The increasing proportion that renewable energy accessing to sending-end power grid leads to the decrease of voltage support capacity, and the problem of transient voltage instability is likely to occur when power grid operates in the mode of minimum generation. Based on Northwest grids as an example, it reveals the problem of transient voltage instability with the typical renewable energy sending-end power grid, and analyzes the advantages of the synchronous condenser and the effects with synchronous condenser locations on the transient voltage stability characteristics after the accident. And further combining prevention and emergency control, through coordinating and optimizing the configuration capacity and location of the synchronous condenser before the accident and the emergency control after the accident, it proposes an optimization configuration method of synchronous condenser for the weak voltage supporting sending-end power grid, which is based on security constraints and aims to minimize the control cost. Finally, the method is validated through the actual typical grid. This work is sponsored by National Key Research and Development Program of China (No. 2017YFB0902000) and Northwest Division of State Grid Corporation of China.