Lightning-induced voltages on power lines: advances in modelling, computational effort optimization and innovative tools for the protection of overhead distribution lines

The continuous growth of MV overhead distribution systems requires a constant improvement in terms of security and power quality. One of the most critical event that can cause the fault of a distribution line is represented by the atmospheric discharges and, among them, the most dangerous one is no-doubt the lightning stroke. In the transmission and distribution systems, lightning transients can be caused by either direct or indirect strikes. Indirect strikes are much more frequent than direct strikes and can cause flashovers, especially when the line insulation level is low. The computation of the lightning induced-voltages (i.e. the one related to the indirect strikes, which represent the most critical issue in distribution systems) is a very complicated task for two main reasons: 1) the number of uncertain parameters is high: it involves a correct representation of the current that flows in the lightning channel as well as a correct representation of the soil conductivity where the power line is located. 2) The computational complexity of the calculations that allow evaluating the final overvoltage is high because in this case we are dealing with the computation of electromagnetic fields and with the effect of such fields on the power line. Concerning the protecting measures the most widely employed are the use of shield wires, surge arrester and the increase of the line insulation level. This thesis aims at improving the problem of the lightning-induced voltages in overhead distribution lines in terms of three main concepts: 1) innovation of the existing models, 2) optimization of the computational effort and 3) introduction of innovative tools for the protecting scheme. In this framework, the thesis proposes a new channel-bae current model (1) , an analytical technique for the electromagnetic fields computation (2) , a new scheme for the lightning-induced voltages computation (2), a new approach for reducing the computational effort of the lightning performance computation (2 and 3) and an innovative approach for the evaluation of the mitigation effect of shield wires on the lightning-induced voltages (3)

Do Wind Turbines Amplify the Effects of Lightning Strikes A Full-Maxwell Modelling Approach

Wind turbines (WTs) can be seriously damaged by lightning strikes and they can be struck by a significant number of flashes. This should be taken into account when the WT lightning protection system is designed. Moreover, WTs represent a path for the lightning current that can modify the well-known effects of the lightning discharge in terms of radiated electromagnetic fields, which are a source of damage and interference for nearby structures and systems. In this paper, a WT struck by a lightning discharge is analyzed with a full-wave modelling approach, taking into account the details of the WT and its interactions with the lightning channel. The effects of first and subsequent return strokes are analyzed as well as that of the rotation angle of the struck blade. Results show that the lightning current along the WT is mainly affected by the ground reflection and by the reflection between the struck blade and the channel. The computed electromagnetic fields show that, for subsequent return strokes, the presence of a WT almost doubles their magnitude with respect to a lightning striking the ground. Such enhancement is emphasized when the inclined struck blade is considere

Approximate characterization of large Photovoltaic power plants at the Point of Interconnection

The aim of the present article is that of proposing a calculation procedure to assess electric quantities at the Point of Interconnection (POI) of large PhotoVoltaic (PV) power plants on the basis of rated data and main design elements of the plant itself. The quantities of inters are active and reactive power available at the POI in order to extrapolate the power plant capability starting from the capabilities of PWM inverters. The procedure also allows evaluating the POI voltage variations, an important element due to the increasing requirements for renewable generation units to participate in voltage regulation. The main interest in such a methodology lays in its simplicity of application, that allows avoiding the usage of dedicated software for load flow calculations, and flexibility, that makes it suitable for the support of the bidding and pre-design phase of large photovoltaic power plants

Primary Voltage and Frequency Regulation in Inverter Based Islanded Microgrids through a Model Predictive Control Approach

A frequency and voltage control strategy based on a decentralized and communication-less approach is proposed in this work and applied to Photovoltaic-Storage-Microturbine islanded Microgrids (MGs). The approach is based on the Model Predictive Control (MPC) technique. Thanks to the use of local measurements, each source can nullify the steady-state voltage and frequency errors by means of a dedicated MPC controller. Consequently, the proposed approach unifies the advantages of classic droop and master/slave controllers due to the absence of communication links among devices and due to the absence of a secondary centralized control loop

Corona Effect Influence on the Lightning Performance of Overhead Distribution Lines

Overhead distribution lines can be seriously damaged from lightning events because both direct and indirect events can cause flashovers along the line. The lightning performance of such power lines is usually computed neglecting the effect of corona discharge along the conductors: in particular, the corona discharge determined by the indirect lightning event is taken into account only by few researchers because it can have meaningful impacts only in few cases. However, when we deal with overhead distribution lines with high Critical Flashover value (CFO) and small diameters, the corona discharge caused by indirect events has to be taken into account. This paper shows the effects of corona discharge in the lightning performance computation of overhead distribution lines. The analysis will involve different configurations in terms of line diameter and air conditions, focusing on the negative effect of corona discharge in the number of dangerous events that determine line flashovers

Analytical Expressions for Lightning Electromagnetic Fields With Arbitrary Channel-Base Current. Part II: Validation and Computational Performance

This article provides the implementation procedure, the validation, and some considerations on the computational efforts of the analytical expressions developed for the lightning electromagnetic fields presented in the companion paper. The validation is presented with different configurations in terms of channel-base current, ground conductivity, and distance to the lightning channel, comparing the obtained results with the numerical integration of the classical formulas. The comparative analysis shows a perfect agreement between the proposed analytical approach and reference numerical simulations. Moreover, the computational effort of the proposed method is discussed, focusing the attention on the choice of the points in which the channel has to be divided in order to maximize the CPU time savings without losing accuracy

A simplified first harmonic model for the Savona Campus Smart Polygeneration Microgrid

Microgrids (MGs) may represent a new answer to many old problems in energy and electric world, such as pollution, high reliability, efficiency and so on. The MG rapid increase implies a fast transformation of the electric power systems that are now characterized by new energy sources requiring dedicated control architectures and simulation tools. Unfortunately, the majority of such new sources are connected to the MG by means of power electronic converters that often make it impossible to replicate some of the concepts, tools and control philosophies commonly used in traditional networks. For this reason, there is a strong need of developing models for MG structures that can be reliable but sufficiently simple to be used for the control system design. In this framework, this paper presents a simplified first harmonic model for the Smart Polygeneration Microgrid (SPM) located in the Savona Campus of the University of Genoa that allows the interface with many controllers. The developed simplified model is validated comparing it with a complete simulation performed in the PSCAD-EMTDC environment that allows to represent each component with a high level of detail

Measurement Campaign and Experimental Results of an Islanded Microgrid

Microgrids (MGs) represent surely the most challenging topic in electrical engineering, promising the decreasing of carbonization, the totally integration of different renewable sources and a cheap solution for those rural areas where electric distribution is economically unsustainable. The most important feature of a MG is represented by the possibility to work in islanded configuration in order to satisfy its own load without the support of the main grid. Literature is poor of experimental results on islanded MG and the research has focused on theoretical approaches on several topics, such as frequency, voltage stability, modeling, software simulations and energy management systems. The purpose of this article is to provide some experimental results such as power supply, voltage and frequency measurement during islanded operation. This test campaign was performed in the Smart Polygeneration Microgrid, a test tool of University of Genova created for experimental validation for grid-connected and islanded MG operation

A new method to evaluate the stability of a droop controlled micro grid

The increasing interest in Microgrids (MGs) leads to the research of more efficient control methods and claims for new methods to assess the obtained working points stability. As the majority of renewable sources are connected to the MG by means of power electronic converters, it is often impossible to replicate some of the concepts, tools and control philosophies commonly used in traditional networks. One of the more frequent control methods in literature is the droop philosophy, whose stability issues have been deeply investigated. In order to evaluate the stability of a droop controlled MG, the present paper presents a new method, whose main advantage is the reduction of the computational effort that reflects in benefits for PLC and control hardware implementation. Simulation tests for validating the suggested technique are provided highlighting its good performances