Modular Multilevel Converter (MMC) HVDC system has become more attractive to apply for medium or high power conversion system due to its highly modularity, scalable features and excellent output voltage. Unlike the traditional LCC technology, MMC does not depend on the AC system to which is connected making this technology appropriate for active and also passive systems. Therefore, significant research has been conducted for dealing with challenges of its modeling and simulation. Detailed models of it with typical two orders of magnitude of switches dramatically increase the computational difficulty. Therefore, different more efficient modeling methods have been developed and proposed. This paper presents the detailed step-by-step modeling approach by using average value model(AVM) of MMC and its controller in PSCAD/EMTDC simulation software. Generic blocks in the software are used. Then a dual hybrid in-feed HVDC transmission system (employing the VSC-HVDC link connected in parallel with an LCC-HVDC link) is established to validate the model and emphasize the leverage of VSC-HVDC to prevent LCC-HVDC transmission commutation failure by providing reactive power to the latter. Both steady state and fault analysis of the hybrid dual in-feed HVDC transmission system has been simulated and discussed.