Lightning performance evaluation of quasi-hemispherical air terminations versus a Franklin rod

This research aims to investigate the effect of adding a quasi-hemispherical object at the top of a Franklin rod. This will significantly reduce corona and hence space charge under the high electric fields experienced during the descent of the downward moving leader. By obtaining the appropriate size and shape of the object at the top of the termination the aim is to send a single streamer at precisely the correct moment for it to undergo streamer to leader transition and intercept the downward moving leader. Research as well as preliminary simulation and experimentation point to a critical radius of approximately 0.3–0.35 m. A test setup with a 5 m air gap has been designed and manufactured with the aim of producing upward leaders from competing air terminations, thus simulating natural lightning conditions more closely. The experimental setup is an inverted rod plane gap and includes all the necessary d.c. biasing circuitry and measuring equipment. Eleven air terminations were tested against the Franklin rod and the results captured with a high speed gated camera. A 1.2/50 μs lightning impulse waveform was used during testing. During testing there was no evidence of upward leader formation. The strikes were to the Franklin rod and electric field enhancement proved to be the dominant factor of breakdown. As such the effect of adding a quasi-hemispherical shape to the tip of a rod could not be evaluated as the mechanism of natural lightning attachment was not accurately replicated.

► Upward leader formation from grounded air terminations is crucial for comparison with lightning in competition tests with long spark gaps in air.
► Competition tests between quasi-hemispherical and spherical air terminations and the Franklin rod under 4.5 m gaps in air were conducted.
► A standard lightning impulse waveform was used and the Franklin rod proved to be the best air termination.
► With a standard lightning impulse voltage and a 4.5 m gap there was no evidence of upward leader formation from the competing air terminations.
► Electric field enhancement at the air terminations dominated breakdown.