Lightning performance evaluation of quasi-hemispherical air terminations versus a Franklin rod
Date
2012-02-01
Authors
Djurdjevic, Igor
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Abstract
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 the 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 air
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 300 – 350 mm. A test setup with a 5 m air gap was designed and
manufactured with the aim of producing upward leaders from the competing air
terminations, thus simulating natural lightning conditions more closely. It 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 in a point to
point breakdown configuration and the results captured with a high speed gated camera.
1.2/50 μs lightning impulse waveform was used during testing and the air gap was 4.5 m
long. Results showed that all of the strikes during competition testing were to the
Franklin rod. There was no evidence of upward leader formation and electric field
enhancement dominated breakdown. The air gap needs to be extended and waveforms
with longer rise times, more energy and hence better chance of upward leader formation
need to be used. The Franklin rod proved to be the best air termination during testing.