The resonance overvoltages in nonsinusoidal modes of extra high voltage transmission lines

Abstract

Due to the rising role of overhead lines extra high voltage there is a need of application of modern electronic devices that allow controlling modes of bulk electricity system. Durable resonance overvoltage may cause damage of electrical power equipment. This kind of overvoltage is caused by properties of the network and can be eliminated by changing the ratio between the parameters of the network and its mode. Unlike switching overvoltage that exist centiseconds, resonance overvoltage not only occur unexpectedly, but they may go on for a long time until the automatic action or action of personnel will not change electrical schemes or operation modes. The resonance overvoltage are neglected when selecting insulation or parameters for high-voltage surge arrester as this protective measure is calculated to limit switching overvoltage, not to extinguish a long resonance process. Therefore, the probability and development of system accidents due to resonance overvoltage is significant. One of the main causes of equipment failure in the main electrical network is overvoltage. Overvoltage are the values which according to the technical requirements exceed the maximum value of normal operating voltage. The reason is that the relatively small insulation reserve provides for the equipment of the electricity grid because of its high cost for a given voltage. Generally, cost of power systems insulation is a significant factor in capital investment. The main source of distortion in extra high voltage networks is nonlinearity of volt-ampere characteristic of magnetic shunt in unloaded power autotransformer. This mode causes the conditions for the significant overvoltage on even harmonics. It should be noted that the processes of the second harmonic overvoltage are generally known, but their specific occurrence conditions depend on many factors of abnormal mode. That is why the aim of the article is developing measure for reduction and prevention of overvoltage in accordance with conditions for their existence. In investigations residual magnetization is neglected because capacitance between autotransformer and circuit breaker is large enough. In the article the features of the operation of SF6 switches that should be considered when introducing them to the power lines of 750 kV. The switching control unit Switchsync F236 is one of such devices which enables to connect and disconnect contacts of poles circuit breakers at required time. This function of Switchsync F236 can be used not only for interruption of short circuit. One of the objectives of application Switchsync F236 is prevention of resonance overvoltage occurrence at closing extra high voltage line on no-load autotransformer. The analysis of electromagnetic switching transients when operating a three-phase auto recloser with really existing power transmission lines has been implemented. The influence of initial conditions of electromagnetic transients based on simulation has been analyzed. Based on the research it is proposed to use suppression equipment components with aperiodic current line switches. The research of recommended exposure settings of the controlled switching device on duration of current aperiodical component has been done. Recommendations on the use pre-insertion resistors and switching units controlled have been proposed. The importance of application and prevention of resonance overvoltage caused research area which is presented in article. Ref. 11, fig. 6.