Safe connection of AC power facilities for parallel operation ensures minimum equalizing currents flow at the time of switching on. Therefore, the following conditions must be met: equality of voltage amplitudes, equality of voltage frequencies, compatibility of the directions of rotation of the voltage vectors, and compatibility of the phase angle between the vectors of the connected voltages. The control of the above-mentioned conditions and the selection of the moment of switching on is carried out by automatic or semi-automatic synchronizers, the former also performing voltage and frequency regulation functions.