## 2 Protection of High-Voltage Substations against Lightning Surges  
Despite the fact that power systems operate mainly in the steady state, they have to withstand and handle overvoltage and overcurrent stresses generated during transient conditions. It is worth noting that the dimensioning of the equipment is dictated by the expected magnitude and the duration of the arising surges. Consequently, the efficient design of a power network in order to transfer and deliver electric power of high quality and reliability, requires a thorough understanding of its transient behavior [1; 13]. The installation of an external lightning protection system, according to the electrogeometrical model [3; 4; 5; 14], is the most common method to protect the substation against direct lightning hits. Accepting the efficiency of the installed external lighting protection system, atmospheric overvoltages that impinge on the connected high voltage line comprise the main reason of insulation faults, degrading the normal operation of the system. The severity of the developed surges in substations is determined by the magnitude and the waveform of the surges impinging on the substation from the connected overhead line, considering the travelling wave behavior of the arising surges. In the case that no surge arresters are installed, the amplitude of the impinging surges is the most important factor, since the slope of the surge and the installation position of the arrester have to be considered, in the case that arresters are used for the protection of the system. A direct lightning strike to the phases of the incoming high voltage line results in the development of two equal travelling waves that propagate in opposite directions; the prospective magnitude of these overvoltages is determined by the peak value of the lightning current and the surge impedance of the phase conductors. The overvoltage wave that arrives at the entrance of the high voltage substation can cause serious damages of the equipment, leading to interruption of the power supply. It is worth noting, that, in the case of a direct lightning hit, the arising overvoltages are not influenced grounding resistance of the towers, so the installation of surge arresters is the most effective protective measure [15]. Modern metal oxide gapless surge arresters are gapless, consisting of ZnO nonlinear resistors. A prerequisite for the efficient operation of the arresters is a grounding system of low earthing resistance to avoid the development of overvoltages higher than the basic insulation level of the system. Added to this, the appropriate placement and the electrical characteristics of the surge arresters and the lightning hit position are crucial parameters that determine the lightning performance of the substation [16; 17; 18; 19; 20]. When lightning strikes the tower or the overhead ground wire of a high voltage line, the overvoltage that arises is a function of the peak value and the slope of the lightning current, the TFR and the induction that the metallic tower presents. The arising surge voltages propagate to the high voltage substation and pose a significant threat to the normal operation of the system. If the magnitude of the developed overvoltage becomes greater than the insulation level of the line, a backflashover occurred; the risk of a backflashover can be limited by reducing the earthing resistance of the tower [21]. Of special interest in respect of insulation assessment and protection are the power transformers, which may be subjected to impulse stresses of high magnitude. To this direction, in order to protect the equipment of the substation, surge arresters are installed at critical positions of the system. Even for overhead lines that are adequately protected by ground wires, the avoidance of backflashover, near the substation, is an issue of priority in an effort to determine the installation position and the number of the demanded surge arresters to accomplish insulation coordination of the substation.