## 1 Introduction
Besides designing a control scheme, optimal ESS scheduling strategies have been proposed to improve contract fulfillment and to minimize the curtailment of renewable energy production under uncertain circumstances [16; 17]. For efficient wind farm operation, it is important to determine the ESS's size and utilize it efficiently. Methodologies for choosing an adequate storage size have been proposed to handle ramping events and mitigate scheduling errors caused by intermittent power generation [18]. Determining an installation site for an ESS is a major consideration in planning the coordinated system under the stochastic nature of power generation and load [19; 20]. Furthermore, an optimal management policy was designed to maximize benefits and select an optimal storage size [21]. In short, the proper operation and planning strategies of an ESS can help wind farm operators and power system operators minimize their operational costs under an intermittent power supply. Korea has encouraged ESS installation on wind farms through the provision of subsidies. According to the revenue agreements for wind power generation and ESS discharging power, revenue is dependent on peak and off-peak periods [22]. The power provided by the PCC is sold at a system marginal price (SMP), and the wind power generation transmitted to the PCC can earn an REC credit at all periods. However, during peak periods, wind farm operators can obtain additional REC credits for power discharged from their ESS. With this incentive policy, wind farm operators might recoup their ESS installation investment. In reality, however, peak-biased incentives affect power system operations since wind farm operators discharge their ESS power only during peak periods when additional revenue can be earned [23]. Although the power output of wind farms can fluctuate at all times, the lack of incentive for utilizing the ESS during off-peak periods may increase power variability as wind energy penetration increases. Hence, considering peak-biased incentive policies, ESSs cannot adequately support the power system at all periods. While power system operators agree that utilizing energy storage resources is important for stable system operation, no study has focused on the effect of peak-biased incentives on wind power fluctuations. To address this, we evaluated the interaction between imposing an REC multiplier for discharging power during off-peak periods and the number of power shortages under Korea's renewable energy policy. This study addresses the following aspects:  
* A coordinated model of a wind farm with an ESS is constructed to model actual behaviors of wind farm operators;
* A revenue optimization problem is modeled as a mixed integer linear programming (MILP) problem based on the settlement rules of wind power generation;
* An objective function is modified to consider additional revenue obtained by discharging in off-peak periods; and
* The effect of imposing an additional incentive in off-peak periods is analyzed according to the REC multiplier magnitude. The rest of this paper is organized as follows: Section 2 describes an optimization model of a wind farm with an ESS. The modified optimization model to induce discharge in off-peak periods is introduced in Section 3. The effects of an REC multiplier in off-peak periods on mitigating wind power fluctuations are analyzed through a case study in Section 4. We conclude in Section 5.