With the increasing expansion of renewable energies, ever more powerful wind turbines are being built, which reach even greater heights. While smaller structures are almost exclusively struck by downward flashes (cloud-to-ground flashes), towering structures from around 100 m (such as modern wind turbines) are mostly struck by upward flashes (ground-to-cloud flashes). The beginning of the downward flashes is characterized by short-lasting, very high so-called impulse currents of over 100 kA, whereas the upward flashes are initiated by long duration currents, so-called initial continuous currents (ICCs). Typically, the (initiating) long duration currents are a few 100 A and have a charge of a few 100 C. They usually last a few 100 ms. These long duration currents can cause damage such as electrical and electronic system failures and malfunctions. In addition, there is metal melting and other damage to equipment and structures (e.g. to gear and blades of wind turbines). Similar damage is observed on aircraft when they are struck by lightning as they approach a thundercloud. In order to protect both (tall) buildings and aircraft from such damage, they are equipped with lightning protection systems. To test the lightning protection systems, the lightning currents and their components are simulated in the laboratory. So-called high current impulse generators are used to simulate impulse currents, while long duration current generators are used to simulate long duration currents. A fundamental requirement here is that the high current impulse and long duration current generators meet the requirements specified in the standard. The essential requirements are defined in the national standard VDE 0185-305-1 (or the international standard IEC 62305-1) and in the standard EUROCAE ED-84. While VDE 0185-305-1 covers the lightning protection of fixed structures, EUROCAE ED-84 describes the lightning protection of flying systems. What both standards have in common is that the hazards from the long duration current are simulated with an approximately constant current with a (maximum) charge of 200 C. In addition to this performance feature, modern long duration current generators are now subject to significantly higher requirements. Recent measurements have shown that higher charge quantities are to be expected on modern, high-rise wind turbines. DIN EN IEC 61400-24 takes these measurements into account by simulating the risk of long duration currents with a charge of 600 C for particularly exposed wind turbines. In addition to the charge, other parameters are important for the risk posed by long duration currents. These are primarily the duration and the current amplitude, which is characterized by the current maximum and the average current. If we look, for example, at the long duration currents of negative upward flashes measured by the University of the Federal Armed Forces in Munich at the Peißenberg Tower, it can be seen that in 5.4 % of cases they have a longer duration than the time of 500 ms specified in VDE 0185-305 1 and in 14.2 % of cases a higher average current than the (maximum) current of 400 A specified there. The difference is even more striking in the case of positive upward flashes, which can have long duration currents of over 20 kA. It is therefore desirable to have a long duration current generator that can simulate both the currents according to standardization and the more extensive requirements due to the naturally occurring lightning currents. With this in mind, a new long duration current generator has been developed that can generate a long duration current which is variable over a wide range in order to be able to reproduce the various influences of the current maximum, the duration and the high charge quantities of natural lightning. In contrast to the usual long duration current generators, which are operated with charging voltages of up to around 1 kV and currents of several 100 A, the new long duration current generator was designed for a charging voltage of up to 2.5 kV. The higher charging voltage enables stable burning of the arc on extended test objects, so that the tests can be carried out with an indirect high-voltage electrode without the disadvantages of the usual use of a direct high-voltage electrode. The high generator voltage is possible because a capacitor bank is used as energy storage instead of the batteries or DC generators that are often used. While the maximum possible long duration current is severely limited by the internal resistance of batteries, for example, the current from the new long duration current generator is only limited by the wiring with corresponding resistors, which is why significantly higher currents can be realized. The new long duration current generator therefore offers the possibility of simulating the extremely high long duration currents of positive flashes with current values greater than 20 kA and charges of over 500 C. The use of a capacitor bank also offers advantages, as it can be automatically earthed and short-circuited, thus improving personal safety. To ensure safe operation and idle mode, the long duration current generator was integrated into the existing safety concept of the high-voltage hall. The high-voltage side with the long duration current generator is separated from the control panel on the low-voltage side by a partition fence. The high-voltage side can be accessed through doors in the partition fence. When the doors are opened, the safety circuit integrated in the high-voltage hall is interrupted. If the safety circuit is interrupted, a control unit automatically switches off and earthes the long duration current generator. The main components of the long duration current generator are the capacitor bank and a variable resistor. The capacitor bank consists of 50 capacitors connected in parallel with a (total) capacitance of 0.22 F. At a maximum charging voltage of 2.5 kV, the charge is 550 C and the generator energy is almost 700 kJ. The variable resistor consists of an internal resistor of around 0.1 Ω and a downstream resistor cascade with which a variable resistance of up to 25.5 Ω can be set. This allows currents of around 100 A to over 20 kA to be realized. The disadvantage of this circuit is that only currents with an exponential drop can be generated. To overcome this disadvantage, current-shaping switches are used which reduce the resistance in stages and thus enable an almost constant current. To finally switch off the long duration current, a short circuit is generated in parallel to the test object. A special switch is used for this purpose, which commutates the current in a parallel circuit to the test object and thus diverts the current around the test object and switches it off in this way. In order to be able to simulate the natural sequence of an initial impulse current followed by a long duration current, the long duration current generator must be combined with a high current impulse generator. An inductance is used for the necessary decoupling of both generators. By using the decoupling inductance, the test object is first subjected to the impulse current and then to the long duration current. The high current impulse generator can be operated with or without a crowbar switch to generate either an oscillating or unipolar current. According to common opinion, only the charge of the long duration current is responsible for melting on metal structures, but not other parameters such as its current strength or duration. In order to verify this assumption, tests were carried out on aluminium plates with a thickness of 2 mm. While the charge was kept constant at 200 C, the long duration current was varied between 100 A and about 5 kA. With uncoated aluminium plates and relatively low long duration currents up to 500 A, only relatively small surface melting was observed, which did not lead to hole formation. If the long duration current was increased to over 1 kA, pitting occurred, which became more pronounced with further increases in current. However, this only applies up to a current value of around 2 kA. At even higher currents, the pitting decreases again, so that interestingly, no more pitting was observed from a current value of around 5 kA. In contrast, significant differences were observed in coated aluminium plates (aircraft paint with a thickness of 300 μm). At relatively low currents (up to around 200 A), only minor melting was initially observed. However, as the current is increased, the melting became more and more pronounced, so that clear hole formation was already visible at current values below 1 kA. In contrast to the uncoated aluminium plates, the hole formation did not decrease at higher currents, but the holes became larger and larger as the current is increased. It can therefore be seen that, in addition to the applied charge, other parameters for melting and hole formation on sheets must be taken into account. These are, on the one hand, the thickness and the material of the applied coating as well as the duration of the long duration current and its current strenght. As an indirect high-voltage electrode was used in the investigations, the melted volume was considerably smaller than in the investigations with a direct electrode known from the literature. This indicates that the melting out caused by long duration currents in natural lightning is significantly lower than previously assumed. However, detailed investigations into this were not carried out as they would far exceed the scope of this study. It would also be necessary to investigate different materials for the sheets with variations in their thickness in order to be able to draw reliable conclusions about the melting processes. This opens up a wide range of possible future research areas.     «

With the increasing expansion of renewable energies, ever more powerful wind turbines are being built, which reach even greater heights. While smaller structures are almost exclusively struck by downward flashes (cloud-to-ground flashes), towering structures from around 100 m (such as modern wind turbines) are mostly struck by upward flashes (ground-to-cloud flashes). The beginning of the downward flashes is characterized by short-lasting, very high so-called impulse currents of over 100 kA, whe...     »