Voltage-frequency coupling effect in power grids and a novel load shedding scheme against it

Frequency stability and voltage stability are two main kinds of stabilities in power grids. Researchers studied the two problems independently in the past. However, few studies have been done on the relationship between these two kinds of instabilities. Moreover, coupling effects of voltage and frequency dynamic on the traditional last line of defense in power grids have not been researched yet. This paper analyses the voltage-frequency coupling effect based on typical system. We found that when the coupling effect is strong, the system frequency response trend cannot be correctly decided only by initial active-power deficit. Initial reactive-power deficit and the ratio of induction motor in loads are two key factors dominating the degree of coupling. Further, based on simulations it is found that the traditional last line of defense in power grids would fail to prevent the system collapse when the coupling effect is significant. The failure cause is analyzed. Finally, we put forward a novel adaptive load shedding scheme using local measurements based on dynamic of the coupling, the control fundamental is strictly proved and the whole structure is introduced in details. Simulations based on typical system validate the effectiveness and adaptability of the proposed method compared with the traditional last line of defense applied in power grids.