The aim of this work is to investigate, in steady state and dynamic performance, the phenomenon of the opposite relationship, already observed at real operation conditions of the Brazilian Electric System, between generators and synchronous compensators excitation voltage and the controlled one. In these situations, the generator / synchronous compensator nominal capacity, for example, would not be useful to keep the voltage controlled. Due the opposite relationship, an increase in the excitation voltage would reduce the controlled voltage. The automatic control would keep acting and reducing more the voltage. This mechanism can lead the system to the collapse. The study of this problem was based in the generator / compensator behavior as a control voltage device, in steady- state and dynamic performance, front of several operation situations of electric power system, like reference voltage (controlled voltage) variation and load changing. The steady state analysis used a load flow algorithm, while the time domain simulation was utilized for the dynamic performance analysis. The real existence of the phenomenon was verified through these analyses, emphasizing the operation region changing in some of them. In other cases, the analyses results in the steady-state were different of the dynamic performance results.