Article by professor Eduard Meerovich Bazelyan "Dissipation array system" for high-rise objects
The history of introduction of “dissipation array system” into the practice of lightning protection can hardly be called lucky. A protective device, patented by Roy Carpenter [1] in the end of the recent century presented itself a construction of metal spokes (similar to an umbrella without fabric) wit the radius of several meters, on which there were sharp needles 10 cm long (fig. 1). The number of needles could be up to several thousands. According to the inventor's idea, they should have been coronizing in the electric field of a thunder cloud, creating a thick layer of a spacial charge, which could neutralize downward lightning leader charge. In the result, the lightning stopped its existance.
The severe critics form the side of the specialists on atmospheric electricity seemed quite justified. At the current of micro-amper level, the corona is not able to create a layer of a spacial charge, able to neutralize the lightning leader with the charge of several C, at the well-developed corona, the prickers stopped to function independently from each other and turned into the system, the current of which was not very different from the current of a single needle. For this reason, the offered lightning protection system was initially acknowledged incapable.
Figure 1. The design of the device "dissipation array system" (DAS)
The situation changed, when the team of Russian specialists joined the analysis of the device action principle. They concentrated on the study of features of non-stationary multi-focal corona in the electric field of a thunder cloud [2-4]. The results of their researches allowed to explain the nature of the protective action of such devices as "dissipation array system" installation of which significantly reduced the number of lightning strikes into high-rise buildings, over which they were installed.
The Gold's hypothesis [5] was put into the base of the analysis, according to which, the strike point of ligtning on the surface of the grounded structure (for example, lightning rod) is defined by the conditions of counter leader development, starting from the top in the electric field of a thunder cloud and downward lightning channel approaching it. It was set, that the following actions have to be carried out for the counter leader start:
- corona charge at the top should shift to a streamer form, for what, corona current from the radius top r0 should incease the critical value
where μi - mobility of the main kind of corona ions and Ecor - corona's ignition threshold;- power of the streamer flash should be sufficient for the warming-up of its stem, for what, the voltage on the streamer wave ~1 m long should be not less than 400 kV;
- the leader, which started in the stem volume, should be stably forming, for what, the average electric field in the leader channel EL cannot exceed the electric field of the atmosphere in the place of leader head dislocation.
Numerical experiments in [4] showed, that the specifics of "dissipation array system" is most manifested in the formation of non-stationary corona. It is characteristic, that presence of many needles (103-104) poorly tells on the value of its summary corona current, which increases the current from the single rod electrode of the same height to some dozens of percents (fig. 2).
Figure 2. The dynamics of changes in time of the corona current from the electrode 100 m high
in the electric field of a thunderstorm cloud, linearly increasing to 20 kV / m in 10 seconds
Ток короны – corona current; Время - time
As the electric field strengthens with the downward current leader charge, corona current from "dissipation array system" outrides the current from the single rod corona electrode in growth speed and becomes bigger for an order of magnitude (fig. 3). However, the effect is seemingly, because on the top of the rod, there is a single corona hotspot, whereas on the surface of the device "dissipation array system", there are thousands of hotspots simultaneously and corona current is divided between them in an approximately equal share.
Figure 3. The dynamics of changes of the corona current from the electrode 100 m high in the electric field
of a thunderstorm cloud 20 kV/m and the downward leader, transporting the running charge 1 mCl/m from the height of 3000 m with the speed 200 m/ms; radial dislocation of the vertical leader channel 100;
count of time from the moment of lightning start
Ток короны – corona current; Время - time; Высота электродов – electrodes’ height; Стержень - rod
In the result, each coronizing needle current is insignificant. It is not able to exceed the cricial value and shift the corona in a streamer form. That is why, the counter leader from the device "dissipation array system" may start only in case, when the electric field of all the surface increases the ionization threshold (about 30 kV/cm in normal conditions). It requires very strong convergence of the downward lightning leader head with the surface of "dissipation array system". Calculated data in fig. 4 demonstrate the dynamics of the field growth on the surface of the device "dissipation array system" with the radius of 5 m for a powerful lightning with the running charge of the leader 1 mCl/m. The leader comes down with the speed 2x105 m/s right along the vertical axis of the protective system. It can be seen, that the threshold field is provided with the downward leader charge, when its head goes down to the level of 150 m and turns out to be at the distance of 50 m from the surface of "dissipation array system". Until this moment, it remains "invisible" for lightning. The protective action of multi-focal corona system is connected with this circumstance.
We should repeat once again, that in the base of the "dissipation array system" system functioning, there lies the principle of splitting up corona current by multiple corona hotspots, in the result of what, the switch of the counter charge to the streamer form, necessary for a counter leader start, is not carried out on any of them.
Figure 4. Change of electric field strength on the surface of the "dissipation array system" device 100 m high
and radius of 5 m in the process of descend along the axis of the downward lightning leader system with the parameters similar to those indicated in fig. 3
Электрическое поле – electric field; Высота головки лидера – height of leader head
According to the method given in [4], the estimation of the expected number of lightning strikes into the concentrated (small square) object with the height h can be held by defining the height of orientation Ho by the counter leader start moment. If then to use principles of equal distances to the ground surface and to the grounded system, displaced horizontally to the set distance R. The equality of these distances defines the radius of lightning contraction Ratt
,
in its turn setting the contraction area, and, consequently, number of lightning strikes in the region with the tunning density of lightning nM
In fig. 5 you see determination of contraction radius Rass by the results of numerical modelling of the corona development process in the electric field of the atmosphere and the downward lightning leader, setting function Ho(R).
Figure 5. To the estimation of downward lightning contraction radius
Электрическое поле – electric field; Высота головки лидера – height of leader head
E. M. Bazelyan, DEA, professor
Energy Institute named after G.M. Krzyzanowski, Moscow
>> Read more "Part 2"
See also:
- Efficiency and reliability of "dissipation array system"
- The design of the device "dissipation array system" (DAS)
- Useful materials for grounding and lightning protection designers
- Free webinars with the leading industry experts
- Real examples of calculations of grounding and lightning protection
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