Enhances the stability of electric drives of continuous production in voltage sags

The short-time voltage dips cause electric drive cutoff and technological regime impairments. The researches on the different metallurgical enterprises show that a voltage dip value distribution has a random character. Generally, these voltage sags are caused by one-phase faults. The technical solutions decreasing an occurrence probability of voltage sags at the OAO “Magnitogorsky metallurgichesky kombinat” (OJSC Magnitogorsk Iron and Steel Works) are described. These technical solutions imply division of 110 kV loop networks of interior plant power supply into independent circuits, essential direct connection to 6-10 kV generator voltage of house-service low-power stations, using free-running generator as independent power supply. The analysis of the cold strip mill synchronous electric drives with AFE frequency converters is performed. The different variants of active rectifier control algorithms with adaptation to asymmetrical power supply voltage are described. In this case the negative sequence voltage is used as a correction signal for forming of three-phase voltages at the input of the frequency converter. The vector diagrams of line and phase voltages for a situation of 50 % voltage drop are shown. The calculation results of the modulation indexes necessary for voltage sag compensation and saving of the frequency converter stability are given. The limits of the frequency converter stability work for one-, two- and three-phase faults are determined. The variant of the stability increasing for active rectifiers by means of modulation indexes equality for each phases are described. It is shown that range of the residual voltages in which the active rectifier preserves stability is expanding. In general, realization of these methods helps to keep converter stability with smaller value of the residual voltages.