Influence of synchronization modes and out-of-phase generator switching on in independent power systems on the overvoltages in excitation windings

Abstract

One way to improve the reliability of power supply is to use several, parallel connected, different power sources. An example is hybrid power systems where the power sources may be a combination of a conventional power system and various types of renewable energy sources. In such systems, parallel operation of synchronous generators is provided. To enable parallel operation, the following three synchronization methods are used: accurate synchronization; self-synchronization; rough synchronization. Switching to parallel operation by the method of accurate synchronization is performed at the moment of sufficiently close coincidence of amplitudes, frequencies and phases of voltages of the connected generator to the network with strict coincidence of these values; synchronization proceeds ideally in the sense that at the moment of switching on the generator there are absolutely no current shocks. However, in independent power supply systems where the voltage and frequency of the network are not strictly constant, it is difficult and sometimes impossible to fulfill the conditions for accurate synchronization. On the other hand, it is impossible to deny the possibility of introducing errors during synchronization with control devices. Therefore, it is of interest to establish the limits of permissible deviations from the conditions of accurate synchronization under which overvoltages in the excitation circuit do not occur. As the practice of setting generators to parallel operation shows, one of the most difficult and long-term operations is to establish the equality of the initial phases of the voltages of the connected generator and the network. Failure to accurately synchronize the initial mismatch angle leads to the appearance of equalizing current in the stator winding depending on this angle. The stator flow created by the equalizing current has an aperiodic component which, when the rotor rotates, induces electromotive force of the main frequency in its circuits. As a result, alternating currents occur in the excitation and damper windings. The variable component causes pulsation of the total current which under certain conditions may become negative.