Measures to keep islands stable after power oscillation or out of step islanding

In China, when an interconnected grid is subjected to large disturbances and becomes out-of-step, splitting the out of step area from the main grid has been used widely as the last resort for preventing widespread blackout. If an area that will be split from the main grid has large generation deficit, low voltage and high frequency may occur at the beginning of the island´s forming. Sometimes, the frequency may be so high that generator overspeed protection acts to trip generators cascading or simultaneously. Thus, measures based on underfrequency are invalid. In addition, the undervoltage load shedding measures can not work quickly enough due to its time delay. At this case, an integrated measure is proposed to keep the island stable. That is, a certain proportion of load is shed when system splitting is being operated; the quantity of the load shed can be set according to the generation deficit of the area. After that, other load shedding measures can act under certain conditions. This can improve the voltage profile, increase the generators´ output power, and limit island frequency. Simulation results show that the integrated measure can keep the island stable.