Reproduction of current pulse of lightning discharge of positive lightning with amplitude of 90 kA on lightning rod using MCG based power source

Grounding, as a protection method of different devices from lightning shock effect, is widely used both at living and industrial buildings. Variety of the protected objects and conditions of grounding electrodes exploitation, and wide range of possible lightning current values do not allow nowadays uniquely determine lightning current dependence of active impedance of the protecting grounding electrodes and, correspondingly, the value of electrical potential, coming to protected object. Experimental investigations on behavior studies of grounding active impedance at current pulses effect of negative and positive polarities were performed by different authors. However, current pulses amplitudes, created by experimental facilities on real grounding electrodes did not exceed 10-20 kA, what is lower of the current amplitude of average lightning with occurrence probability of 50%. Maximum recorded current pulse amplitude in lightning discharge is 800 kA. However, occurrence probability of such lightning is very small. Data on current discharge parameters with occurrence probability from 1% and 50% are presented in the paper. Amplitude of lightning current pulse at occurrence probability of 1% is 200 kA. At increase of the probability up to 50% lightning current amplitude decreases up to 30 kA. Current pulse duration at half-height amplitude of average lightning is equal 50 /spl mu/s, and for current pulse with amplitude of 200 kA reaches 500 /spl mu/s. Action integral q=/spl int/I/sup 2/dt for 30 kA lightning is 5/spl middot/10/sup 4/ A/sup 2//spl middot/s, and for 200 kA -3/spl middot/10/sup 6/ A/sup 2//spl middot/s. For lightning with negative polarity current pulse rise time is about 1 /spl mu/s, and for current pulses of positive polarity could reach several tens of microseconds. To increase data reliability about protected objects resistance on lightning discharge effect, it is necessary to reproduce possible lightning with maximum close currents on real current grounding electrodes. If we take into account that active impedance of very branched grounding devices is not less than several Ohms, to reproduce lightning current with the amplitude up to 200 kA it is necessary to use power sources with power capacity of more than 10 MJ. In this case value of electrical potential on the grounding electrode could reach several hundreds kilovolts (/spl sim/0.5 MV).

I. SOME INFORMATION ON VERTICAL GROUNDING ELECTRODES

Vertical grounding electrodes are widely used for objects protection from lightning discharges effect. Value of active impedance of direct current, in the condition that electrical field strength in the ground does not exceed the value, at which non-linear processes appear in the ground and, consequently, smaller than the strength value of the beginning of spark processes, according to [3] is equal to:

$${R_0} = {\rho \over {2\pi l}}ln{{2l} \over {{r_0}}} \eqno{\hbox{(1)}}$$ where –

ground specific impedance;

–

length of metal rod or tube from the ground surface;

–

radius of metal grounding electrode.