A brief review of technical solutions for the implementation of controlled 'flexible' connection of asynchronously operating power systems was made. Transmission of active power between power systems with the same nominal frequencies, linked via phase-rotating transformer is examined. The device is based on hexagonal scheme, which provides symmetrical 12-phase output system of voltages, wherein the various phases of it are connected to different phases of three-phase receiving system by means of semiconductor switching elements. To reduce the oscillations of transmitted active power, caused by step switching of the phase angle between the voltages of the connected systems it is proposed to use pulse-width modulation method, which provides a smoother angle switching. It was shown that starting from a certain length of the connection line and at sufficiently high modulation frequency, the level of harmonic distortion of currents and voltages on the buses of receiving power system could meet the requirements imposed on higher harmonics content in high voltage electrical networks without the use of additional filters.