Analysis of Lightning Overvoltages Due to Direct Strikes on Hybrid Overhead Lines

This paper evaluates the characteristics of overvoltages associated with direct lightning strikes to an HOHL. The study utilizes electromagnetic transient (EMT-type) simulations, based on a time-domain solution of the transmission line equations, validated against measurements performed on a 1:20 scale model. The analysis examines the impact of lightning current front time and grounding impulse impedance on overvoltages, with a particular focus on the mediumvoltage (MV) circuit, which employs a spacer cable configuration. It further clarifies the roles of the overhead ground wire (OHGW), the underbuilt shield wire (USW), and the low-voltage

(LV) circuit in improving the performance of the HOHL compared to conventional distribution lines.

The stroke current front time plays a major role in determining the magnitude of the overvoltages. Longer front times lead to a noticeable reduction in the absolute voltage peaks for both phase-to-ground and phase-to-messenger insulation. The tower footing impedance also has a substantial impact, with both voltages increasing with the impedance value. The MV circuit of the HOHL exhibits significantly better lightning performance than a conventional distribution line. The OHGW is primarily responsible for this enhanced performance by shielding the MV circuit from direct strikes, while the close electromagnetic coupling between the phase conductors and the grounded messenger wire further limits the potential rise across the insulation. This effect is reinforced by the USW and the LV circuit, which introduce additional coupling paths that help attenuate transient surges. The LV circuit provides more effective mitigation than the USW due to its stronger electromagnetic coupling with the MV phases. This effect is further enhanced by flashovers on the LV phases, which effectively transform those conductors into additional grounded paths.