CONTINUING CURRENT IN TOWER-INITIATED LIGHTNING Helin Zhou 1 , Gerhard Diendorfer 2 , Rajeev Thottappillil 1 , Hannes Pichler 2 and Martin Mair 3 1 Division of Electromagnetic Engineering, School of Electrical Engineering, Royal Institute of Technology (KTH), Stockholm, Sweden 2 Austrian Electrotechnical Association (OVE), Dept. ALDIS (Austrian Lightning Detection & Information System), Vienna, Austria 3 Central Institute for Meteorology and Geodynamics, Vienna, Austria helin.zhou@ee.kth.se , g.diendorfer@ove.at , rajeev.thottappillil@ee.kth.se , h.pichler@ove.at , martin.mair@gmx.net ABSTRACT We investigate 172 upward-initiated negative lightning flashes containing at least one leader-return stroke sequence observed to the Gaisberg Tower (GBT) in Austria from 2000 to 2009. A geometric mean value of 8.75 kA of the return stroke peak current is determined, followed by a continuing current of 1.48 ms with a charge transfer of 0.786 C. We find that in case of the continuing current duration larger than 40 ms, usually defined as long continuing current, or when the continuing current charge transfer is greater than 5 C, the initiating peak return stroke current did not exceed 20 kA. Moreover, continuing currents with durations of less than 20 ms or with a charge transfer smaller than 5 C, those continuing currents were initiated by return strokes with peaks of any range. 1 INTRODUCTION Continuing current is one of the charge transfer modes to ground, which is occurring after the return stroke. It is assumed to be the cause of burned holes in wind turbine blades and of other serious lightning damaging effects. Based on photographic and electric field records, Kitagawa et al. [1] and Brook et al. [2] reported that 50% of the multiple-stroke flashes involved at least one subsequent stroke, that was followed by a long continuing current (>40 ms), which lowering about half of the charge transferred to earth. Shindo and Uman [3] concluded that electric field peaks associated with return strokes preceding short (10-40 ms) and long (>40 ms) continuing currents, are smaller than those having “questionable” continuing current (1-10 ms). Ballarotti et al. [4] defined as very short continuing current a duration less than or equal to 10 ms but greater than 3 ms. They found that the histogram of very short continuing current exhibits a lognormal distribution. Fisher et al. [5] presented a variety of continuing current waveforms based on direct current measurements from triggered lightning. Saba et al. [6] used a high-speed video camera to analyze the relation between lightning return stroke peak current and following continuing current. In this paper, we analyze the relationship between correlated peak current of return strokes, continuing current duration and continuing current charge transfer based on the GBT current measurements. The latter two include the duration and charge transfer of return stroke itself, respectively. 2 INSTRUMENTATION AND DATA A current-viewing shunt resistor of 0.25 mΩ having a bandwidth of 0 Hz to 3.2 MHz was installed at the base of the air terminal on the top of the GBT and used to measure the overall lightning current waveforms. Two separate fiber-optic channels of different sensitivity (±2 kA and ±40 kA) are used to measure low amplitude current like the initial continuous current and large return stroke peak currents, respectively. These signals are recorded with a sampling rate of 20 MS/s by an 8 bit digitizing board installed in a personal computer. The total record length with each trigger was 800 ms with a pre-trigger recording time of 15 ms. A 250 kHz cut-off frequency digital filter and appropriate offset correction is applied to the current records before the lightning parameters (peak current, charge transfer, action integral, etc.) are determined. More details of the lightning measurement program at the GBT and the deployed instrumentation can be found in [7]. A total of 172 upward-initiated negative lightning flashes (713 return strokes) to the GBT from 2000 to 2009 and containing at least one leader-return stroke sequence were employed in our investigation of continuing current. 3 ANALYSIS AND RESULTS 3.1 Histograms of peak current, continuing current duration Figure 1 shows the distribution of peak currents of all negative return strokes (N=713) in upward-initiated flashes to the GBT from 2000 to 2009. The geometric mean value of peak current is 8.75 kA, consistent with the value reported by Diendorfer et al. [7]. Figures 2, 3 and 4 show very short (1-10 ms), short (10-40 ms) and long (>40 ms) continuing current, respectively. Due to the difficulty of discrimination between the return stroke tail and the beginning of the 1143-1 30th International Conference on Lightning Protection - ICLP 2010 (Cagliari, Italy - September 13th -17th, 2010)