[show abstract][hide abstract] ABSTRACT: This paper presents experimental results obtained at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida during the summers of 2002 and 2003. Currents induced by triggered and natural lightning events were measured at the terminations of a buried power cable, in the cable shield, and in the inner cable conductor. Measurements of the horizontal component of the magnetic field above the ground surface for both natural and triggered lightning are also presented. For distant natural lightning events, locations of ground strike points were determined using the U.S. National Lightning Detection Network (NLDN). Based on the theoretical developments presented in Part I of this paper , the field-to-buried cable coupling equations are solved in both the time domain and in the frequency domain. The obtained experimental results are then used to validate the numerical simulations provided by the relevant developed codes.
IEEE Transactions on Electromagnetic Compatibility 09/2005; · 1.33 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, we present a review of theoretical methods to compute lightning induced currents and voltages on buried cables. The evaluation of such induced disturbances requires the calculation of the electric field produced by lightning along the cable path. We show that the Cooray's simplified formula is capable of predicting accurately the horizontal electric field penetrating the ground, at distances as close as 100 m. Regarding the parameters of the buried cable, a comparison of several approximations of the ground impedance is presented. We show that the Pollaczek expression corresponds to the Sunde general expression, when the displacement current is neglected. The analysis shows also that all the proposed approximations provide very similar results for the considered range of frequencies (up to 30 MHz). Most of the approximate formulas neglect the contribution of the displacement current and, therefore, predict values for the ground impedance which tend to infinity at higher frequencies. This corresponds in the time domain to a singularity of the ground transient resistance at t=0. By analogy to the Sunde approximation for the ground impedance of overhead lines, we propose a logarithmic approximation for the ground impedance of a buried cable. In addition, unlike most of the considered approximations, the proposed formula has an asymptotic behavior at high frequencies; therefore, the corresponding transient ground resistance in the time domain has no singularity at t=0. It is also demonstrated that within the frequency range of interest, the wire impedance can be neglected, due to its small contribution to the overall longitudinal impedance of the line. The ground admittance, however, can play an important role at high frequencies (1 MHz or so) especially in the case of poor ground conductivity. The ground admittance needs to be taken into account in the calculation of lightning induced currents and voltages on buried cables. This is in contrast with the case of overhead lines in which its contribution is generally negligible, even in the MHz range. We also investigate the time-domain representation of field-to-transmission line coupling equations. The coupling model includes the effect of ground admittance which appears in terms of an addi-
tional convolution integral. An analytical expression for the ground transient resistance in the time domain is also proposed which is shown to be sufficiently accurate and nonsingular. Finally, we present a time domain solution of field-to-buried cable coupling equations using the point-centered finite difference time domain (FDTD) method, and a frequency domain solution using Green's functions. In our companion paper (Part II), we compare both solutions to experimental waveforms obtained using triggered lightning.
IEEE Transactions on Electromagnetic Compatibility 09/2005; · 1.33 Impact Factor
[show abstract][hide abstract] ABSTRACT: Simulation results relevant to realistic configurations of medium-voltage (MV) and low-voltage (LV) distribution systems provide useful information to evaluate both the expected level of power quality problems on LV networks due to lightning and the benefits achievable by a proper installation of protection devices. The paper describes a procedure to evaluate lightning overvoltages induced on MV lines by lightning strokes hitting the ground on the line vicinity and that are transferred through the MV-LV transformer on the LV network. To show how the high-frequency model of the transformer play a crucial role in the simulation of transferred overvoltages, the paper presents the results obtained by using two different models identified by means of laboratory measurements.
[show abstract][hide abstract] ABSTRACT: The paper focuses on bidding selection mechanisms based both on offer price and electricity-quantity strategies. The aim of the paper is to present a procedure that allows the use of both game theory and of detailed cost-based unit-commitment computer codes. The procedure is conceived in a way to be as much as possible independent of the specific market rules. The results of a preliminary analysis are shown, concerning simple test cases that incorporate some distinctive characteristics of electricity markets and power plants.
[show abstract][hide abstract] ABSTRACT: New general expressions relating lightning return stroke currents and far radiated electric and magnetic fields are proposed, taking into account the effect of an elevated strike object, whose presence is included as an extension to the transmission line (TL) model. Specific equations are derived for the case of tall and electrically short objects. The derived expressions show that, for tall structures (when the round-trip propagation time from top to bottom within the tower is greater than the current zero-to-peak risetime), the far field is enhanced through a factor with respect to an ideal return stroke initiated at ground level. The enhancement factor can be expressed in terms of the return stroke wavefront speed v, the speed of light in vacuum c, and the current reflection coefficient at the top of the elevated strike object. For typically negative values of this top reflection coefficient, lightning strikes to tall towers result in a significant enhancement of the far electromagnetic field. Expressions relating the far electromagnetic field and the return stroke current are also presented for electrically short towers and for very long return stroke current wavefronts. For the case of return strokes initiated at ground level (h=0), these expressions represent a generalization of the classical TL model, in which the reflections at the ground are now taken into account. We describe also simultaneous measurements of return stroke current and its associated electric and magnetic fields at two distances related with lightning strikes to the 553-m-high Toronto Canadian National (CN) Tower performed during 2000 and 2001. The derived expressions for tall strike objects are tested versus obtained sets of simultaneously measured currents and fields associated with lightning strikes to the CN Tower, and a reasonable agreement is found. Additionally, it is shown that the peak of the electromagnetic field radiated by a lightning strike to a 553-m-high structure is relatively insensitive to the value of the return stroke velocity, in contrast to the lightning strikes to ground.
IEEE Transactions on Electromagnetic Compatibility 03/2005; · 1.33 Impact Factor
[show abstract][hide abstract] ABSTRACT: SOMMARIO La memoria esamina, con riferimento al panorama italiano, il contributo che impianti termoelettrici a molteplicità di ciclo possono offrire al processo di ripristino del sistema elettrico dopo un black out; ciò in particolare con il fine di qualificare le risorse di primo ripristino procedendo ad una classificazione ed ordinamento delle stesse in vista di una flessibilizzazione della procedura di riaccensione. Il gestore del sistema deve infatti fare affidamento, per l'attivazione di una procedura di ripristino, di risorse costituite da impianti non di sua proprietà e la cui disponibilità potrebbe essere soggetta alle risultanze di un mercato competitivo del servizio di black start. Il tentativo di superamento della rigidità dei piani di riaccensione codificati è ulteriormente suffragato dalla proposta di un sistema di supporto alle decisioni, concepito nel tentativo di fornire, all'operatore del centro di gestione, un costante aggiornamento circa le contingenti condizioni del sottosistema riacceso, allo scopo di poter frequentemente mutare la tattica di proseguimento delle operazioni con minimizzazione del rischio di fallimento del processo e del tempo di recupero.
[show abstract][hide abstract] ABSTRACT: The statistical assessment of the lightning-performances of distribution lines must take into account the presence of protection devices such as surge arresters and shielding wire/neutral groundings. Indeed, the presence of those devices does modify the indirect lightning-response of the line. A statistical investigation based on the Monte Carlo method is carried out on a distribution line with realistic configuration, aimed at assessing the indirect lightning-performance of the line. Accurate models are used to evaluate the lightning performance of the line equipped with surge arresters, in order to estimate the effects of arresters spacing and characteristics. A preprocessing procedure based on the simplified Rusck formula for a first evaluation of the lightning-induced voltages of the randomly generated events is implemented in order to reduce the computational time.
Probabilistic Methods Applied to Power Systems, 2004 International Conference on; 10/2004
[show abstract][hide abstract] ABSTRACT: Voltage transients are the main causes of complaints from medium-voltage customers. For this reason, utilities and independent authorities are focusing their attention on investigation techniques capable of providing information useful for classifying voltage transients depending on their origin. Distributed measurement systems for power quality monitoring able to detect, measure, and classify impulsive transients can be used in coordination with lightning location systems to assess whether there is a relation between a given transient and a lightning event. In this paper, a criterion for evaluating the above correlation is proposed and a numerical example, which is based on real accuracy specifications, is presented. The results show that attempting a correlation between lightning events and induced transients is quite reasonable.
IEEE Transactions on Instrumentation and Measurement 09/2004; · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper describes a procedure based on the Monte Carlo method to obtain the statistical distributions of lightning current parameters at ground level starting from those inferred from experimental data recorded by means of tall instrumented towers. As a matter of fact, it is generally accepted that these distributions are affected by the presence of the tower; lightning current amplitudes, in particular, are "biased" toward higher values, as the tower ability to attract lightning flashes tends to increase for flashes with larger currents. Concerning the current peak distribution, the obtained results match with those presented in previous studies on the subject; for instance, for the Berger et al. distribution, they provide a median value decrease ranging from 20%-40%, depending on the attractive radius expression adopted in the calculation. The proposed procedure is more general than others presented in the literature for the same purpose. It can be applied, indeed, to whatever model is adopted to represent the exposure of the tower to direct strokes and, further, it allows to quantify the tower effect on the statistical distributions of all lightning current parameters of interest, and not only of the peak value one. The statistical distributions at ground, calculated with the proposed procedure, should be considered for power system insulation coordination studies, chiefly for a more representative and significant evaluation of the indirect lightning performance of distribution lines. The obtained results can also be used to assess the performance of lightning location systems concerning the relevant current statistical distributions.
IEEE Transactions on Power Delivery 08/2004; · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: This paper presents a new approach to model a transmission tower for lightning performance studies. It consists of representing each part of the tower by equivalent vertical and/or horizontal transmission lines as required. While horizontal line parameters are obtained from standard line formulas, specific expressions are derived for the parameters of vertical lines. Moreover, mutual coupling between any two parallel vertical transmission lines is taken into account. Transient waveforms obtained using the proposed method are compared with experimental data obtained using a reduced-scale model and field experiments. The agreement between simulated results and experimental measurements is satisfactory.
IEEE Transactions on Power Delivery 05/2004; · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, we investigate the effect of periodically-grounded shielding wires and surge arresters on the attenuation of lightning-induced voltages. We discuss the adequacy of the commonly made simplification of assuming the shielding wire at ground potential, instead of being treated as one of the conductors of the multiconductor system. We also compare then the mitigation effect of shielding wires with that achievable by the insertion of surge arresters along the line. The computation results are first validated by means of calculations obtained by other authors referring to a simple line configuration, and then by means of experimental results obtained using a reduced-scale line model illuminated by a nuclear electromagnetic pulse (NEMP) simulator. One of the main conclusions is that the effectiveness of shielding wires and surge arresters depends mostly on the spacing between two adjacent grounding points or surge arresters.
IEEE Transactions on Power Delivery 02/2004; · 1.52 Impact Factor
[show abstract][hide abstract] ABSTRACT: Voltage transients are the main causes of complaints from medium-voltage customers. For this reason, utilities and independent authorities are focusing their attention on the investigation techniques capable of providing information useful to classify voltage transients depending on their origin. Distributed measurement systems for PQ monitoring able to detect, measure and classify impulsive transients can be used in parallel with Lightning Location Systems to assess whether there is correlation between a given transient and a lightning event. In this paper, we shall analyze the requirements for distributed systems for PQ analysis in order to investigate on the above correlation.
IEEE Transactions on Instrumentation and Measurement 01/2004; 53:1202-1208. · 1.36 Impact Factor
[show abstract][hide abstract] ABSTRACT: The evaluation and the analysis of the lightning electromagnetic pulse (LEMP) response of distribution networks require the availability of accurate models of LEMP-illuminated lines and their implementation into software tools able to calculate lightning-induced electromagnetic transients in distribution systems having complex configuration. This paper deals indeed with a computer code, LIOV (lightning induced overvoltage code), and with two interfaces recently realized between it and (a) the DCG/EPRI EMTP96 on the one hand and (b) the SimPowerSystems program in the Matlab–Simulink environment on the other hand. The aim of these interfaces is to extend the simulation capabilities of the LIOV code, which computes lightning-induced voltages on a single multiconductor transmission line above lossy ground, to the case of distribution systems characterized by complex yet realistic configurations. Models/code validation by means of a comparison with experimental results is also presented.
[show abstract][hide abstract] ABSTRACT: Statistical distributions of lightning current amplitude, time-to-peak and other lightning current parameters used in power system insulation coordination are based on experimental data obtained by means of tall instrumented towers. It is, however, generally accepted that these distributions, which we will call ‘classical’ ones, are affected by the presence of the tower, as the tower ability to attract lightning flashes tends to increase for flashes with larger currents. Current amplitudes are thus biased towards higher values with respect to those which refer to flashes at ground. In this paper, we adopt a procedure recently proposed by the authors to infer the statistical distributions of lightning current parameters at ground level starting from those obtained from data measured using tall instrumented towers. The procedure is first applied to classical statistical distributions of lightning current parameters to quantify the tower bias, and this for various models that have been proposed in the literature to represent the exposure of the tower to direct strokes. Then, the statistical distributions at ground, calculated by applying the proposed procedure, are used to calculate the indirect-lightning performances of an overhead line above an ideal and a lossy ground. The results are also compared with those obtained using the original distributions of lightning current parameters, i.e. by neglecting the bias introduced in the distributions by the presence of the instrumented tower, and a significant difference is found.
European Transactions on Electrical Power 10/2003; 13(6):365 - 372. · 0.63 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper, we present the experimental results obtained at the International Center for Lightning Research & Testing (ICLRT), Camp Blanding, Florida, where currents induced by triggered and natural lightning events were measured at one end of a shielded buried cable, both in the cable shield and in the inner conductor. The horizontal magnetic field was also measured. For the natural lightning events, the recorded waveforms are correlated with the data obtained by the U.S. National Lightning Detection Network (NLDN). The paper provides also a discussion of the correlation between the horizontal magnetic field and the induced current, as well as of the effect of lightning stroke position and distance from the buried cable on the induced disturbances.
Power Engineering Society General Meeting, 2003, IEEE; 08/2003
[show abstract][hide abstract] ABSTRACT: The appraisal of the response of distribution networks to the excitation of lightning electro magnetic pulse (LEMP) requires the availability of accurate models of LEMP-illuminated lines. These models should be able to take into account the presence of shielding wires or neutral conductors and of their relevant groundings. Aim of this paper is the estimation of the influence of the line grounding, with particular reference to the grounding electrode model, on the calculated values of lightning-induced voltages in overhead power distribution lines.
Power Tech Conference Proceedings, 2003 IEEE Bologna; 07/2003
[show abstract][hide abstract] ABSTRACT: The aim of the paper is the analysis of the dynamic behavior of a thermoelectric power plant equipped with a steam unit and a gas turbine during the early phases of a black-start-up maneuver for the network restoration. Models of different detail level for two production units and the relevant regulators have been implemented into two simulation tools. The paper contains the description of the different models, identifies the cases in which the two simulation codes predict similar results and presents the main characteristics, as well as the fields of application of the two codes. As an example, the paper presents also an application of simulators developed for the planning and the preparation of island operation and start up field tests of a power station.
Power Tech Conference Proceedings, 2003 IEEE Bologna; 07/2003