This article has been accepted for inclusion in a future issue of this journal. Content is final as presented, with the exception of pagination. IEEE TRANSACTIONS ON SUSTAINABLE ENERGY 1 Qualification and Quantification of Reserves in Power Systems Under

Abstract—The presence of high levels of renewable energy resources (RES) and especially wind power production poses tech-nical and economic challenges to system operators, which under this fact have to procure more ancillary services (AS) through var-ious balancing mechanisms, in order to maintain the generation-consumption balance and to guarantee the security of the grid. Traditionally, these critical services had been procured only from the generation side, yet the current perception has begun to rec-ognize the demand side as an important asset that can improve the reliability of a power system, offering notable advantages. In this study, a two-stage stochastic programming model, repre-senting the day-ahead market clearing procedure on an hourly basis and the actual minute-to-minute operation of the power sys-tem, is developed comprising different services that specifically address various disturbance sources of the normal operation of a power system, namely intra-hour load variation, intra-hour wind variation, as well as generating unit and transmission line outages. Index Terms—Ancillary services, contingency reserves, demand response, load-following reserves, stochastic programming. NOMENCLATUR