With the centralized access of large-scale new energy sources and the increase of DC output, the sending power grid is faced with transient overvoltage and steady-state overvoltage after an accident. The safety effect including transient stability, power flow organization and adequacy effect are comprehensively evaluated based on the UHVDC operating characteristics of the sending power grid. On this basis, first, the supporting effect of a thermal power unit on the static voltage of a multi-circuit DC converter station is analyzed. Then the dynamic voltage supporting effect of the DC predictive fault set is obtained by integrating the unit reactive power over time under the fault. Combined with the supporting effect of the static and dynamic voltage, the optimal startup sequence of the DC supporting unit is determined, and the DC stable control tripping capacity is taken into consideration. Finally, the minimum startup method of the DC near-area is determined and verified by an actual grid example. This work is supported by National Key Research and Development Program of China (No. 2017YFB0902000).