Abstract: The voltage source converter based high voltage direct current（VSC-HVDC） technology is a potential transmission method for new energy grid-connection and DC grids. A DC/AC converter for VSC-HVDC transmission system is designed and constructed in Zhangbei data center. The designed DC/AC converters adopts the structure of paralleled multi-converter+z-type grounding transformer. In order to suppress the output voltage unbalance and output voltage distortion caused by load characteristics in the off-grid mode, an unbalance control strategy and a harmonic suppression strategy of converter output voltage are proposed to ensure the quality of power supply. In order to ensure the uninterrupted power supply and improve the power supply reliability of the equipment, an active current-limiting control strategy is proposed to limit the current actively when the power system has a short-circuit fault in the off-grid power supply mode. Finally, a 2.5 MW DC/AC converter is built for experimental research. The experimental results verify the effectiveness of the proposed control strategy. The designed converter has been applied to the flexible substation in Zhangbei data center.