Atmospheric overvoltages in the power grids
The transmission of electrical energy from producers to consumers is carried out mainly through step-up transformer substations with 50 (60) Hz alternate current, 110-1150 kV electricity transmission lines and step-down substations. Each substation and each tower of the overhead power lines (OPL) is equipped with an earthing device (ED). It comprises a network of horizontal electrodes located under the Earth’s surface. Vertical electrodes are connected to the nodes of this network, which are driven into the ground at a depth not less than 3 metres. The area of such an ED in the substations totals tens of thousands of square metres. The maximum resistance of the ED at the substations is 0.5 Ω, and of the OPL towers – 30 Ω. The outdoor switchgears (OS) of the substations are protected against direct lightning strikes through rod-fitted lightning conductors, whose current collectors are connected to the ED of the substations. The screens of the cable lines (CL), the neutrals of the transformers and everything on which the insulation of the high-voltage devices stands, as well as everything on which the insulator garlands are hung are connected to it.
The OPL conductors are protected from direct lightning strikes through a lightning protection cable hung over them. This cable, together with the OPL tower, is connected to the ED on the tower, and in the substations – to the ED of the substations. An OPL may be shut down, but it is impossible to be broken, so in practice ED on the entire continent are connected through lightning protection cables of OPL in a common network.
The construction of lightning protection devices is based on the blind faith that lightning strikes fall from the sky and carry a negative charge. From then on everything is simple – the lightning current is collected by the lightning conductor and flows into the ground through the ED. A voltage drop during the passage of the lightning current through the ED resistance is found to be applied to its vertical electrodes.
At first sight everything is taken into consideration and the possibility of failures is excluded, but practice shows that in reality everything is just like in the Armenian anecdote, “Have you heard Kirkor won 100,000 from the lottery? Yes, I have. But I understood that he didn’t win but lost, and not from the lottery, but at cards, and it wasn’t Kirkor, but Garabed”. Because the current of the main lightning discharge can’t have negative charge – this is impossible because of the negative charge of the Earth. The main discharge can only be positive. That’s why it does not go into the ground, but comes out of it. It is generated by the electrons collected by the ED. The voltage drop during the passage of the lightning current turns out to be applied not to the vertical electrodes of the ED, but to the insulation of the facilities.
As it was already noted, during the occurrence of a leader discharge, negative charges accumulate on the objects protruding over the Earth’s surface. Their energy may not be big, but the potential of the lightning conductor, installed on the portal of an outdoor switchgear (OS), may reach tens and even hundreds of millions of volts. This voltage leads to pulse reverse current overlapping from the earthed parts located in immediate proximity to the lightning conductor to the conductive parts. To avoid the overlapping, it is necessary to increase the insulation clearances and the number of insulators in the garlands. Although the pulse strength of an insulator chain of three insulators is 300 kV, in OS – 110 kV, the amount of insulators in the garlands reaches six, and at times seven-eight pieces.
It turns out that the real role of each ED is to create atmospheric overvoltage. Its value depends on the level of the solar activity, but anyway, the bigger the ED area and the deeper its electrodes in the ground, the greater the lightning current and the higher the voltage on the earthed parts of the substations. Through the earthed neutral of the transformer and its coils this voltage goes to the OS buses. Its value is relatively low, but as the power of the electron flux is enormous, it can turn every pulsed reverse current overlapping into a power arc. Most often its victims are the transformers and the CL. Surge arresters are used for their protection, which return part of the electrons back into the ground. From the side, this looks as follows:
The construction of the ED in the substations is tantamount to putting up a poster: “Welcome, dear guests!” When the electrons reach the substation bus through the earthed neutral of the transformer and its coils, they are met by the shouts of the surge arrester, “Go back, you darned devils, you were the last thing we missed here!” In the end it turns out that the lightning protection revolves the electrons in a senseless closed circle.
In the OPL sections located far from the substations, the atmospheric over voltages are created by the towers. Upon passing through the earthing device of the tower, the main lightning discharge current generates voltage whose value may reach millions of volts. This potential also causes inverse arch ceiling air gap, and the electrons collected by the ED of the substations make it an earth fault. There is no salvation from that as the electrons should go to the clouds and shall use every opportunity provided to them to that end.
In a nutshell, the existing state of affairs in the energetic seems as if an enormous army of specialists deals only with the way to help our planet perform its galactic obligations. Some of them, by constructing ED, do their best to help the electrons go out on the Earth’s surface. Others erect the OPL towers and hang on them the lightning protection cables with the only purpose to cause the leader discharge of the lightning and thus help the electrons go to the ionosphere. Even the failures caused by this activity are unable to stop them. Maybe it is because their removal is at our expense.