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Wind turbines (WTs) can beseriously 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 considered. [ABSTRACT FROM AUTHOR]

Lightning electromagnetic (EM) fields are typically computed by assuming the current propagation within a straight vertical channel of negligible cross section. However, it is well known that the lightning path is tortuous and this can significantly affect the radiated fields and the lightning-induced voltages on power lines. Lupò et al. (2000) proposed an analytical method evaluating the EM fields radiated by a tortuous channel assuming a step current source; to consider an arbitrary (realistic) current source, a slow-converging convolution integral was applied. The aim of this article is to reduce the computation time of such procedure by extending to the tortuous channel case the approach proposed by Brignone et al. (2021) for the evaluation of the lightning EM fields with arbitrary current source, originally developed for a straight vertical channel. The performances of the present method are evaluated by means of comparison with the solution proposed by Lupò et al. (2000), showing an excellent agreement with a significant reduction of the CPU effort. This in principle will allow the computation of lightning-induced overvoltages at both individual and statistical level without a prohibitive computational effort. [ABSTRACT FROM AUTHOR]

The Fourier transform (FT) of the electric field (EF) radiated by a lightning return stroke is shown to include a distributional term at zero frequency, which prevents numerical algorithms, such as fast FT, from working properly. Inspired by the finite duration of any field measurement in real applications, we introduce a time-windowing strategy, whereby the integration time-interval of the FT becomes bounded. As a result, the distributional term is removed from the FT, whereas the functional part of the FT is modified throughout the whole range of frequencies, in such a way that a good performance of the numerical algorithms can be restored. The dependence of the modified spectra of the EFs on the length of the time window is also investigated and three numerical experiments with different channel-base currents are performed to validate the whole resulting framework. [ABSTRACT FROM AUTHOR]

Inverter based islanded MicroGrids (MGs) are very fascinating but challenging ways to integrate Distributed Energy Resources (DERs) in electric networks. Active and reactive power regulation is a major issue due to the necessity of optimally exploiting the sources characteristics and controlling voltage and frequency. Focusing the attention on the reactive power control, the main needs are to regulate voltage at suitable values and ensure a proper reactive power sharing among the energy sources. The conventional control for reactive power presents relevant drawbacks and so an Advanced Droop Control strategy (ADC) is presented to approach primary voltage control and reactive power sharing among Grid-Supporting inverters in islanded MGs. The ADC strategy is presented defining its control law and the control scheme with a stability analysis. Then, an analytical procedure is developed to set the controller parameters. Following, a comparison between the ADC and droop control is performed with dedicated simulations on a common benchmark Microgrid in order to highlight the advantages of the proposed control strategy. Finally, a comparison with other advanced control techniques is proposed in order to show that ADC is able to guarantee improved performance in terms of the combined regulation of voltage and reactive power. [ABSTRACT FROM AUTHOR]

Dealing with lightning performance computation, researchers usually consider only negative downward first return strokes and the relevant channel-base peak current distribution. The aim of this article is to account for the lightning flash mechanism, which means to include also downward negative subsequent strokes and downward positive strokes, taking into account available distributions of the number of strokes per flash (i.e., the flash multiplicity) and of the distance among the impact points of the strokes within the same flash (i.e., the channel terminations distance). In this regard, to the best of the authors’ knowledge, the present study is the first one taking into account the multiple ground channel terminations in the lightning performance assessment. The obtained results show that both the negative subsequent strokes and multiple channel termination determine a significant increase in the estimated number of flashovers per year, and they need to be considered in the lightning performance assessment. On the other hand, considering positive downward strokes, a less remarkable increase in the estimated number of flashovers per year is observed. [ABSTRACT FROM AUTHOR]

This article proposes a method for the derivation of a formula that links the maximum lightning-induced-overvoltage occurring on an overhead distribution line with the variables that characterize the lightning return stroke, such as the peak current, front duration, and stroke location. The coefficients of such formulas are obtained thanks to a limited number of lightning field-to-line coupling simulations, thus guaranteeing a significant computational time saving with respect to already existing methods. The method is presented first with a preliminary analysis to get some insight into the functional relationship between each of such variables and the overvoltage. Then, the method is presented and validated against numerical calculations performed with a dedicated code, both in terms of punctual evaluations and overvoltage distribution. [ABSTRACT FROM AUTHOR]

The article provides analytical expressions for the electromagnetic fields generated by a lightning return stroke characterized by a channel-base current with arbitrary time waveform, in presence of either a perfectly conducting or a lossy ground, assuming the transmission line model for the current along the channel. In this second case, a time-domain analytical expression for the Cooray–Rubinstein formula is presented. The main idea that leads to the derivation of analytical formulas consists of dividing the channel into intervals in which the distance between the field source point and the observation point can be approximated with a linear function of the time, and of the spatial coordinates of both points. In the companion aticle, a detailed comparison is proposed with the classical (numerical) approach highlighting an excellent agreement both at close and far distances, considering all the values of practical interest for the ground conductivity. Moreover, the method guarantees a meaningful improvement in computational performance. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article provides a new channel-based current function, which aims at faithfully reproducing the salient parameters of lightning currents. All the model parameters can be set in a fully analytical way, guaranteeing the possibility of changing them (according to the available measured data) without any numerical procedures. Closed-form solutions of its integral, Fourier transform, and specific energy are proposed, as well as a detailed comparison with the most widely used models, focusing on the advantages of the proposed method. Finally, the specific parameters of the new function to fit with the commonly used measured data sets and standards are provided. [ABSTRACT FROM AUTHOR]

One of the main issues that has to be faced when evaluating lightning-induced overvoltages on a realistic network configuration is to implement the algorithm that solves the field-to-line coupling problem into an electromagnetic simulator that is able to represent with a high level of detail all the power system components. On one hand, numerical stability conditions of the finite-difference time-domain schemes have been deeply investigated in the presence of boundary conditions known analytically; on the other hand, there is not so much literature on the situations in which such conditions are provided in a numerical way through an external electromagnetic simulator. The study reveals that the commonly used linear extrapolation of currents at the line extremities might rise to numerical instabilities. An efficient solution using the method of characteristics is presented and validated. [ABSTRACT FROM AUTHOR]

This paper proposes a novel methodology for emergency control acting on the mechanical power in a power grid containing steam power generators that experiences a contingency. The control strategy is based on model-predictive control (MPC), a model-based control that is widely adopted for the regulation of linear and nonlinear dynamical systems in various fields of application. The MPC is applied in a decentralized formulation, where only local measurements are necessary. The performances of the proposed control strategy are assessed by means of simulations on a Western Electricity Coordinating Council test case network and on the 39-Bus New England network. The results highlight both the good behavior of the MPC control technique, even in conditions that traditional emergency control would not support, and the robustness of the proposed structure against power system modeling approximations. [ABSTRACT FROM AUTHOR]

Gas turbines are raising nowadays growing interest in the power generation industry. Being able to regulate their power production faster than any other form of conventional source, they are extremely valuable in compensating stochastic power fluctuations deriving from renewable energy sources in large-scale electrical grids, preserving grid stability, etc. In this context, gas turbines’ dynamic performance plays a key role and it needs to be enhanced (however preserving the machine lifetime at the same time). Nonlinear model based control systems can effectively deal with this problem, exploiting the system structure to obtain optimal dynamic performance. Therefore, this paper aims at developing a sliding mode model based control for gas turbines. The results obtained from its application on a heavy duty Ansaldo gas turbine model (AE94.3A4) are compared to the ones obtained to the traditional control employed in these applications. [ABSTRACT FROM AUTHOR]