Assessment on Voltage Sag in Distribution Network Considering Action Time of Relay Protection

In traditional assessments on voltage sag are simply assumed that the duration obeys some distribution of random or is a constant value. In order to obtain a more accurate assessment of voltage sag, the more accurate duration of voltage sags are needed. The duration of voltage sag caused by fault is depended on the fault clearing time of the protection system. Time characteristic of stepped current protection and protection scope of instantaneous current protection are taken into consideration, the two demarcation points of the protection operation on line are determined, the protection operation time of every segment is analyzed. An example is simulated by PSCAD/EMTDC software, and it is shown that more reasonable assessment is obtained by precise duration of voltage sag.