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The appropriate analysis of the response of distribution networks against Lightning Electro Magnetic Pulse (LEMP) – originated by nearby strikes – requires the availability of accurate coupling models in order to reproduce the real and complex configuration of distribution systems. The above models represent a fundamental tool for estimating the number of protective devices and their most appropriate location in order to guarantee a given minimum number of flashovers and outages per year. When dealing with real networks, such an optimization could require huge computational efforts due to the vast number of power components and feeders. This thesis thoroughly analyzes many of the possible engineering simplifications that, without losing accuracy, can be adopted in the statistical evaluation of the lightning performance of distribution networks in order to limit computational times. Particular attention is devoted to the effect of the ground conductivity on the LEMP and on the line parameters; two new analytical expressions for the evaluation of the inverse Laplace transform of the ground impedance matrix elements of multiconductor overhead lines are derived. The first expression is the inverse Laplace transform of Sunde’s logarithmic formula and is given in two equivalent forms. The second expression is the inverse Laplace transform of Sunde’s general integral expression. Finally, a procedure able to evaluate the lightning performance of a real medium-voltage distribution network, which includes several lines, transformers and surge protection devices is developed and proposed for the analysis of some real cases. Such a procedure allows inferring the characteristics of the statistical distributions of lightning-originated voltages at any point and phase of the network. The analysis aims at assessing the expected mean time between failures of transformers caused by both direct and indirect lightning strikes., L'analisi della risposta di una rete elettrica di distribuzione a un campo elettromagnetico esterno generato da una scarica atmosferica richiede l'utilizzo di accurati modelli in grado di riprodurre la reale e complessa configurazione della rete. Tali modelli rappresentano uno strumento fondamentale per la stima del numero di dispositivi di protezione ed il loro appropriato collocamento al fine di garantire il numero minimo annuo di “flashovers” e interruzioni. In una rete di distribuzione reale, tale ottimizzazione può richiedere sforzi computazionali proibitivi a causa dell’elevatissimo numero di componenti di potenza e linee presenti. Questa tesi analizza in maniera esaustiva molteplici semplificazioni ingegneristiche adottabili, al fine di ridurre i tempi computazionali, nella valutazione statistica del numero annuo di guasti di una rete di distribuzione. Particolare attenzione è dedicata agli effetti della conducibilità finita del suolo sul campo irradiato dal fulmine e sui parametri delle linee. Nella tesi sono derivate due nuove espressioni analitiche per il calcolo della trasformata di Laplace inversa dell’impedenza del terreno. La prima è la trasformata di Laplace inversa dell’espressione di Sunde logaritmica ed è proposta in due forme equivalenti. La seconda è la trasformata di Laplace inversa della più generale espressione integrale di Sunde. Infine, si è sviluppata una procedura in grado di valutare la “lightning performance” di una rete di distribuzione in media tensione avente configurazione realistica, che comprende “feeder” principali e laterali, pali, cabine secondarie e dispositivi di protezione contro le sovratensioni. La procedura messa a punto, basata sull’applicazione del metodo di Monte Carlo, permette di calcolare l'ampiezza delle tensioni indotte da fulminazione in qualsiasi punto e in ogni fase della rete. L’attività ha riguardato anche la valutazione del tempo medio fra i guasti (MTBF) di ogni trasformatore MT / BT causati da fulminazione indiretta e diretta, parametro di fondamentale rilevanza per l’ente distributore.

The article proposes an improved method for the calculation of the electromagnetic field radiated by a lightning return stroke over a perfectly conducting ground. Specifically, the proposed time domain expressions allow a closed-form solution for the field produced by an impulse-current propagating upward along a vertical path with a given velocity. The expressions are valid for a wide class of engineering return stroke models and can be adopted also in case the return stroke velocity varies along the channel. The solution for an arbitrary current waveform can be obtained by the convolution of the current at the channel base with the electromagnetic field due to the impulse-current along the channel. The accuracy and advantages of the method are assessed by the comparison with those obtained by a finite-element method computer model. The expressions are implemented in the LIOV-EMTP-RV code for the calculation of the induced voltages in an overhead distribution line.

This paper presents a multistage approach for the day-ahead scheduling of a local energy community (LEC) aimed at considering the uncertainty associated to both the generation from renewable sources and the energy consumption. The considered LEC corresponds to a community of prosumers connected to the same distribution power network. In general, the considered prosumers are equipped with photovoltaic generating units, energy storage units, and local loads. The scheduling of the LEC is defined by a distributed optimization procedure based on the alternating direction method of multipliers (ADMM), which minimizes the total energy procurement costs. The adopted approach adjusts the operational planning according to the current operative conditions by means of a scenario-based multistage procedure. This paper describes the scenario tree generation and an intraday decision-making procedure based on the day-ahead solution. Moreover, numerical tests are presented for several operating scenarios.

The paper deals with the response of distribution networks to direct and indirect lightning. The response of the network is analyzed with the aim of inferring the mean time between failures of the connected transformers. This is accomplished taking into account the voltage at the utility frequency, the flashovers occurrence in the line insulators, the location of surge arresters, and the shielding provided by nearby buildings. Two different transformer failure criteria are assumed: a critical value and a disruptive effect model. The results make reference to multi-conductor overhead systems with two different topologies: simple straight line and a real feeder with laterals. It is shown that such a detailed lightning performance analysis is useful in order to select the most appropriate strategy for the installation of surge arresters, which may allow the achievement of the desired mean time between failures at affordable costs.

This letter deals with the calculation in time domain of the transient ground resistance matrix of an overhead transmission line (TL). Each element of the matrix is evaluated by solving analytically the inverse Laplace transform of the general integral expressions of the ground impedance in frequency domain proposed by Sunde. The presented expressions are suitable for the direct implementation in an electromagnetic transient program (e.g., EMTP-like ones) based on a time domain solution of the TL's equations.