Chapter 12 On Control of Energy Storage Systems in Microgrids Yu Wang a* , Sidun Fang a , Yan Xu a a School of Electrical and Electronics Engineering, Nanyang Technological University, Nanyang 50 Ave, 639798, Singapore. Corresponding author: Yu Wang Email: wangyulz@hotmail.com. Tel: +65 97365935 12.1 Introduction The traditional energy structure highly depending on fossil fuels such as coal and oil has become a major concern of climate change and air pollution in modern society. These environme nta l concerns and energy crises of fossil fuels lead to the rapid development of renewable energy technologies [1], [2]. A large number of renewable energy sources (RESs) together with energy storage systems (ESSs) have been penetrated into existing power systems especially distributio n sides through power electronics interfaces. The power generation becomes more and more decentralized other than the conventional centralized generation, which results in the concept of distributed generation. In power distribution systems, a cluster of demand-side loads and distributed energy resources can be connected and disconnected from the main grid to operate in grid-connected or islanded mode. These small-scale power systems are named as microgrids. The original idea of microgrids emerges at the beginning of this century, which aims to benefit the integration of distributed generators (DGs) and enhance grid resilience [3]. In grid-connected mode, the microgrid can be viewed as one entity, which exchanges power with the main grid to realize energy trading and provide ancillary services [4], [5]. In the islanded mode, the major target of the microgrid is to maintain local generation/demand balance, while providing stable and high-qua lity power supply [6]. According to the current flow, microgrids are usually categorized into alternative current (AC), direct current (DC), and hybrid AC/DC microgrids [7], [8]. The residential, commercial, and industry microgrids have been built all over the world to benefit the renewable penetrations, grid resilience enhancement as well as traffic electrification [9]-[11].