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A HVDC grid based on voltage source converters (VSC) is an attractive solution for the grid-integration of offshore renewable energy, interconnection of AC grids, facilitation of power markets, and energy emergency support. However, the power flow within the DC grids is determined by the line resistance, and cannot be fully controlled by the VSCs only. Thus, Power flow control of DC grids are needed not only to realize the economic operation and power markets, but also to prevent overload of DC lines in post-fault conditions. Three types of control devices, DC transformer, the variable resistor and series voltage source, are presented to achieve the power flow control. The topologies and modeling of these control devices are presented. Their control performance is evaluated through simulations using a 5-terminal DC grid. Their effectiveness, feasibility, and applications are discussed. Through the preliminary comparison, the series voltage source is considered as the best solution in terms of the rating, power losses, and control performance.