Michael Rock

Applying of surge arresters in power electronic network components

The increased use of renewable energy sources and connection of energy storage systems to energy distribution networks of the future on the basis of DC voltage demands new power electronic modules with a higher degree of efficiency, reliability and with materials saving. In this article, the overvoltage protection concept on the basis of metal oxide varistors (MOV) for such network components as pulse-width modulation (PWM) modules and DC-to-DC converters as well as a new patented, innovative power-electronic topology concept [1, 2], which allows the regenerative energy sources connect direct to the medium-voltage distribution system is introduced. The distorted voltages from converters and from other power electronic network components are characterized by high-frequency pulsating voltages which generate leakage currents because of MOV capacitance. This leakage current in its turn can overheat the MOV and accelerate its thermal ageing. The main focus of this work is to study the distorted voltages in already mentioned network concepts and its influence on installed MOV and vice versa. The concept of overvoltage protection for some network components is presented. Additionally, an effort to combine the overvoltage protection system with some other useful functions for converter, such as definition of reference potential of the internal ground, was fulfilled.

Initial investigation of influence of wind farms to lightning events

Wind turbines are very exposed structures to lightning due to their height and installation in unshielded areas and are therefore expected to influence the local lightning activity. However, it is unknown in what quality and quantity lightning is influenced. The following paper presents a study conducted at wind farms in Germany showing to what extent lightning activity is influenced by wind turbines. Conducting the analysis all lightning strokes of more than 50 wind farms over the period of 10 years were analyzed before and after the construction of the considered wind farms using the lightning detection system BLIDS [1]. Thereby both onshore and offshore wind farms are investigated. Examining lightning density, amplitudes and probability distributions of the considered wind farms a significant increase in lightning frequency was observed inside the impact area of the wind turbines both for onshore and offshore wind farms. With regard to amplitudes of negative lightning strokes no specific conclusion could be drawn for onshore wind turbines while amplitudes at offshore wind farm increased extremely. The data obtained during current initial investigation can be used for further study to estimate the average number of lightning flashes to a single or a group of wind turbines. That can help to design a sufficient lightning and overvoltage protection system for wind turbines. However, one should take into account that detection efficiency (DE) of lightning detection systems for ground-to-cloud flashes (upward lightning) in special cases is reduced compared to cloud-to-ground flashes (downward lightning). In [2] a DE of 43 % is reported and explained by the fact that the ground-to-cloud lightning currents often are an initial continuous current only (ICCOnly), free from any superimposed impulse currents and therefore cannot be detected by lightning detection system at all. However, this type of flashes could pose a risk to wind turbines because of their enormous high transfered charge values which can easily exceed 300 As. This specific type of ground-to-cloud flashes cannot be considered in this study because of reason mentioned above.

Specifics by measuring of lightning current with large Rogowski Coil

In this paper our experience of designing and using of a large Rogowski Coil (RC) for measuring of lightning currents is described. Some inconveniences regarding the use of large RC with circumference about 6 m to 20 m, namely closeness to resonance frequency during measuring, loss of low-frequency components of the current impulse tail part can be eliminated by presented here measuring circuit. Basically, this circuit represents a new approach of lightning current measuring, which was tested in laboratory and during real lightning events on a communication tower. The results are depicted and discussed in detail in this paper. Moreover, the possibility to measure both impulse and continuous current (CC - upward lightning from high measuring object) from the same RC is considered. Some time and frequency domain network simulations were carried out in EMTP-ATP and compared with transient and frequency response measurements. Our scientific investigations were simplified by using scripts, initially developed in FAMOS and in MathCAD. The algorithms and results of their work are presented here as well. Also, in order to obtain precise measurements, some measures against electromagnetic interferences have been realized.

Electronic circuit for accurate measuring of lightning continuous currents sensed by Rogowski coil

This paper is dedicated to the question of measuring lightning continuous currents on the basis of a Rogowski coil. A precise capturing of lightning continuous current (CC — upward lightning from tall objects [1]) via Rogowski coil is one of the most difficult issues. First of all, the parameter ranges of CC should be known in order to design of precise measuring equipment on the basis of Rogowski coil. One of such important parameter of CC is its range of rate of change di/dt. The necessary data of CC has been obtained by real measurements carried out at ORS Gaisberg communication tower in Austria [2], [3], [4]. Also, the common difficulties by measuring of CC on the basis of Rogowski coil, such as superimposing of lightning impulse currents during CC, appropriate time constant for integrator, are discussed. Initial investigation by designing of appropriate measuring circuit was done with help of network simulation software. Then the optimized circuit was built and tested in the lab. The results of simulations as well as measurements are presented.

Investigation of lightning parameters occurring on offshore wind farms

The following study conducted on offshore wind farms (WFs) located in the North Sea and in the Baltic Sea. The lightning strokes of 13 WFs over the period of about 8…10 years were analyzed before and after their construction using the lightning location system (LLS) of EUropean Cooperation for Lightning Detection (EUCLID) [1]. Many WFs were excluded from the observation because they were constructed just recently. A detection efficiency (DE) of any LLS for ground-to-cloud flashes (upward lightning) in some cases is very low compared to cloud-to-ground flashes (downward lightning). In [2] the DE of 43% is reported and explained by the fact that the ground-to-cloud lightning currents are often an initial continuous current only (ICConly), free from any superimposed impulse currents and therefore cannot be detected by LLS at all. This type of flashes is often occurred at tall structures (> 100 m) and could pose a risk to wind turbines (WTs) because of their enormous high transferred charge values which can easily exceed 300 As. This specific type of ground-to-cloud flashes are not covered by this study because of the limitation of the LLS. The data obtained during current initial investigation can be used for design and test of lightning protection system (LPS) for WT. The IEC 61400-24 Ed.2.0 (CD) [3] recommends to consider the same maximum values of lightning parameters for offshore WT as for onshore WT, since “there is no evidence of lightning parameters being significantly different offshore”. Our investigation clearly shows that this is not the case. Also, the distance from the coast has an influence on the lightning parameters. Finally, cumulative probability distributions of lightning current peak values obtained for offshore WFs are compared with corresponding curves for onshore WFs presented in [4].

Approach for evaluation of lightning current distribution on wind turbine with numerical model

Wind turbines are increasingly becoming an important source of energy for many power systems worldwide. Reliable and safe operation of the turbines is becoming more and more important. Due to their height and exposed location wind turbines are defined as very exposed structures to lightning. A reliable lightning protection system (LPS) for wind turbines is therefore essential. Designing the overvoltage protection system of a wind turbine the lightning current distribution and overvoltages inside the turbine needs to be analyzed. For this purpose the real structure of a specific standard wind turbine is considered. Based on the geometry and material parameters an equivalent circuit model is developed in EMTP-ATP. Using this model, the current distribution and overvoltages can simply be calculated. In the following paper this model is used to investigate the voltage stresses occurring inside the turbine due to direct lightning strike into a rotor blade. Based on the simulation results different overvoltage concepts for the electrical and IT systems are analyzed including the investigation of the power electronic converter of a full converter wind turbine. Ultimately an overvoltage protection concept is suggested that reduces voltage stresses for the electrical and IT system as well as the power converter according to standard [1].

Evaluation of lightning protection systems proposed for small structures by electromagnetic simulation

This paper validates the performance of low-cost lightning protection systems for small structures proposed by researchers in the past. Such structures have an acute demand in the countries with very high lightning ground flash density, yet the affordability of the mass public is quite limited due to the struggling economies of the countries. The protection systems for small housing structure, and a standalone protection structure for one or few people have been investigated by implementing the structures in HFSS/ANSYS software which employs finite element method. By applying current waveforms to represent first negative return stroke, subsequent negative return stroke and positive return stroke, the electric field and potential gradient of the entire space and the current density and the thermal profile of the protective structure have been computed. The objectives were to find whether the voltage distribution in the wake of a lightning strike could initiate side flashes, generate touch potential and step potentials that exceed dangerous levels, drive current densities due to which the structure collapse under thermal effects. It has been found that the proposed protective structures could suppress side flashes and harmful effects due to step potential while the structure will be able to withstand the heat generated. However, the touch potential could still be beyond the human injury thresholds, thus a minimum separation from the lightning current passage should be advised to the occupants.

Detailed calculation of interception efficiencies for air-termination systems using the dynamic electro-geometrical model — Practical applications

Interception efficiency (IE) is the most important parameter to show the effectiveness of air-termination systems. The dynamic electro-geometrical model (DEGM), a numerical method, is capable of calculating such interception efficiencies. This model is purely based on international accepted models, parameters, deviations and dependencies, which are also comprised in the IEC 62305. So far it has been used to calculate the interception efficiencies for rod-type air-terminations. This paper discusses applications using the DEGM. A detailed analysis shows that for a rod-based air-termination system the IE is much better than expected. This leads to the idea of a comparison analysis between an air-termination system purely planned according to the standardized “rolling-sphere” method and an “optimized” air-termination system based on the DEGM.

Development of a head-phantom and measurement setup for lightning effects

Direct lightning strikes to human heads lead to various effects ranging from Lichtenberg figures, over loss of consciousness to death. The evolution of the induced current distribution in the head is of great interest to understand the effect mechanisms. This work describes a technique to model a simplified head-phantom to investigate effects during direct lightning strike. The head-phantom geometry, conductive and dielectric parameters were chosen similar to that of a human head. Three layers (brain, skull, and scalp) were created for the phantom using agarose hydrogel doped with sodium chloride and carbon. The head-phantom was tested on two different impulse generators, which reproduce approximate lightning impulses. The effective current and the current distribution in each layer were analyzed. The biggest part of the current flowed through the brain layer, approx. 70 % in cases without external flashover. Approx. 23 % of the current flowed through skull layer and 6 % through the scalp layer. However, the current decreased within the head-phantom to almost zero after a complete flashover on the phantom occurred. The flashover formed faster with a higher impulse current level. Exposition time of current through the head decreases with a higher current level of the lightning impulse. This mechanism might explain the fact that people can survive a lightning strike. The experiments help to understand lightning effects on humans.

Relationship between self-inductance and mutual inductance of down-conductors

The relation between lightning current steepness di/dt and the inductive voltage into installation loops formed by down-conductors of lightning protection system (LPS) and electrical installations as well as the voltage generated by the self-inductance of down-conductors itself is investigated.