Performance evaluation of impedance-based synchronous generator out-of-step protection in the presence of unified power flow controller

Flexible ac transmission system (FACTS) controllers change the transmission line voltage and current signals measured with protection relays. These devices affect the operation of impedance-based relays and can lead to relays under/over-reaching. This study investigates the effects of a unified power flow controller (UPFC) as one of the most important shunt-series compensators on the synchronous generator out-of-step (OOS) protection. A detailed model of the UPFC is used for the purpose of the study and its performance has been simulated in three operational modes of the shunt-series compensator including static compensator (STATCOM), static synchronous series compensator (SSSC) and UPFC. The results show that all the three modes of the UPFC change the impedance trajectory and disrupt the diagnosis of the OOS protection. Moreover, an analytical approach is developed to eliminate the negative effects of the devices. The approach only requires the UPFC voltage and current synchro-phasor data to calculate the modified impedance. The results show that the presented method properly eliminates the negative effects on the OOS relay and prevents it from under-reaching in all the UPFC operation modes. The most important advantage of the modified algorithm is that it can be generalized to other types of FACTS devices.