Loss of excitation (LOE) protection is one of the most important protections of synchronous generators. Nowadays, application of an impedance relay with two negative offset operation zones to protect LOE is prevalent; this relay measures the output voltage and current of generator to detect LOE. On the other hand, utilization of flexible AC transmission systems (FACTS) devices in power systems for different applications is soaring; these devices cause changes in voltage and current signals in

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... e utilized power system. Therefore, this paper is dedicated to investigate the amounts of changes in the current and voltage signals measured by the LOE relay when the static synchronous compensator (STATCOM) and static synchronous series compensator (SSSC) are used. Research results show that the changes in the measured signals manipulate the operation of the relay. It is also shown that partial LOE culminates in an under-reach of the relay. In surveys conducted, various operational conditions of the power system and FACTS devices are considered as well.