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The paper presents a methodology with a new simplified approach, as a set of procedures, aiming to optimize the insulation level when compacting 400-kV overhead transmission lines (OTLs) in order to increase their operational reliability. The procedure initiates from presupposed requirements for compaction, as it is becoming increasingly more difficult to carry out reconstruction, revitalization, or construction of new OTLs while keeping the original design criteria. The main purpose of this approach is to reduce transients on the insulation, which are caused by direct lightning strikes to the tower top, and influence switching overvoltages using an additional shield wire on OTLs. The insulation leader progression model E 50% and the dynamic ionization model E ion for grounding with the constant ratio between the tower grounding resistance R g and specific ground resistivity ρ g are used to evaluate the lightning transients. For the analysis, the switching overvoltages phase domain transmission line model is used, where OTLs are divided into a number of short segments. For the purpose of this investigation, a mathematical model of a double-circuit compact OTL is implemented.

Installation of line surge arresters (LSA) on overhead lines is normally accomplished with aim of line performance improvement. Co-ordination insulation procedure carried out during the line design in most cases doesn't take into account a suppression of overvoltages by using any overvoltage limiting devices. The decision supported by poor line performance to install LSA is usually made later. The reduction of overvoltage level depends mainly on the number of LSA installed, but in any case it is considerable. It means that insulation regarding dielectric strength might this way be over dimensioned. This fact could be considered for reduction of phase-to-ground and phase-to-phase distances, which lead to greater level of line compaction. This is of the most importance for lines with post insulators, since shorter insulators could be used and consequently narrower right of way achieved. Our target was to investigate the possibility of reducing lightning overvoltages by using LSA’s attached parallel to the insulators. For the analyses purpose we implemented a mathematical model of a single-system 110 kV compact overhead line. The Sigma Slp software was used for computer simulations. The line was investigated from the perspective of lightning over-voltages as they are the factors the most severely stressing the insulation. We focused to lightning strikes at a tower top for the case of single-phase (C) LSA installation configuration. The main influencing parameters were varied such as lightning current, surge shape and tower earthing resistance according to their most probable values to obtain overvoltage properties. We concluded, that adoption of LSA’s does not only reduce the overvoltage amplitude in that phase but also changes its shape, which has to be taken into account for test impulses generation. The physical model of wooden pole with composite line post insulators was built for testing in high-voltage laboratory. Power frequency and impulse tests were performed on insulators (phase-to-ground) and between phase conductors (phase-to-phase) in dry and wet conditions when necessary. Besides standard also non-standard impulse shape tests were accomplished to obtain withstand voltages for different arching distances. Based on our comparison with the simulation results we concluded that - speaking in terms of lightning overvoltages - it is possible to use insulators for the insulation level Um = 72,5 kV (140/325 kV), if LSA’s in phases A, B and C would be installed. To verify the conclusion in all other respects, further analyses are needed.

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