|In this paper, the authors have structured a com-plete algorithm for calculating the self-excitement of the generating groups in the NPS power plants, a phenomenon that can occur in the operative manoeuvres performed in 220 kV, 400 kV and 750 kV electrical networks. This shows how with simple parameter calculations can be evaluated parameter approximate values for long lines switching re-gimes, the safety measures to be taken when manoeuvres are appropriately performed and their sequence in order to avoid dangerous overvoltages that would lead to non-admissible stresses of the energetic equipments. In order to increase the degree of generality of these calculations, the relative size method was used, for which a computational program developed in the Mathcad programming environ-ment was designed. This calculation algorithm was custom-ized for a double-circuit overhead electric line LEA 220 kV case study. Also are presented the practical algorithms for calculating the voltage jump that occurs when connecting and disconnecting the long electric transmission lines from the power system. The overvoltages caused by the switching of these lines are also evaluated. These values must be taken into account by the managerial and operational staff to properly perform and coordinate manoeuvres. For the con-sidered double-circuit overhead power line 220 kV LEA, the calculation algorithms for both no load and under load switching were customized, and two scenarios were studied, namely when the line is not compensated and when the line is compensated by a compensation coil.