and the changing needs for developing standards and recommended practices at national and international levels. Co-operation with other CIGRE committees and with external organizations is a key factor in this regard. SC C4 has established strong ties to CIRED and IEC, particularly in the activities related to Power Quality and Electromagnetic Compatibility. Cooperative relationships have also been established with IEEE PES, UIE, ITU-T and EURELECTRIC. A key focus of the planned activities of SC C4 is on technical challenges related to power system physical performance that need to be addressed by system planners, equipment suppliers, network operators. The technical activities of SC C4 have been organized in terms of the five distinct areas related to power system performance. Advisory Groups have been set up to assist in the planning and steering of the activities in each of the five areas. The following are the current and future planned activities in each area.

In this paper, simultaneous GPS time-stamped measurements of the electric and magnetic fields at three distances and of the return stroke current associated with lightning strikes to the Toronto CN Tower during the Summer of 2005 are reported. The lightning return-stroke current was measured using a Rogowski coil installed at a height of 474 m AGL (above ground level). The vertical component of the electric field and the azimuthal component of the magnetic field were measured simultaneously at distances of 2.0 km (Pratt building of the University of Toronto), 16.8 km (Environment Canada Building), and 50.9 km (Grimsby, ON) from the CN Tower. Preliminary discussion on the current-field relation and propagation effects is also provided.

This paper deals with the evaluation of lightning-induced overvoltages in a typical light-rail DC traction overhead power line. In particular, results provided by a new ad-hoc full-wave formulation are compared with those obtained by classical field-excited transmission line (TL) methods and by the general purpose NEC code based on the Method of Moment. Advantages and disadvantages of the methods are illustrated and discussed. It is shown that the full-wave formulation can be used in a circuit simulator in conjunction with a detailed representation of the light rail feeding system.

The paper illustrates a procedure based on the continuous-wavelet transform (CWT) for the analysis of voltage transients due to line faults, and discusses its application to fault location in power distribution systems. The analysis carried out shows that correlation exists between some characteristic frequencies of the CWT-transformed signals and specific paths in the network covered by the traveling waves originated by the fault. The paper presents a procedure for determining fault location in MV distribution systems, which exploits the above-mentioned correlation. The MV distribution system analysed in the paper is accurately represented by means of an EMTP model; various fault types and network characteristics are examined. The paper presents also the basic concepts of a measurement and fault location prototype system with distributed architecture

La prima parte della memoria riporta i risultati di un’analisi della capacità complessiva del parco di produzione elettrica italiano, nella sua composizione attuale, a reagire a squilibri di potenza. Viene fatto riferimento ad alcune situazioni reali d’esercizio verificatesi di recente. La seconda parte è motivata dal previsto rapido diffondersi delle unità operanti in ciclo combinato gas-vapore. E’ illustrato un modello di un impianto di produzione a ciclo combinato, implementato in ambiente Matlab-Simulink, che consente lo studio preliminare del comportamento di tale tipo d’unità in occasione di variazioni di frequenza improvvise e severe. I risultati ottenuti mostrano l’importanza delle prescrizioni tecniche attinenti alla connessione dei generatori alla rete elettrica, al fine di garantire in maniera durevole la capacità del sistema di reagire adeguatamente agli squilibri di potenza.

The paper deals with the stresses of cable insulations due to lightning induced overvoltages for the case of distribution networks composed by MV overhead lines and buried cables. We are here interested only in the conditional probability of the cable breakdown in absence of line flashovers, which means that flashovers along the overhead line are disregarded. We illustrate a procedure based on the application of the Monte Carlo method and on a simulation tool for the accurate computation of lightning-induced voltages on an overhead line, namely the LIOV code. A frequency dependent parameter cable model is used for the calculation of the overvoltages stressing the cable insulation, which is assumed not to be illuminated by LEMP. We associate to each calculated overvoltage a breakdown probability for the cable, by using experimentally obtained probabilistic distributions of the breakdown voltages of the cable insulation for different families of voltage wave shapes. These probabilities, together with the expected number and characteristics of the lightning events, allow us to infer the yearly number of expected failure events in the above assumptions. The breakdown behavior of extruded cable insulation is represented by a 2-parameter Weibull distribution obtained from experimental tests on cable models, manufactured with the same materials and same technologies of real cables, by applying the well-known statistical “enlargement law”.

For an improved assessment of the indirect-lightning performances of distribution systems, the real configuration of the system, which means the topology, the presence of protection devices such as surge arresters and shielding wire/neutral groundings, must be taken into account. In this paper, the indirect lightning performance of a distribution line with realistic configuration is evaluated by means a statistical procedure based on the Monte Carlo method. Accurate models are used to evaluate the lightning induced overvoltages, in order to estimate the effects of arresters spacing and characteristics. Results relevant to a line length of 100 km, as it is often presented in the power/lightning literature, appear to be not as useful as those relevant to the real system of interest. ©2006 IEEE.

The paper deals with the subject of the source-identification of transient voltage disturbances in distribution system buses. In particular, a statistical procedure is proposed for the evaluation of the probability that a lightning flash detected by a Lightning Location System (LLS) could cause a fault and, therefore, relay interventions, generally associated with voltage dips. The proposed procedure is based on the coordinated use of the information provided by the LLS and the availability of an advanced simulation tool for the accurate simulation of lightninginduced voltages on complex power systems, namely the LIOV-EMTP code. The uncertainty levels of the stroke location and of the peak current estimations provided by the LLS are discussed and their influence on the lightning-fault correlation is analyzed. © Copyright KTH 2006.