At present, the networks of extra-voltage received the SF6 switches with the auto-compression principle of extinguishing the arc. The experience of operation showed the inability of these switches to perform commutation-type quick "on/off" for lines with a high degree of compensation of charging power due to the existence of a long durable aperiodic component in the current flowing in the switch. The results of investigation of the causes of accidents of SF6 circuit breakers during commutation of compensated transmission lines of 750 kV are presented. In the article have been reported, that the presence of a large aperiodical component of current prevent the current from crossing zero for several milliseconds, resulting in serious damage to one pole of circuit breaker.In the work the analysis of abnormal modes of extra high voltage transmission lines with different measures is performed. The article deals with the cycle of three-phase automatic re-inclusion on the 750 kV line with two groups of shunt reactors installed. Electromagnetic processes in compensated transmission lines depending on parameters of the line are investigated. The moments of a sharp change in parameters of transient processes during switching in extra-high voltage lines are revealed. Measures to limit the duration of the existence of the aperiodic component of the current are considered.In the main power Ukrainian grids have replaced air switches for SF6 gas switches. This upgrade was caused by the fact that modern electrical industry does not produce air switches and in the event of damage to repair and restore power would be impossible. Compared to SF6 gas air circuit breakers have certain advantages, the main ones are speed and high breaking capacity, and the possibility of an extra device controlled switching. This upgrade was carried out from 2010 year to 2014.Nevertheless new SF6 breakers have a lot advantages, but some nuances were discovered in the process of using 750 kV on long overhead power lines with shunt reactors installed on them, due to which there were several accidents at the substations. Damage 750 kV circuit breakers, which in recent years have occurred on several foreign substations and also in Ukraine, served as an excuse for starting the necessary development of normative documents, establishing regulations and norms of operation.