Hardware-in-the-loop (HIL) simulation plays an important role in the research and development of HV/UHV DC systems, of which the key technology is the real time simulation model of MMC-HVDC. In this paper, the real time modeling technology of MMC-HVDC is studied. Especially, a gate signal equivalent method is proposed for modeling of a half and full bridge hybrid MMC applicable for UHV HVDC system. Simulation results show that the proposed method provides high accuracy, while requiring no extra simulators. A three-terminal UHV HVDC system is modeled, and a series of HIL tests are carried out, which show the effectiveness of both the simulation method and control strategies.