THE INFLUENCE OF MOTOR REACCELERATION ON VOLTAGE SAGS

The assumption that a voltage sag is rectangular is not correct in a s ystem with large induction motor toads. The motors decelerate during t he short circuit. After fault-clearing they will accelerate again, dra wing a high reactive current from the supply, causing a prolonged post fault voltage sag. This is aggravated by the removal of branches by th e protection. The resulting shape of some voltage sags in an example s ystem is shown and discussed. For the example system, a stochastic vol tage sag table is determined. This table gives the expected number of sags of different depth and duration. The influence of faster protecti on and of reduced transformer impedance on the table is presented. A s imple motor model is implemented in a method for including interruptio ns due to voltage sags in the reliability analysis of power systems. T his model is presented briefly and used to show the influence of motor parameters on the number of sags that lead to an interruption of plan t operation.