The increased interest in using ultra-high-voltage (UHV) transmissions has stimulated the research on UHV operation characteristic and correlative relay protections. Since the operational experience in UHV system is limited, there is a need to model UHV transmission systems and test the performance of traditional protection devices already in use on high-voltage and extra-high-voltage systems. This paper presents the development of a test environment based on a real-time digital simulator of a UHV power system model and the results of testing a distance relay using the model. It reveals that the protective zone is enlarged in the UHV system with shunt reactor compensation and is reduced without the compensation. To resolve these issues further investigation is needed and ways of resolving them, including the development of new algorithms, should be examined.