Application of the Leader Progression Model to evaluate the lightning performance of AC and DC EHV transmission lines

摘要

This paper presents a comparison of the expected lightning performance of EHV transmission lines evaluated by the traditional Electrogeometric Method (EGM) and some Leader Progression Model (LPM) methodologies. LPM is yet in discussion, with several proposed methodologies that present a wide variation on the results. For EHV transmission lines, EGM presents optimistic results, what is not desirable in the design of a complex electric system. EGM and LPM were considered for typical 230, 550, 765 kV AC and 600, 800 kV DC lines. In EGM, the point of the lightning impact on the transmission line depends only on line geometry, whereas in LPM, the power frequency voltage influences the inception of the upward leaders at each conductor. An upward leader may start from a phase conductor well before the starting of an upward leader from the shielding wire. Thus, a stroke, which EGM indicates to hit a shielding wire, may impact a phase conductor when using LPM. The effect of power frequency voltage appears on 765 kV AC and 800 kV DC lines of regular height. For these lines, when energized, the upward leader inception occurs in the phase conductors earlier than in the shielding cables and thus the lightning stroke hits a phase conductor, characterising a shielding failure. With the line de-energized, the upward leader starts earlier in the shielding cables and the discharge is attracted to the shielding. This phenomenon increases the shielding failure rates of the 765 kV AC and 800 kV DC lines. Backflashover rates decrease when the line is energized, since a very large amount of strokes, which were supposed to hit the shielding wires, is deviated to a phase conductor. This presents another important problem: shielding failures may occur for discharges of very high current magnitude. The influence of tower height on the lightning attraction was investigated for 525 kV AC lines. For very tall towers, the lightning impacts due to shielding failures increase, in a similar manner observed for 765 kV AC lines. This may be attributed to the effect of power frequency voltage on the upward leader inception. Thus, even for 525 kV AC lines, EGM presents underestimated outage rates and the use of LPM methodologies may be recommended.