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The uncertain and variable nature of renewable energy sources in modern power systems raises significant challenges in achieving the dual objective of reliable and economically efficient system operation. To address these challenges, advanced scheduling strategies have evolved during the past years, including the co-optimization of energy and reserves under deterministic or stochastic Unit Commitment (UC) modeling frameworks. This paper presents different deterministic and stochastic day-ahead UC formulations, with focus on the determination, allocation and deployment of reserves. An explicit distinction is proposed between the uncertainty and the variability reserve, capturing the twofold nature of renewable generation. The concept of multi-timing scheduling is proposed and applied in all UC policies, which allows for the optimal procurement of such reserves based on intra-hourly (real-time) intervals, when concurrently optimizing energy and commitments over hourly intervals. The day-ahead scheduling results are tested against different real-time dispatch regimes, with none or limited look-ahead capability, or with the use of the variability reserve, utilizing a modified version of the Greek power system. The results demonstrate the enhanced reliability achieved by applying the multi-timing scheduling concept and explicitly considering the variability reserve, and certain features regarding the allocation and deployment of reserves are discussed. [ABSTRACT FROM AUTHOR]

Variable and uncertain conditions associated with increasing renewables introduce prominent challenges in the optimal power system operation and necessitate the reinforcement of flexible capability potential provided by the eligible resources. To address these challenges, some U.S. ISOs have incorporated specific “flexible ramp products” in their short-term scheduling processes. This paper proposes an enhanced modeling approach to underpin flexibility in European Central Dispatch systems; the model employs a “multi-timing” day-ahead scheduling method, which allows for the optimal procurement of such ramp products on an intra-hourly (“real-time”) basis, when concurrently optimizing energy and reserves over hourly time intervals. Additionally, a probabilistic method for the quantification of the flexibility requirements is proposed, to align the real-time system ramping needs with the intra-hourly optimization intervals. The model is evaluated using the Greek power system for three market configurations; results demonstrate enhanced flexibility features in terms of market efficiency and system reliability. [ABSTRACT FROM PUBLISHER]