## 1 Introduction  
As global energy policies try to reduce the use of conventional fuel-based generators, the rate of renewable energy penetration has increased. Following global trends, South Korea has adopted a renewable portfolio standard (RPS) to promote the utilization of sustainable energies. In 2012, renewable promotion policies, such as feed-in tariffs (FITs), subsidized renewable energy owners, regardless of the actual power generation. The shift from FITs to the RPS was due to a financial shortage. In order to solve this problem, the RPS was designed to impose renewable energy generation obligations on large suppliers instead of offering unconditional energy subsidies [1]. Within this policy, renewable energy owners can retrieve their investment by obtaining a renewable energy certificate (REC) credit for the actual generation and selling it in the REC market. Among many South Korean regions, Jeju Island has been aggressively adopting renewable energy as a way of achieving carbon-free power generation by 2030. With these efforts, renewable energy in Jeju exceeded 48.7% of the total energy generation in the winter of 2018 [2]. Although the increasing renewable energy supply is eco-friendly, the high fraction of energy produced by intermittent renewable energy sources, such as wind and solar, affects the power system's stability and reliability [3, 4]. The increase in renewable energy use can cause a reduction in the power system's inertia that is necessary for reliable operation. This reduced inertia can easily result in large frequency fluctuations. Jeju, where electricity has been mostly supplied by high-voltage direct current (HVDC) connected to the mainland, is especially vulnerable to the variability in energy supply caused by wind power [5; 6]. Wind farm operators should follow a grid code, defined by regulators, to connect their wind farm to the power grid safely. The grid code includes requirements for additional controller installation to control the voltage at the connected node and support the grid with a contingency energy reserve [7]. Among the terms of the code, power variation at the point of common coupling (PCC) due to wind power fluctuations is a major concern of wind farm operators and power system operators. European countries have set power gradient limits in their grid codes as the contribution from renewable energy generation increases [8]. Furthermore, though renewables are considered to be the cheapest energy sources, excess renewable energy generation has to be curtailed for operational stability [9]. The ramping that occurs at the PCC should also be considered since the reserve and ramping components are secured to prevent power shortages [10]. Although there are no requirements stipulated in the grid code for wind power fluctuations in Korea, it is expected to recommend that the power variation at the PCC in a 5 min period does not exceed 5% of wind farm capacity. Among the methods to mitigate power fluctuations, utilizing an energy storage system (ESS) is considered to be a key solution for the power system operation of large wind farms. [11]. An ESS can handle the power variability with flexible charging and discharging. In many studies, performance analyses were conducted under intermittent power systems with various ESS types, including flywheels, superconductors, and fuel cells. Among energy storage sources, the battery-based ESS (BESS) demonstrated a better performance [12]; its advanced technology and large market share were favorable for large offshore wind farms [13]. With these advantages, the operation and the planning methods for an ESS have been introduced to improve wind farm operations. The coordinated control method for ESS integration with wind turbines was proposed to reduce wind power fluctuations and to extend ESS lifespan [14]. An adaptive supervisory control scheme was designed for wind turbine-integrated systems and ESSs to improve power qualities [15].