## 1 Introduction  
The lightning performance of high-voltage substations is primarily concerned with the distribution and transmission network operators, since lightning surges contribute to the dielectric stress and the ageing of the insulation. This can lead to failures of the various components of the substations, may result in interruption of the power supply, and are related to dangerous repercussions for the safety of the equipment as well as the personnel [1]. Thus, for the above reasons as well as for reliability issues, it is becoming an increasingly common requirement the improvement of the lightning performance of high-voltage substations and the reduction of the lightning failure rate.  
The design of the lightning protection system of high-voltage substations intends to limit the expected outages of substantial equipment of the substations due to lightning overvoltages. It is worth noting that in the case of gas-insulted substations, the severity of the potential consequences due to a lightning overvoltage is more intense, compared to air-insulated substations, considering the repair or/and replacement cost [2]. The combination of an external lightning protection system with surge arresters at the entrance of the substation is the common practice for the reduction of the lightning faults [3; 4; 5]. However, various factors influence the protection level and the effectiveness that the applied protective measures provide such as the lightning hit position, the length of the underground cables for the connection of the incoming transmission line and the transformer, the number and the placement of the surge arresters, the grounding resistance. These are parameters that determine the range of the arising surges and have a great impact on the lightning performance of the substations. In [6] the lightning performance of an air-insulated substation that connects a hydro power plant with network is presented, considering different overvoltage protection schemes. The influence of various parameters (i.e., surge front times, soil resistivity, distance of the cable that connects the transformer and the busbar) on the lightning performance of a combined overhead line/cable connected gas-insulated substation is examined in [7], since the role of the earth grid is revealed in [8]. Moreover, the adequate number and the appropriate placement of a surge arrester is also crucial in attempts to upgrade the lightning performance of the power substations [9; 10; 11; 12].  
In the current work, a method is developed for the design of high-voltage substations intending to improve their lighting performance. A heuristic method is used in order to adjust the critical factors of high-voltage substations that determine their lightning performance, in an effort to reduce the substation's outage rate. The developed method provides the most efficient configuration for the proper planning and developing of high voltage/medium voltage substations, considering the tower footing resistance (TFR), the length of the underground cable that connects the line with the transformer, and the installation position of the arresters, in order to keep the lightning outage rate below a defined threshold. The developed method can be proved useful to power utilities, in order to design or upgrade high-voltage substations and reduce the expected annual lightning failure rate.