This paper proposes an extension of the traditional ac Optimal Power Flow (OPF) model to incorporate the Voltage Source Converter based Multi-terminal High Voltage Direct Current system (VSC-MTDC). In the extended OPF model, the detailed VSC-MTDC system model is included, and the key technical limits of VSC (e.g., the maximum VSC valve current and the maximum dc voltage) are used as the operating constraints of the VSC-MTDC system. The extended OPF model is applied to a case study using the Nordic 32-bus system to evaluate the contribution of the embedded VSC-MTDC system on the overall system loss reduction. The study results show that the overall system loss of an ac transmission grid is not necessarily reduced by embedding a VSC-MTDC system into it. The reduction of the system loss by embedded VSC-MTDC system depends on their configuration and location, and the VSC station loss ratio as well.