Serial controllable FACTS devices, such as a Controllable Series Compensator (CSC) and a Static Synchronous Series Compensator (SSSC), can have a significant impact on the voltage stability of an electric power system. This paper presents a mathematical derivation for the voltage dependency of a CSC and a SSSC in a single-load infinitive-bus model. The comparison of the impact of a CSC and a SSSC on the voltage stability in a five-bus system confirmed the new analytical equations. According to theoretical considerations, from the point of view of voltage controllability, each device has its own advantages, but in the case of low voltages or low loads a SSSC is superior to a CSC.