This paper presents a study of the impacts of the Flexible AC Transmission Systems (FACTS) devices on the transmission line impedance measured by numerical impedance protection relay when applied to protect a shunt compensated transmission line. Most of the FACTS devices are installed on existing transmission lines to enhance their capacity and performance. The Static Synchronous Compensator (STATCOM) is a shunt device of the FACTS family using power electronics to control power flow and improve transient stability on power grids. This paper investigates the effect of STATCOM operation conditions during voltage fluctuations and faults on the impedance measured by numerical impedance relay. Matlab with SimPowerSystems toolbox is used to model and simulate a 500 KV, 300 Km shunt compensated transmission line. The effects of different fault types and different voltage fluctuation cases on the numerical impedance relay are investigated of compensated and uncompensated transmission line