## 2 Optimization without Consideration of the Reliability Criteria  
Considering that the future perspectives of the DER market will be revised in South Korea in October 2021, which will also review the availability of resources aggregated by BESSs and renewables, the resources in one of the power complexes were selected for this study, as illustrated in Figure 1.  
Currently, the DER in the complex does not contribute to the REC market via long-term contract with a fixed SMP and price of the REC. Thus, profit maximization is designed to allow the BESS to operate at its discretion with greater flexibility. Excluding emergency generators, the resources eligible for participation in REC markets are the WPG, the PV coordinated with a BESS, and the WPG coordinated with a BESS. In this operating scheme, BESSs are only dedicated to receive charge from the renewables when coordinated to do so. The total power injected into the external grid can be measured at each PCS installed.  
Considering the market settlement of the REC by KPX (Korea Power Exchange), an AO is eligible for two revenue streams. One refers to the SMP revenue, which is the total power injected and measured at the PCS. The other refers to the stream from the REC multiplier when it is discharged during the applicable windows. The scheduling system in the targeted site consists of only coordination between the WPG and the BESS, without flexible operating schedules of the BESS, which has fixed periods of operation. Furthermore, the BESSs in the site are operational without profit maximization, and it therefore has been fully discharging once it is charged.  
Figure 2 illustrates a comparison between the prior scheduling method and the scheduling method proposed. As mentioned in the Introduction, reliability constraints were added to the prior scheduling methods.  
Figure 1: Available resources as a DER in the subject power complex.  
The input variables for the model proposed are the same as the variables required for the prior scheduling methods, because the data required for DA scheduling in the local ISO were restricted to be the SMP determined one day ahead by the SO settlement team, the output forecast based on the data publicly available from the state weather forecast agency, prices of the REC, the multiplier after types of DER have been coordinated, and the technical data of the DER which differs between installations.  
One of the major differences between systems is the flexibility of the BESS coordination. The BESS scheduled by the scheduling method proposed was programmed to charge and discharge during the intervals where it would not be eligible for settlement of the REC with the multiplier, considering the total revenue to be generated from all the scheduling intervals, whereas BESSs scheduled in previous methods have operated in accordance with fixed intervals.  
This model assumes that the wind farm operator is not responsible for controlling resource variability because there are no penalties or incentives dependent on fluctuating wind power generation in the current policy. The major technical reason for coordinating renewables is to not to curtail the output of the resources, and clearly confirming the contribution from the BESS. Another reason is due to economic issues from the perspective of profit, because it is more profitable from the AO's perspective to leave renewables continuously contributing to the market without curtailments under current settlement mechanisms, unless the renewables have been registered to be dispatchable resources for the local SO.