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Significant penetration of distributed generation in many distribution systems has opened an option of operating distribution systems in island mode for economical and technical reasons. However, balancing frequency of the islanded system is still an issue to be solved, especially when the demand exceeds the generation in the power island. This paper presents a strategy to shed an optimal number of loads in the island to stabilize the frequency. The load shedding strategy is based on frequency information, rate of change of frequency, customers' willingness to pay, and loads histories. Different scenarios have been simulated with results showing that the proposed method is effective in shedding the optimal number of loads while stabilizing frequency.Index Terms-Distributed generation (DG), islanding, rate of change of frequency, underfrequency load shedding, willingness to pay (WTP).

The mainly used islanding detection techniques may be classified as active and passive techniques. Passive techniques don't perturb the system but they have larger nondetection zones, whereas active techniques have smaller nondetection zones but they perturb the system. In this paper, a new hybrid technique is proposed to solve this problem. An average rate of voltage change (passive technique) has been used to initiate a real power shift (active technique), which changes the real power of distributed generation (DG), when the passive technique cannot have a clear discrimination between islanding and other events in the system. Simulation results show that the proposed method is effective in detecting islanding of distribution systems with DGs.