中文说明：
利用逐步倒推法进行电网规划。分第一和第二两个阶段，第一阶段是通过编程获得最小经济网络；第二阶段是通过编程获得最后的最优网络。
English Description：
The step-by-step backward method is used for power grid planning. The first stage is to obtain the minimum economic network through programming; the second stage is to obtain the final optimal network through programming.