# Research on Energy Storage Optimization Operation Schedule in an Island System
## 1 Introduction  
In the past 20 years, the power generation cost of renewable energy and power storage systems (PSS) has been greatly reduced. This represents the use of renewable energy to replace a certain percentage of traditional thermal units around the world. In particular, power generation costs of the island systems are higher than those of large-scale power systems because their smaller scale is more economically competitive. It can be expected that more renewable energy will be added to island systems in the future. Because island systems use diesel generators, the energy cost is usually higher than that of large-scale power systems. In addition, island systems have special geographical environments. The following are important economic issues for island systems [1]. The overhead power lines are susceptible to salt injury. Therefore, some island systems will use underground cables for the power supply. Since underground cables are capacitive, generators need to absorb additional reactive power. If the diesel engine absorbs too much reactive power, it may cause the protection relay to trigger and let other generators trip, and eventually cause power outages. The addition of renewable energy will increase the uncertainty of the power system. Therefore, the island system prediction technology of renewable energy becomes more important. The planning and economic analysis of PSS are also important research topics. Only a few generators operate when the island system is off-peak. According to the N-1 principle, if one of the generators fails, it may cause serious power system accidents, so it is necessary to explore the dispatch strategy. In the power generation system, unit commitment (UC) has great significance, as it is the determination of committed units during a scheduling period that is crucial to power system operation [2]. Mixed-integer linear programming (MILP) is a common approach to solve this mixed-integer combinatorial problem, but it does not traditionally schedule the PSS. Many works of literature have studied the situation of island systems combiningrenewable energy and PSS. Some consider the fast response characteristics of storage systems to maintain the stability of renewable energy by adding several types of reserves in the system [3,4]. Others use PSS to reduce the curtailment of renewable energy [5,6], and likewise, its volatility [7]. The PSS is also not a new peak-shaving solution. The PSS can be used by itself [8] or can be combined with movable loads [9] to reduce the total operating costs of the system through peak-shaving. However, studies on the use of the PSS in the spinning reserve are still insufficient. Although the paper of [PERSON] and [PERSON] [10] studied its use as an additional ancillary service, they only considered it to solve the error in demand forecast. [PERSON] et al. [11] included the PSS in the reserve but only set limited participation from it in the spinning reserve. This paper aims to study the scheduling system of the penetration of renewable energy in Taiwan's Kinmen Island to reduce power system accidents, including unexpected situations, small-scale disturbances, and changes in load or renewable energy output during normal operation, by allowing full participation from the PSS in the reserve schedule. This study simulated two cases with two load conditions, minimum and maximum, to analyze the system's robustness in handling both up- and down-spinning reserve constraint to mitigate power system accidents. In the case of safe and stable power system operation, this paper considers the scheduling of both generators and PSS to minimize the economic costs of the system. The research architecture is shown in Figure 1. The input of the model was the load and PV profiles, from which the net load was computed, and the generator constants. The optimization problem included the objective function and system constraints.