The solar activity and the electrical grid failures
As a whole, power engineers became used to the thought of the inevitability of the occurrence of lightning-induced overvoltage and learned to remove the failures caused by them. However, the abnormally high solar activity at the end of the last century helped to discover such a weak spot in the electrical grids that judging by the publications, energetics is ready for unconditional surrender. These are the power transformers. They comprise the most complex and expensive elements of the network and at the same time they are the most vulnerable to lightning-induced over voltages.
The transformers are protected by the surge arresters and the surge suppressors from the surge waves coming from OPL. However, they have no protection from their rear, where the neutral connectors of their coils are connected to the ED of the substations. There is nothing to prevent the passage through its coils of the main lightning discharge current, which can reach hundreds of thousands of amperes. From this side transformers are practically defenceless. According to data from ORGRES of the Russian Federation, 68 power transformers and 124 instrument transformers were damaged from 1997 to 2001 in the territory of the RF as a result of atmospheric influences.
In the very moment the leader discharge of the lightning approaches the OPL tower, the main lightning discharge current rushes to this very point too. It comprises a flux of electrons collected by the substation ED. They could reach the meeting point with the leader discharge by using the lightning protection cable, but there are several circumstances hindering that: the cable resistance exceeds a number of times the resistances of the phase conductors, the powerful transformers operate with a dead-earthed neutral, and the active resistance of their coils is only a few ohms. What is more, the inductive resistance of the transformer coils during the flow of lightning direct current becomes equal to zero. Namely these reasons make the main lightning discharge current use the transformer coils to perform its impending tasks. As a result of the increase of the voltage in the neutral and the flow of enormous direct current through the transformer coils, overheating of the coils and the core occurs. As the current transformers do not respond to the flow of direct current, the relay protections are idle and a signal for switching off the transformer from the network can be sent only by the gas protection. However, if the lightning current is sufficiently high, the total time of starting the protection and the switch can be sufficient for melting of the coils to occur and for the transformer to go out of order completely.
On the whole, rather intuitively than with the help of a logical analysis, the experts have come to the conclusion that the reliability of the electrical grids depends on the solar activity.
Here is one of NASA’s communications: “On 13.03.1989 the coronal mass ejections from the Sun led to the melting of transformers in Quebec and New Jersey and millions of people remained without electricity for a period of nine hours. Abnormalities were observed in the entire territory of the USA”.
Conclusion can be made about the amount of electrons penetrating the terrestrial bowels based on the size of the territory encompassed by the Aurora borealis. Here is what Kаthryn Sullivan, deputy director of the National Oceanic and Atmospheric Administration [NOAA], said: “Since 2000 there have been 12 magnetic storms of higher class on the Earth. They led to the non-functioning of elements of the energy system, radio communication shut-off and aurora, which was observed as far as at the Equator. The last storm occurred on 11.09.2005.”
The greatest failure, as a result of which approximately 10 million people in Canada and 40 million people in the USA remained without electricity, occurred on 14 August 2003. The commission reached to the conclusion that the failure was caused by an earth fault when the OL cables sagged due to the thermal expansion and drew near to the branches of the trees underneath them.
It is hard to agree with the commission’s conclusion, at least because of the fact that the electric strength of the air is very high. It equals 0.5 x 106 V/m. However, to put everything in its right place, it is necessary to see the information about the solar activity, published regularly by NASA. On August 10th, M-class solar flares started, and on August 14th one of them even developed into the highest Х-class. Aurora borealis was observed far from the Arctic Circle – in Maine, Michigan and Wisconsin. As it was established earlier, an enormous amount of electrons penetrated the terrestrial bowels as a result of these flares, and the respective amount of ions accumulated in the proton radiation belt. In their striving to join at any cost, they created the high potential and abrupt inhomogeneity in the electrical field on the tree tops under the OPL conductor wires. This caused reverse overlapping of the air gap, and the electron flux, concentrated by the substations ED, in turn converted this pulse into a power arc, which led to the shutting down of the electricity transmission line.
Of course, the non-functioning of the transformers as a result of solar flares are unpleasant as our entire life to a greater or a lesser extent depends on electricity, but they are not an immediate threat to people’s lives. However, this does not apply to cable networks in Bulgaria, in which the electricity supply of the transformer stations is provided directly from the 110 kV substations.