upper change point的意思|示意

Well, the change in f is the value of f at the upper point here, which is x0+delta x, and minus its value at the lower point P, which is f of x0, divided by delta x.

f的变量就是f在上面的点时的值， x0+delta x，减去f在下面的点P时的值，x0，再除以delta x。

The lower-level problem is to maximize the loading margin under a load change direction and the upper-level problem is to find the closest critical point subject to the constraints of load change.

下层问题求解某个负荷变化方向上的最大负载裕度，上层问题考虑负荷变化方向的约束，寻找最近的电压稳定临界点。