I. Introduction
As A Clean and sustainable energy, renewable power generation has received wide attention and rapid development around the world. However, most wind power or solar generation operates at maximum power point tracking (MPPT) mode and decouples from system frequency. As a result, it leads to very limited inertial response and primary frequency regulation compared to conventional generators [1], [2]. Thus large-scale renewable power integration brings significant challenge to fast frequency control of power system [3], [4]. Following a fault such as a generator trip, frequency of a system with high penetration of renewable power generation is likely to drop rapidly and then gets to a low nadir. When the frequency drops below the preset threshold for under frequency load-shedding (UFLS), a certain amount of load will be shed immediately and automatically so as to maintain system security. However, it decreases the reliability of power supply and results in large economic losses.