Currently there is a need to synchronize operation of the electric power system in the remote areas and increase of existing lines transmission capacity. The construction of new power transmission lines involves high economic expenditures. Well-known papers consider the issues of application of controlled series compensation devices only for long-distance power transmission lines with voltage of 500 kV and higher to increase the transmission capacity and the level of stability. The aim of the study is to increase the stability and the limit of the transmitted power when controlled series compensation devices are installed on 220 kV lines. It is necessary to develop a criterion of aperiodic steady-state stability of an electric power system with a 220 kV-controlled power transmission line. Methods of mathematical modeling of electric power system, the theory of long-distance power transmission lines and electromechanical transients, and methods of analyzing electric power system stability were used. A.M. Lyapunov’s first approximation method was used to develop a simplified mathematical model. We applied the developed software as a simulation tool. An analysis was carried out to study the influence of series compensation devices regulation coefficients on the aperiodic steady-state stability of the electric power system and the transmission capacity of 220 kV power transmission lines. A change in the modulus of voltage drop at the power transmission and the angle characteristics under the influence of the regulation coefficients of the series compensation device was revealed. A criterion of aperiodic steady-state stability has been developed for systems of this kind with controlled series compensation. It differs from traditional ones by considering the changes in the voltage drop in the power transmission and it allows more accurate estimation of the proximity to the stability threshold. An assessment of aperiodic steady-state stability according to the formulated criterion for an electric power system with a controlled series compensation device on a 220 kV line was obtained. The values of the control coefficients of the series compensation device have been determined. No violation of the steady-state stability occurs under the given values. The results can be used to solve the issues of increasing the transmission capacity of transmission lines to improve the stability of the system.

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According to the rules of installation of electricity-generating equipment, synchronous machines (generators, compensators, electric motors) must be equipped with automatic excitation control devices. Their application has a positive effect on the stability indicators and electrical power-engineering system modes. Currently, the development of industry and an increase in the number of consumers require transmission capacity growth of existing 220 kV power transmission lines. The use of controlled series compensation devices can significantly increase the transmission capacity of a power transmission line, however, there is a problem of stable operation of the electric power-engineering system. To choose the methods for control parameters of automatic excitation control and controlled series compensation device, it is advisable to analyze the oscillatory steady-state stability of the electric power-engineering system that contain a controlled 220 kV power transmission line when regulating the excitation of its generators. Methods of mathematical modeling of the electric power system, the theory of long-distance power lines and electromechanical transients, methods of analyzing the stability of electric power systems are used. The original software in the C++ programming language has been used as a modeling tool. The authors have analyzed the influence of controlled series compensation of high voltage transmission line and generators of automatic excitation control on oscillatory steady-state stability of electric power system. The parameters value of regulation of the controlled series compensation device and the automatic excitation control are determined, considering restrictions while maintaining the positive influence of these devices. Zones of stability of the examined electric power-engineering system are formed depending on setup variable of the devices under consideration. The obtained results can be used to enhance oscillatory steady-state stability of electric power system with controlled series compensation device and automatic excitation control systems.