Lightning-induced voltages.

The LISEP activity is coordinated with that of CIGRE Working Group C4.1 ‘Lightning’, with that of the JWG CIGRE C4 – CIRED 05 ‘Protection of MV and LV networks against lightning’ and with the IEEE WG ‘Lightning performance of distribution lines’ and is carried out within the framework of an international research collaboration among the University of Bologna (Department of Electrical Engineering), the Swiss Federal Institute of Technology (Laboratoire de Reseaux d’énergie électrique) and the University of Roma ‘La Sapienza’ (Department of Electrical Engineering). In the last years, LISEP has extended the collaboration to other Italian and foreign universities, research centers and companies: University of Florida, Lisbon Polytechnic, University of Uppsala, ENEL-CESI, University of Armed Forces in Munich, University of Arizona, EdF-Electricité de France, University of Sao Paulo, University of Pisa and University of Naples.
 

	Lightning Electromagnetic Pulse (LEMP) characterization The research activity concerns either the frequency domain [1] and the time-domain [2] LEMP characterization. One of the goals is the development of engineering lightning return-stroke models for the spatial-temporal description of the lightning current along the channel during the return-stroke phase. The “Modified Transmission Line (MTL) model” [3] is one of the results of such an activity. Such a model, whose first experimental validation is presented in [4], has been adopted either by ENEL and by EdF for the LEMP calculations. It is compared with other ones presented in the literature and with available experimental results in [8,9]. The research has been aimed also at gathering new experimental results for LEMP characterization and model validation: they have been obtained in occasion of some experimental campaigns carried out in collaboration with the Swiss Federal Institute of Technology and with the University of Florida [29,39,41,45]. The MTL model has been extended in [10] to take into account the decrease of the return stroke velocity during the return-stroke phase, and used for the calculation of the horizontal component of the electric field in [11]. In [23] it has been further extended to the case of an elevated strike object, representing instrumented towers; its validation by means of experimental results gathered at the Munich University of Armed Forces is presented in [48]. Additional analysis of Electromagnetic fields radiated by elevated strike structures is presented in [66]. In [32] some results on the subject, obtained within the framework of Cigré WG ‘Lightning’ are presented.
	Critical analysis of LEMP-to-transmission line coupling models and their extension to overhead distribution lines above a lossy ground. The research activity is aimed at: Comparing the existing coupling models for the estimation of LEMP-to-transmission line coupling (models which appear formulated in different ways), an issue which has been and still is of great interest in the power/lightning literature. Of the several contributions presented at international conferences and published on reviewed journals [13,14-16,17,20,21,22,24,26,31,35,38,106,115,116,120,122], the most significant are considered to be: [20] in which the theoretical and experimental activity carried out shows that only one of the proposed models — and its equivalent formulations — is adequate for the problem of interest; [38] where it is clarified that the electromagnetic field components appear to have a different role in the coupling mechanism depending on the adopted formulation, and [35], which provides additional clarification on the induction mechanism. Analyzing the role played by the soil resistivity [37,47,49]; the effect of such a parameter is thoroughly discussed in [42] e [57], where it is concluded that depending on the stroke location and on the observation point the soil resistivity can result in an enhancement of the induced voltage amplitude with respect of the case of a lossless ground, due essentially to the its effect on the exciting LEMP. In [61] a simplified method is proposed, aimed at the evaluation of the induced voltages for multiconductor lines above a lossy soil. The critical analysis of the multi-conductor line resistance matrix expressions proposed in the literature is carried out in [95,110,118], where a new analytical expression for time-domain solution of the coupling equations is proposed.
	Analysis of the lightning- induced voltages and development of computer codes for their evaluation on power networks with realistic configuration.   LIOV code The main effort of the activity is devoted to the development of a computer code, LIOV (“Lightning-induced overvoltages”), which allows for the calculation of lighting-induced voltages on multiconductor lines above a lossy soil as a function of line geometry, lightning current wave shape, return-stroke velocity, soil resistivity, etc.. The theoretical background of the LIOV code is presented in [15], [42], [33], [43] and [50]. For the LEMP calculation, the developed MTL model is used [3]; concerning the electromagnetic coupling, the model implemented, based on the transmission line theory and on the formulation by Agrawal et al. is described in [13,42,33,50,69]. Experimental validation of the LIOV code is performed and presented in [5,13,20,65]. Application of the code for the evaluation of LEMP-induced voltages in electrical systems of different type can be found in [6,7,12,16,18,37,43,44,47,70]. In [43] some issues concerning the effectiveness of shielding wires (assumed grounded only at the line terminations) as well as some theoretical aspects of the problem are discussed and clarified. Leader induction effects are discussed in [25,50,58]; while the influence on corona effect is analyzed in [33,46,59,69,101,125]. Both leader and corona effect are shown to be important for particularly close or severe strokes and both can result in amplitude enhancement of the induced surges. The LIOV code has been suitably modified to deal with the above cases. A further enlargement of the code which allows for the treatment of non linear and/or capacitive line terminations has been accomplished in cooperation with CESI [19]; in [28,36] the LIOV code has been employed, in cooperation with ENEL, to perform a first analysis of the response of MV overhead lines to lightning induced overvoltages. A further improvement of the LIOV code has been achieved by implementing a modification to the model by Agrawal et al., which allows for the treatment of LEMP-illuminated multiconductor overhead lines above a lossy ground with multiple groundings of the shielding wire or the neutral conductor (if any) or of the phase conductors through surge arresters or spark gaps [76,87]. The experimental validation of such an improved LIOV version, based on a 2nd order finite difference integration scheme, has been accomplished by means of tests carried out on a reduced scale model illuminated by the NEMP simulator of the Swiss federal institute of technology 'SEMIRAMIS'. [71,113].   LIOV-EMTP code Although the LIOV code can be in principle enlarged and adapted to deal with any type of network configuration, it has been considered it more straightforward to link it with the Electromagnetic Transient Program (EMTP), making thus it possible to deal with distribution networks any complex. This research, initiated in cooperation with ENEL-CESI, has brought to the development of the so-called LIOV-EMTP code [27], which has been used within the framework of an international Cigré-Cired Task force for the analysis of the LEMP-induced disturbances on LV distribution networks having complex topology and, in cooperation also with the University of Pisa, for the estimation of the overvoltages transferred from MV to LV through distribution transformers [30,34,51,59,53]. The LIOV-EMTP code, has been then improved, still in cooperation with ENEL-CESI, to take into account the presence of the stationary industrial-frequency voltage [54], and of such an activity it has been referred at the Cigré general session in Paris, in 1996 [44]. The LIOV-EMTP code has been compared with a similar version of it, subsequently developed at EDF [27], and a joint cooperation research project aimed at interfacing the recent version of EMTP (EMTP96) with LIOV has been presented in [62]. The new version of the LIOV-EMTP code (LIOV-EMTP96) has been realized at the University of Bologna, in cooperation with the EPFL [71,76,80,85]. Compared to the previous version of the LIOV-EMTP, the new code is provided with a more stable integration routine, does not imply any modification to the source code to be realized, and allows for a more straightforward construction of the simulation case. A MATLAB version of it has been realized too (Mat-LIOV) which exploits the capabilities of the Power System Blockset [74,76,80]. These codes have been tested versus experimental results obtained through an experimental campaign carried out for such a purpose in Switzerland [82,85]. LISEP participates to the experimental activity carried out at the ICLRT (International Center on Lightning Research Tests), Camp Blanding, Florida, each year. During the two summers of 2002 and 2003, additional experimental results useful for code validation have been gathered on a unenergized distribution line, and some tests on underground cables have been accomplished too [89]. The results of the research activity have been presented also at invited lectures and tutorials at various international conferences [79,100,119,124,126,129].
	Insulation and protection coordination of overhead distribution lines with reference to indirect lightning for power quality improvement. This research activity is aimed at developing a procedure, based on previously mentioned models and on the Monte Carlo method, which allows the estimation of the lightning performance of a distribution line, expressed in terms of number of flashover per year VS the line critical flashover voltage [60,64,105,107], and the choice of the most appropriate means for reducing the number of flashovers (use of shielding wires, surge arresters, etc.). The procedure developed allows for the estimation of the lightning performance of a distribution line as a function of several parameters, such as the soil resistivity [60,64], the lateral distance expression [112], the influence of the return stroke speed [67], taking into account possible correlations between such a parameter and the current amplitude. The proposed method is compared with others presented in the literature and with the one proposed by IEEE (Std. 1410) [60,102,114,121] showing its more general capabilities. In [86] the influence of the line grounding electrodes modeling on the line performance has been discussed. Clearly, the activity takes advantage of the availability of the LIOV-EMTP96 code, described above. Another topic faced within this research activity is the determination of the correlation between voltage sags and lighting events in power distribution networks, which is carried out also in cooperation with CESI. There are a number of reasons for voltage sags in distribution networks: there is some evidence, however, that in electrical systems located in regions with high value of isoceraunic level, lightning can cause the majority of voltage sags. Data from Lightning Location Systems, which provide an estimation of both lightning flash location and return-stroke current amplitude, can then be used to understand whether lightning is indeed the real cause of circuit breaker operation during thunderstorms – which means, in turn, of voltage sags – or not. Due to the complexity of the problem, the information coming from LLS (lightning location and estimate of lightning current amplitude) are, in general, not enough to infer the origin for voltage sags. It is necessary to suitably integrate them with data from system monitoring, e.g. relevant to the intervention of circuit breakers in primary substations, and with simulation results from of accurate models for computing lightning–induced voltages on complex power networks. First results on this line of research are published in [75].
	Critical reassessment of statistical distributions of lightning current parameters. Statistical distributions of lightning current parameters are one of the fundamental inputs needed for the statistical assessment of the lightning performance of transmission and distribution lines. Several researchers have devoted many efforts in the past years to obtain meaningful statistics, all based on direct current measurement data gathered by means of instrumented towers. They still serve as conventional input for insulation coordination studies (e.g.: Berger et al., Electra, 41, 1975; R.B. Anderson and Eriksson., Electra, 69, 1980). Nowadays, some questions concerning the need for revisiting these statistical distributions, especially concerning the current amplitude, appear to exist, and this for a number of reasons, among which: - the large amount of data gathered by means of lightning location systems provides statistical distributions of the current peak amplitude that do not agree, in general, with the above mentioned ones; - current waveforms directly measured using instrumented towers could have been ‘altered’ in some cases by current reflections occurring at both tower top and bottom [72,76,82,90,96,97,99,109,128,123]; - ‘conventional’ statistical distributions of lightning current peak amplitude have been gathered using tall-instrumented towers relative to flat ground or so, and are therefore ‘biased’ towards higher amplitudes. A procedure based on the Monte Carlo method, which allows 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, is proposed in [88]. The proposed procedure is more general than others presented in the literature for the same purpose, in that it can be applied whatever model is adopted to represent the exposure of the tower to direct strokes, and because 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, have been applied in [78] for the assessment of the lighting performance of distribution lines. The obtained results find also a useful application for the assessment of current statistical distributions based on data from lightning location systems. Also for this subject the results of the research activity have been presented also at invited lectures and tutorials at various international conferences [98,108]

Distributed measurement system for power quality monitoring.

This activity is carried out in close cooperation with the Electrical and Electronic measurement group, within the framework of a research program supported by the University of Bologna (Funds for Selected Research Topics).

The research activity is developed within the research project entitled ‘Study, project and realization of an automatic measurement system prototype with innovative characteristics for the power quality analysis’. Aim of this research is the realization of a distributed measurement system able to perform measurements, relevant to the power quality, in the nodes of a distribution power system. In particular the LISEP is involved in the analysis and characterization of a-periocal disturbances. Within the research activity the LISEP has collaborate to the development of the hardware relevant to the instrumentation having complex and distributed architecture for the analysis of the power quality. In [92] are presented the theoretical and technological aspects relevant to the to the realization of measurement systems having distributed architectures. It is analyzed the possibility of use of virtual instrumentation with complex architecture in which the measurement hardware can be composed by different commercial instruments and data acquisition systems suitable interfaced and managed by a central unit which provide the on-line and off-line elaboration and analysis of the acquired signals. In the research activity published in [93] is presented a method aimed at the simultaneous use of data provided by distributed measurement system (having the above mentioned complex and distributed architecture) and data provided by lightning location systems for correlating lightning events with distribution systems a-periodical disturbances.