Webinar "Origin of Lightning Protection Zones, Their Actual Capabilities, and Negative Impact on External Lightning Protection Optimization". Page 2

The nineteenth webinar of "Earthing and Lightning Protection: Design Issues and Problems" series.

Webinar text. Page 2

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Laboratory research in Russia

– The serious work was started in the end of the 1930s, in Moscow, in the All-Union Electrotechnical Institute. Artur Akopyan, a very decent researcher, guided this work. This is what he did: he used a pulse generator; in the gaps of about 1.5 meters long, the model of a facility was installed, and hundreds of sparks were sent that passed along different paths. The portion of these sparks got into the lightning arrester, the other portion got into the facility, and the third part got into the earth, and the models were thus tested. What did Artur see but did not want to understand? He saw the following. It turns out that, with the positive and negative voltages, the discharges had completely different paths. Almost all negative sparks got into the lightning arrester, while the positive sparks got God knows where. Finally, when they used the pulse generator that provided a pulse with a steep front, the protection zones of one type were obtained. But when, instead of such generator, they used a source of constant voltage or industrial frequency voltage, the protection zones were different. And they have made a simple conclusion: it is better to choose the worst zones, as the worst zone provides a positive voltage pulse with the short front, and we will use these zones. And they used these zones. And nobody knows the reason why exactly these zones but not any other.

Protection zone according to Akopyan

– As a result of these experiments, the protection zones were generated with the envelope curves of this section drawn for the unclear reason as either parabola or hyperbola. But the most important point is that this zone began from the top of the lightning arrester in any case. There was a hard and very unpleasant discussion about how to draw the boundary for this zone: as a straight line as shown here or as a hyperbola or parabola, or a curve, as shown by Akopyan. The discussion had been active for several years. It ended up with nothing, but both zones protruded from the top of the lightning arrester. And today, the IEC protection zones also protrude from the top of the lightning arrester. Why do they protrude from the top of the lightning arrester? Let's reason this. Imagine that I have installed a lightning arrester having some height. Near the arrester, I have installed one more stick of about the same height and called it the facility as opposed to the lightning arrester. Their heights are about the same. What will happen then? A half of strikes will get into the stick labelled with L (lightning arrester), and the second half of strikes will get into the stick having almost the same height, which was called "a facility". And what will be their protection efficiency? 0.5. It means that if I want the protection reliability more than 0.5, e.g., 0.9 or 0.99 or 0.99, I have to make the facility lower than the lightning arrester, but it is surely more correct to place the lightning arrester over the facility.

Does the protection zone have a specific configuration?

– Then, the analysis started: can we generate the protection zone? We got used to that the protection zone is built as a cone. Why should it be in the form of a cone? Why cannot we make a protection zone of any other configuration, e.g., rectangular? What do we know about the protection zone? The only thing is known to us is that the space it delineates is protected against the lightning with a certain, specific reliability. But if I determine the space using only one value, I cannot determine the shape of such space. Generally, the shape corresponding to the reliability may be any that is descending from the top. And we understood that the protection zone was a formality, and it was not clear what to do with it.

How can we determine the protection zone dimensions?

– The issue of determining the protection zones occurred again and with the strong wording, in the 1960s. In the 1960s, it became clear that we had an opportunity to create generators with the voltage of 3 million V, 5 million V, or 7 million V instead of 1.5 million V as Artur Akopyan did. And it became clear that those crazy sparks in terms of their length of dozens meters go by undefined paths, but not by the shortest distances as it was believed in the 1930s. Look at these two pictures. On one side, a well-known photograph of the Ostankino Tower is shown, wherein the lightning missed the top for 202 meters, and on the other side, the discharge in Novosibirsk that was obtained in the voltage generator of 5 million V is provided, wherein the spark did not pass along the shortest path to the earth by breaking through 34 meters, but flew away for 150 meters and got into the power lines located at the distance of more than 100 meters from this generator. They had to do something with it. They had to understand what exactly determines the spark path and how it is formed.

Searching for the orientation mechanism

– And in the 1960s, a large series of experiments was conducted that was not aimed at determining the protection zones. They wanted to understand how the lightning discharge chooses its path. And the following idea came up: take a lightning path or take a path of a spark length (you can see the path of the long spark that was obtained in the test bench in Leningrad, which was not named Saint Petersburg yet, 12 meters long. You can see that it is randomly curved). And then, they thought: let's make many paths, capture them, make cross-sections of these paths, and calculate the "+" deviations to the one side, and the "-" deviations to the other side. Then, if all of those paths are formed randomly, the averaged path will be a simple vertical line. And if these paths have some targeted deviations, initially this path will be vertical, and then it will begin deviating to the side where, for example, the lightning arrester is installed.

Determining protection zones in the laboratory

– The second point that was defined is as follows: this targeted deviation during the laboratory experiments varied depending on the experimental conditions. It varied as follows: the longer the gap, the more part of the averaged path was vertical and only after that the deviation began. Here is a gap of 10 meters and there is a gap of 0.5 meter. Can you see how they differ? This one goes in a targeted manner, and this is the averaged path that goes like this. And it turns out that if we conduct the experiments in different scales in the laboratory, we will obtain different results. But that is not all. Another result was also obtained. It turned out that the nature of this path depends on the nature of the voltage pulse. This voltage pulse has got a very short front of about 5 mcs. The path intentionally goes directly to the lightning arrester. And here, I have made a shallow pulse. It has got a 200 mcs front. Can you see that? If I conducted such experiments with the negative polarity, I would have obtained more different values. And based on this, we can only make one conclusion: we can qualitatively study the process in laboratory, but we cannot obtain the protection zone in the laboratory and transfer it to the real life because the result depends heavily on the conditions of the experiment.

Probabilistic method modified by ENIN

– We had to do something, so we had the following idea in our institute: we had decided to make a computational model on the basis of our experiments, which could have defined the protection zones with any significance while using a minimum amount of available experimental data for this. What did we think about? We have done the following thing: the lightning orientation process begins with some height that is called an orientation height and that is, according to the approved Gold's hypothesis, begins in the point when the opposed discharge starts from the top of the lightning arrester. This is the first point. The second point is that the process goes as two independent events. The first event is associated with that the general lightning path is chosen. Where will the lightning go? Will it get into the "lightning arrester – facility" system or to the earth? And the second process relates to the lightnings that are oriented this way. During this process we called a choosing process, a point is chosen on the surface of the lightning arrester or on the surface of the facility.

Initial method provisions

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