Simple and reliable control is conducted by PLC in lower level, and by adopting virtual instrument in upper level for complex loading control.

下位机采用PLC进行简单可靠控制，上位机采用虚拟仪器进行复杂的加载控制。

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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。

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Though the Taliban has earned praise for deftly handling the crisis, two French hostages say they saw their captors loading pistols after meeting lower-level members of the tribal militia.

CNN: Operation Bungle?