Creeping Discharges in the Lightning Protection. Page 2

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Numerical modelling of the spark channel growth dynamics

Численное моделирование динамики роста искровых каналов - Numerical modelling of the spark channel growth dynamics
Сопротивление заземления, Ом - Earthing resistance, Ohm
Ток 70 кА - Current 70 kA
ρ = 100 Ом м - ρ = 100 Ohm m
8 каналов - 8 channels
Время, мкс - Time, mcs
Динамика изменения сопротивления заземления во времени в компьютерном расчете - Change dynamics of the earthing resistance over time in computer calculation

The first point that appears to be is that these channels are a very good thing. Why? This is because, due to the channel, the electrode length varies, to which in addition to metal electrodes, these plasma electrodes are added along which the current flows. As a result, the earthing resistance varies significantly over time. For example, the metal electrode provided the earthing resistance 42 Ohm, and when eight channels have grown that can be seen in the photo, the earthing resistance reduced to about 5 Ohm, i.e. by 8 times. This seems to be a very good effect, and such effect can probably be used somehow.

Lightning current amplitude impact

Влияние амплитуды тока молнии - Lightning current amplitude impact
Предельная длина канала, м - Limit channel length, m
Сопротивление заземления, Ом - Earthing resistance, Ohm
Длина каналов - Channel length
Сопротивление заземления - Earthing resistance
Импульс тока 5/100 мкс - Current pulse 5/100 mcs
Амплитуда тока, кА - Current, kA
Удельное сопротивление грунта 200 Ом м; 4 канала - Soil resistivity 200 Ohm m; 4 channels
Заземлитель опоры 10 Ом - Earthing device of the support 10 Ohm

But the point is that it is the most important thing for me to avoid these channels going to the facilities to be protected. Why? This is because I have installed lightning arresters, I direct the lightning current on these lightning arresters away from the dangerous zone. And here is the point: the spark channels grow for a very significant length. As a result of the fact that they grow significantly, I noticed the following thing: the lightning current, when it bypasses these lightning arresters... see how their length varies depending on the current amplitude. If I have a very good soil with the resistivity 200 Ohm*m, if four channels grow, then with the current of 100 kA, the length of these channels grows for 20 meters. And it means that the lightning arrester is placed at the distance of 20 meters from a flammable or explosive object, then the lightning arrester works pointlessly because the channel will reach this length anyway.

Chart

Длина канала, м - Channel length, m
Время, мкс - Time, mcs
Импульс тока 5/100 мкс - Current pulse 5/100 mcs
Ток 100 кА - Current 100 kA
Ток 30 кА - Current 30 kA
Сопротивление заземления, Ом - Earthing resistance, Ohm
Время, мкс - Time, mcs
Удельное сопротивление грунта 200 Ом м; 4 канала - Soil resistivity 200 Ohm m; 4 channels
Заземлитель опоры 10 Ом - Earthing device for the support 10 Ohm

Now, the second question: is there anything useful in it? What does usefulness mean? Usefulness means the following point: channels reduce the earthing resistance, and it seems to be a useful event. But the question is as follows: When do I need to reduce the earthing resistance? It is important that the earthing resistance had time to reduce to the lightning current amplitude. What amplitudes do lightning currents have? In general, the greatest amplitude of the negative lightning is about 20 mcs; it cannot be more. And for these 20 mcs, with the current 100 kA, the channel grows for only 6 to 7 meters, and with the current 30 kA, it will be only 2 to 3 meters. As a result of this, the earthing resistance with these time values reduces not very much; it reduces only 2-fold at the best case. For the majority of practical tasks, it has no principal meaning.

Channel growth dynamics

Длина канала, м - Channel length, m
Время, мкс - Time, mcs
Импульс тока 5/100 мкс - Current pulse 5/100 mcs
Ток 100 кА - Current 100 kA
Ток 30 кА - Current 30 kA
Сопротивление заземления, Ом - Earthing resistance, Ohm
Время, мкс - Time, mcs
Динамика роста канала в высокоомном грунте ρ = 1000 Ом м - Channel growth dynamics in the high-ohmic soil ρ = 1000 Ohm m
4 канала, сопротивление заземления опоры 30 Ом - 4 channels, earthing resistance for the support 30 Ohm

And there is one more point: how is it done in the high-ohmic soil? I have used the same solution for the soil with the resistivity 1000 Ohm*m. It is a bad soil, i.e. sand, some dry clay loam. And then it appears that, in these soils, the channel grows again for only about 5 meters with a very significant current, for 2.5 meters with a weak current 30 kA, and the earthing resistance reduces again a little bit, for only about several dozens of per cent. And it happens due to the fact that, in high-ohmic soils, small currents flow, and these channels grow slower. They would be willing to grow, but much time is required for this.

For calculating the spark channel length

And if we are talking about the fact that this channel breaks through to the object, it may break through for any amount of time. And if we are talking about how the earthing resistance is reduced, then only the earthing resistance is important which takes place in the pulse front. And here, such a slow process has no time to make anything worthy. This is the situation.

New experimental research

Новейшие экспериментальные исследования - New experimental research
МИК ГИН - MIC IVG
Напряжение - 1,2 МВ Энергия - 2 МД - Voltage: 1.2 MV Energy: 2 MJ
Ток, кА - Current, kA
Напряжение - Voltage
Ток - Current
Время, мкс - Time, mcs
Напряжение, кВ - Voltage, kV

При расчете длины искровых каналов, формирующихся в течение первых 20 мкс воздействия тока молнии, можно принимать во внимание только вариацию его амплитудного значения, а в отношении временных параметров ориентироваться на средние значения длительности и времени фронта первого компонента отрицательной молнии. - When calculating the lengths of spark channels formed during the first 20 mcs of the lightning current exposure, only variations in its amplitude can be taken into account, and with regards to the time parameters, we should consider average values of the front duration and time of the first components of the negative lightning.

Now, I am going to talk about one more story that has existed until the recent times. I have already talked about that, almost one hundred years ago, these spark channels were known. They were recorded. For example, a channel of 200 meters long was recorded in a rocky soil in Norway. But they said: "Guys! Do not worry! If your soils are good, if these are low-ohmic soils, then nothing bad can happen. It is because such channels are not formed in low-ohmic soils". There was such an idea. And we were sure that it was like this, and nobody had a doubt in it. Several years ago, the combined team of three institutes: TRINITI is the Troitsk Institute of Innovative and Thermal Nuclear Research and the Combined Institute of High Temperatures and our Krzhizhanovsky Energy Institute decided to produce a generator that allows to see the actual lightning currents and how they are flowing. The generator made by this combined team is the generator having the superior parameters. Two cells, each of them with a 1.2 MV voltage, and this cell is installed on the ground. The accumulated energy is 2 MJ; it is about 1 kg of explosives.

Measurement results

Результаты измерений - Measurement results
Скользящие искровые каналы эффективно формируются в грунтах высокой проводимости - Creeping spark channels are efficiently formed in the high-conductive soils
Сопротивление заземления, Ом - Earthing resistance, Ohm
Ом м - Ohm m
Ток, кА - Current, kA

And when the field tests of these generators have begun, and these field tests have been started intentionally, taking into account the high-conductivity soils, it appeared that the soil with a very high conductivity, even 100 Ohm*m, is the black soil, and the resistance due to such channels varies by about 5 times. And the myth on that if you have good soils and you will have no spark channels, you should ignore this, has disappeared. It does not exist anymore. It appears that such soils act completely the same with relation to the spark channels. Such spark channels may be formed in the high-ohmic and low-ohmic soils.

Managing a creeping discharge channel

Управление каналом скользящего разряда - Managing a creeping discharge channel
1. Снижение сопротивления заземления в месте удара молнии - 1. Reduction in the earthing resistance in the lightning strike location
р = 200 Ом м - ρ = 200 Ohm m
Длина каналов, м - Channel length, m
Импульс тока 5/100 мкс амплитудой 100 кА - Current pulse 5/100 mcs with the amplitude 100 kA
Rg (исходное значение), Ом - Rg (initial value), Ohm

And a justified question appears: What do we have to do with them? We have to struggle with them somehow. These spark channels should be reduced. How to reduce them? The first point that comes to my mind right now is to reduce the lightning current, but we cannot make the lightning current weaker. However, we can spread this current along several earthing devices. But it appears that the effect that will be achieved in this case is not so good. It is because with the current 10 kA, i.e. the lightning current 100 kA is spread by ten paths.

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