The growth of power systems, increase of distributed energy resources and its associated complexity make power system simulations a tedious task, which leads to the emergence of new software, hardware and technologies. The non-compatibility between technologies represents an obstacle, which can be solved by co-simulation. However, some research facilities have a lack of equipment to investigate particular test cases. This paper aims to establish a link between two different geographically distributed simulators into a real-time simulation, by means of the co-simulation environment OpSim. A delay assessment is performed and a definition of limits and boundaries for the co-simulation environment is established. Finally, a Centralized Voltage Controller is linked to an OPAL-RT simulator through OpSim and its functionality is verified.