Instrumentation
The system installed at the CN Tower since 1990, for the purpose of measuring the lightning current derivative, utilizes a 3-m, 40 MHz Rogowski sensing at the 474-m above ground level (AGL). The coil is connected, via a triaxial cable, to a 10-bit, 10-ns, computer- controlled double channel digitizer with segmented memory, placed at the 372-m AGL. (For further enhancement of the vertical resolution, the current derivative is usually measured using both channels of the digitizer set at different scales.) While the Rogowski coil has a risetime of 8.7 ns, the overall risetime of the measuring system is estimated to be about 20 ns.
During the summer of 1997, a noise-protected current derivative measuring system was installed at the Tower. This system features a 6-m, 20-MHz Rogowski coil surrounding the whole steel structure of the Tower at the 509-m AGL and is connected to the recording station via an optical fiber link. The new current sensing system measures a much better signal-to-noise-ratio than the old system because it captures the whole current and utilizes an optical link.
The two current sensors were installed as far as possible from Tower’s main structural discontinuities (the Tower’s tip, the Space Deck, the Main observation Deck and the ground level) in order to capture the current waveform free from reflections, at least for fast-rise waveforms. This also enabled the proper estimation of reflection coefficients at different discontinuities [4] and would eventually facilitate the recovery of the original current waveform.