The scope of this thesis lies in the development of a Unit Commitment (UC) optimization model, in order to conduct analysis of its results regarding the flexibility issue. Due to the increasing penetration of variable renewable generation and its mandatory injection to the system, the daily net load demand curve shows sharp fluctuations, especially during hours of high solar generation. Therefore TSO commits CCGT (Combined Cycle Gas Turbine) plants, in order for them to provide ramping and

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... lary services. Due to their technical limitation of minimum stable generation period, CCGT plants are usually committed hours before these abrupt load changes occur, staying at minimum stable generation level, being deprived the possibility of being SMP(System Marginal Price) makers. Considering the fact that SMP is mostly set by Lignite Plants, it occurs that CCGT plants are remunerating poorly with SMPs lower than their marginal costs. Especially Independent Power Producers (IPP) of CCGT plants face the difficulty of economic survival in Greek Electricity Market, despite the providing ramping service and if we consider the increasing penetration of Renewable Sources, a flexibility issue arises. The UC model, developed in this thesis, is formulated as a Mixed Integer Programming (MIP) Mathematical Model and is solved in GAMS environment. It contains constraints, referring to the technical characteristics of production units (technical minimum generation, minimum stable generation period, ramping rates etc.), as well as limitation regarding power balance and ancillary services requirements. Each producer bid a step-wise supply function and a price for a certain amount of ancillary service. The model has an annual solving time horizon and an hourly time resolution (1-8760 hours). Furthermore, solving the corresponding Relaxed Problem, enables the determination of the hourly SMP. The numerical example presented is based on Greek Electricity Market, projecting years 2014, 2018 and 2020 and examines three different scenarios. A base or reference scenario for years 2014, 2018 and 2020, a RES scenario, for year 2020, imposing higher penetration of renewable generation (up to 40% of the annual demand) than in year 2020 of the base scenario and a high Imports scenario for year 2014, 2018, 2020. The last scenario is based on the two assumptions. Firstly the increase of Imports and secondly the increase of bidding offers, related to the IPPs, in order for them to face the economic difficulties, as described above. This thesis includes the obtained results of each scenario, as well, as the corresponding conclusions.