Lightning Protection of Large Wind-Turbine Blades

We discuss in this chapter the salient issues related to lightning protection of large wind turbine blades. Lightning protection of modern wind turbines presents a number of new challenges due to the geometrical, electrical and mechanical particularities of turbines. Wind turbines are high structures and, like tall towers, they not only attract downward flashes but initiate upward flashes as well. The proportion between these types of flashes depends on many factors such as the structure height and the local terrain elevation. The rotation of the blades may also trigger lightning and result in considerable increase in the number of strikes to a wind turbine unit. Since wind turbines are tall structures, the lightning currents that are injected by return strokes into the turbines will be affected by reflections at the top, at the bottom, and at the junction of the blades with the static base of the turbine. This is of capital importance when calculating the protection of internal circuitry that may be affected by magnetically induced electromotive forces that depend directly on the characteristics of the current in the turbine. The presence of carbon reinforced plastics (CRP) in the blades introduces a new set of problems to be dealt with in the design of the turbines’ lightning protection system. One problem is the mechanical stresses resulting from the energy dissipation in CRP laminates due to the circulation of eddy currents. The thus dissipated energy is evaluated and recommendations are given as to the number of down conductors and their orientation with respect to the CRP laminates so that the dissipated energy is minimized. It is also emphasized that the high static fields under thunderclouds might have an influence on the moving carbon fiber parts. Representative full scale blade tests are still complex since lightning currents from an impulse current generator are conditioned to the electrical characteristics of the element under test and return paths. It is therefore desirable to complement laboratory tests with theoretical and computer modeling for the estimation of fields, currents, and voltages within the blades

On the evaluation of the effective height of towers ::the case of the Gaisberg tower

In this paper, we present the calculation of the effective height of the Gaisberg tower using several models in which the Gaisberg Mountain is represented by either simplified geometrical shapes (hemisphere and hemiellipsoid), or by using the actual 3D topography. The procedure to estimate the effective height based on the comparison of the electric field at the top of the tower located on flat and mountainous terrains is studied in detail for the three considered representations of the mountain. For each case, the electric field is computed numerically using the finite element method. We show that the use of the actual 3D topography of the mountain surface results in a very low value of the effective height in the range of 200 - 300 m, which results in an underestimation of the total number of flashes to the tower. The obtained results give some concerns about the applicability of this method of estimation for the effective height

A model for the evaluation of the electric field associated with the lightning-triggering rocket wire and its corona

In this paper, we analyze the electric field at ground level during the first stage of triggered lightning experiments, i.e., during the rocket ascent and prior to the lightning initiation. At distances of some tens of meters from the triggering wire, the electric field decreases significantly, while at distances of several hundred meters, there is only a very small decrease of the electric field. Two effects determine the level of the electric field reduction: the corona layer at ground level and the corona sheath around the triggering wire. We present an analytical solution based on the charge simulation method to study the phenomenon. The model is validated by comparing its results to those obtained by numerical simulations using the finite elementmethod. A ground space charge layer and a corona sheath around the rocket-triggered lightning wire are included in the simulation. It is shown that, depending on the charge distribution, the change of the sign of the electric field is correctly predicted by our model. The obtained reductions of the electric field are consistent with simulations and experiments presented in the literature. Moreover, the proposed analytical solution is faster, and it allows studying the influence of several parameters simultaneously, i.e., the radius of the corona sheath and the space charge layer parameters. The described analytical model allows the estimation of the corona sheath radius if the parameters of the space charge layer are known from experiment

An analysis of the initiation of upward flashes from tall towers with particular reference to Gaisberg and Säntis towers

In this paper, we present an analysis of the lightning events preceding initiation of upward lightning flashes from the Gaisberg and the Säntis Towers. It is found that the majority of upward lightning discharges from both towers are initiated without any preceding lightning activity. We show also that the results of the presented studies on the initiation of upward flashes from tall structures might be affected by the selected parameters of the study, namely the time and distance intervals used to identify the triggering events. Preceding events had the same polarity as triggered flashes in the case of the Säntis Tower and had opposite polarities in the case of the Gaisberg Tower. The effect of seasonal and temperature variations have been also analyzed

Correlation vs. causality in other-triggered upward lightning in tower flashes

Upward lightning flashes from towers are sometimes preceded by other lightning activity in the vicinity of the tower. This observation has led to a classification of upward tower lightning into self-initiated, which are not preceded by nearby lightning activity, and other-triggered, for which one or more CG or IC flashes occur within a given preceding interval and within a given distance from the tower. The causality relation between other-triggered flashes and the preceding activity has not been established. In this paper, we hypothesize that at least some of the activity prior to other-triggered tower flashes can be explained as being due to chance rather than causality

Influence of the lightning triggering rocket wire and its corona on the electric field at ground level

In this paper, we present analytical calculations and numerical simulations of the vertical component of the electric field close to a rocket-triggered lightning site during the ascent of the rocket and prior to the lightning initiation, taking into account the corona layer around the wire. In the analysis, the background electric field was modeled according to the available experimental data. The influence of the space charge layer extending upwards from the ground level was modeled and analyzed. The obtained values of the electric field reduction are shown to be consistent with experimental observations. The presented model can also be used to estimate the charge distribution in the rocket wire and the current flowing into the wire from the ground