This paper addresses issues of large power fluctuations and low voltage levels in 100% renewable energy power systems without synchronous power support. The control mechanism of energy storage converters is analyzed, and a reactive power optimization model is established, considering energy storage control and DC transmission characteristics. A reactive power optimization method based on energy storage converter control is proposed. This method utilizes energy storage converter control to optimize the system's reactive power, enhancing the voltage regulation capability while participating in active power balance and ensuring economical operation. Additionally, a mathematical model based on DC channel operational characteristics is established to meet the demands for DC transmission in renewable energy systems. The optimization results can be applied to DC transmission curve planning. Finally, a case study is conducted on a county-level power grid in Northwest China without synchronous power support. The active and reactive power decision variables at each node in the grid are solved using the Yalmip platform in MATLAB, verifying the effectiveness of the proposed method.