Frequency of Damage of Low Voltage Apparatus Due to Lightning Flashes to Ground Nearby HV Overhead Lines

The paper deals with a simplified approach to evaluate the frequency of damage of low voltage apparatus powered by a in housing HV/LV transformer against overvoltages due to nearby lightning flashes (source of damage S4 according to IEC 62,305 standard). The approach is based on computer simulations with validated models according to the current state of the art. The paper evaluates: the overvoltages stressing the HV/LV transformer due to lightning-induced over-voltages on the supplying HV overhead line; the voltage transferred to low voltage circuit through the power transformer and the influence of the transformer characteristics and of the LV circuits feeding the apparatus; the influence of the characteristics and the number of LV circuits on the frequency of damage of the apparatus. The different contributions to the voltage at the apparatus terminals, namely, the voltage transferred by the HV/LV transformer and the voltage induced by lightning current in the circuit downstream the transformer, are recognized. The voltage drop along the earthing system between the points where the transformer and the apparatus are earthed is, in the case considered, not effective because the apparatus and the transformer are bonded at the same point to the earthing system of the electrical installation. Even if these voltage components exhibit different shape and time at peak, for safety, they are added. When the resulting voltage is higher than the rated impulse voltage of apparatus insulation, damage of apparatus occurs. The evaluation allows to conclude that the frequency of damage of the LV apparatus supplied by circuits in a multipolar cable is about a thousand times lower than the one relevant to circuits in plastic conduit. If the tolerable frequency of damage of the LV apparatus is kept in the range of 0.01 damage/year, the adoption of protection measures against overvoltages caused by the source S4 is practically not necessary, except for the case of long circuits in conduit, powered by long HV overhead lines in areas with high values of lightning flash density. As this matter has not yet been considered in the IEC 62305 standard series, the results presented in this paper will be useful in the light of the revision of requirements of this standard.