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In order to protect the transmission lines against lightning strikes, it is important to investigate the tower grounding system. In this paper, a recent method called hybrid continuous circuit-trapezoidal integration method is used to calculate the impulse impedance of the grounding system. Moreover, conventional structures of the grounding systems have been simulated and the results show that, with the same wire length, the square with additional wire system has the least impulse impedance as compared to counterpoise, square and crow's foot. Moreover, the effects of soil resistivity and lightning current rise-time on the impulse impedance of these grounding systems are investigated. It is concluded that the design of the grounding system depends on the geographic location of the site in terms of soil resistivity and isotropic characteristics of the area in terms of lightning current rise-time. [ABSTRACT FROM AUTHOR]

Integration of wind resources into the power systems has increased the technical and financial concerns in transmission expansion planning. In this paper, a stochastic structure is demonstrated for transmission expansion planning with allocation of fixed series compensation under wind and load uncertainties. Fixed series compensations have the ability to increase the transfer capacity of transmission lines. However, their significance benefit for transmission expansion planning is their higher effectiveness in power dispatching, which results in lower investment costs, compared with planning without fixed series compensations. The proposed planning model simultaneously optimizes targets such as the investment cost, the congestion cost, and the expected energy not supplied in each stage. The presented planning model is a complicated nonlinear optimizing problem. For introducing group of nondominated optimal solutions, multi-objective gray wolf optimizer algorithm based on probabilistic optimal power flow is used. Then, fuzzy approach is used to lead to the best response. For demonstrating the feasibility and susceptibilities of the proposed algorithm, the planning methodology has been illustrated on the IEEE 24-bus and Colombian test systems. The results acquired from the simulations monitor capability of the proposed structure in order to satisfy various planning indices. [ABSTRACT FROM AUTHOR]

• Lightning strikes are the major reason for outgoing of overhead transmission lines. • A compact formula based on simulation processes is proposed for BFR calculation. • Tower footing grounding length and insulation level are considered in the formula. • The proposed formula contains the soil resistivity. • Economic analysis of BFR calculation is done to help transmission line designers. This paper aims to present a novel proposed formula, along with technical viewpoints, to solve the problem of transmission lines outage rate due to back-flashover phenomena in terms of soil resistivity, the number of disc insulator units in the string, and tower grounding system structure. Improving the tower grounding system (reducing the impedance) and developing the insulation level are appropriate solutions to prevent the occurrence of the back-flashover, dependent on many factors. To the best of our knowledge, the trade-offs between these two parameters, in respect of soil resistivity, to obtain the desired back-flashover rate (BFR) has not been presented via a compact formula yet. In this paper, it will be shown that in a specific soil resistivity and grounding structure, there is a minimum required insulator unit number and grounding conductor length to achieve the desired BFR, which could be calculated by the proposed formula, straightforwardly. Also, the economic selection between increasing the insulator unit number and developing the tower grounding system in terms of soil resistivity and BFR is investigated. The simulation procedure is coded with Matlab and linked to EMTP/ATP program to analysis the BFR. This method is done for a typical 150 kV overhead transmission line, however, it can be applied to any voltage level of transmission line and significantly facilitate the cost effective design of them. [ABSTRACT FROM AUTHOR]