Analysis of Performance of Adjustable-Speed Drives during Voltage Sags

The induction motors are the most common electric machines on industrial systems and with extended applications when adjustable speed drives (ASD) are used. The speed drives are based on power electronic devices and therefore they are highly sensitive to electric disturbances such as voltage sags, interruption, etc. Voltage sags has become one of most common power quality problems in the electrical systems, producing negative effects mainly in loads with power electronic technology. In this paper, the analysis of the effects produced by voltage sags in the ASD and the induction motor are presented. The electric system used for the analysis is conformed by an induction motor, an AC drive with V/Hz control scheme and a step down transformer connected in Yd. The voltage sags were produced by faults in the electric system with a time duration of 6 cycles (0.1 s). The whole electric system was modeled and simulated in Matlab/Simulink environment. The operating conditions of the induction motor was 80% of nominal speed and full load. The obtained results show high sensitivity of the drive, mainly to the dc-link voltage drop, resulting in a motor speed drop and overcurrents in the drive feeders at the ending sag. The adopted parameters used as a limit for the speed drive disruption were 5% of variation in the motor speed and 1.5 p.u. for the peak current. The most severe effects occur with sags type A and G due to three-phase and two-phase to ground faults respectively. The effect of these sags produced a dc-link voltage drop higher than 30% and therefore the drive disruption as a result of the operating limits exceeded. With voltage sags type C and D, caused by single-phase to ground and two-phase faults respectively, the effects produced in the drive and the motor are negligible.