The high number of photovoltaic systems raises questions regarding their operational security even under extremely weather conditions like thunderstorms. Therefore a simplified model of a photovoltaic system, consisting of typical arrangement of solar-panels, was investigated. The influence of lightning strike was simulated by different lightning current shapes and striking points into the solar-panels. It was found, that besides the construction size and shape of the lightning current the striking point has a main influence on the value of induced overvoltages, occurring on the connection point of the solar-panels. The simulation results were confirmed by measurement with an impulse current. The results demonstrate the dangerousness of lightning currents with very fast rise time e.g. subsequent impulse as well as the reducing of induced overvoltages by an appropriate earthing connection. This has to be considered at the design of the lightning protection system.

Bio-based carbon 3Synthetic ester, Figure 1 [5] 4,5Natural ester The mineral oil-based transformer oil 1 is a mixture of paraffinic and aromatic hydrocarbons (C 15 -C 50 ). The more biodegradable mixture 2 based on hydrocarbons from renewable sources contains almost no aromatics (see Section 2.1). The synthetic ester oil 3 is a tetraester based on pentaerythritol with linear and branched fatty acids (C 5 -C 10 ) as shown in Figure 1.In liquid-filled transformers, the oil fulfils two important functions in that it provides electrical insulation and removes the heat generated by Joule losses mainly from the windings. The dielectric properties, such as permittivity, dissipation factor, and field strength of the electrical breakdown are important parameters when selecting oil for use in HV power transformers. As an important criterion for the insulation quality, breakdown voltage (BDV) and the partial discharge behavior (PD) of the oils are evaluated and compared in this work. The breakdown field strength, which is difficult to calculate for arbitrary geometries, is usually estimated by measuring the breakdown voltage using standardized test procedures. [3] Some insulation quality tests are required after the transformers have been manufactured, but also during their operating time. In most cases, insulation faults are associated with partial discharges.The heat transfer in transformer oils depends on the thermal conductivity and convection. We determined the viscosity and specific heat capacity that have the greatest impact on heat transfer.This work examines the aging mechanisms of the oil and the solid material used to insulate the windings, which also play an important role in the selection of a suitable insulating fluid. Traces of water in the oil have a significant influence on the long-term stability of Kraft paper. [9] A good indicator of the age of the oil is its acidity, as free carboxylic acids are released through hydrolytic cleavage and oxidation. We therefore examined the water content, the acid number and the peroxide number before and after the high-voltage tests. Results Chemical Composition of Transformer OilsIn order to reveal chemical and structural changes in the molecules as a result of repeated exposure to high voltage, the oil samples were carefully analyzed before and after the long-term Against the background of environmental compatibility and unrestricted technical usability, synthetic, and natural transformer oils are critically compared in long-term tests under high voltage using electrical and instrumental analytical methods. Synthetic alkyl esters of pentaerythrol appear to be sustainable and sometimes superior substitutes for mineral oils in terms of chemical stability, viscosity, permittivity, and heat transport. Natural esters of unsaturated fatty acids are found to be unsuitable for equipment exposed to moist air. Adsorbed water appears to be a general problem in transformer oils. The aging mechanisms and molecular changes during long-term operation include radical reactions, and th...

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