The influence of motor re-acceleration on voltage sags

The assumption that a voltage sag is rectangular, no longer holds in a system with large induction motor loads. The motors decelerate during the short circuit. After fault-clearing they will accelerate again, drawing a high reactive current from the supply, causing a prolonged post-fault voltage sag. This is aggravated by the removal of branches by the protection. The resulting shape of some voltage sags in an example system is shown and discussed. For the example system a stochastic voltage sag table is determined. This table gives the expected number of sags of different depth and duration. The influence of faster protection and of reduced transformer impedance on the table, is presented. A simple motor model is implemented in a method for including interruptions due to voltage sags in the reliability analysis of power systems. This model is presented briefly and used to show the influence of motor parameters on the number of sags that lead to an interruption of plant operation