Operational planning is an integral part of maintaining economic operation of the bulk power system and keeping the lights on. Traditionally, power systems have been designed without ramping capability concerns, such that operational planning needed only focus on capacity adequacy, while assuming sufficient ramping capability. Today, the integration of renewable energy sources has led to increased variability and uncertainty in the power balance, thus warranting the need to carefully assess the ramping adequacy of resources on top of capacity requirements. Moreover, the advent of energy-constrained resources, such as energy storage and demand response, has warranted the need to plan power systems in the face of energy scarcity, in addition to ramping and capacity scarcity. Consequently, traditional operational planning is becoming less capable of handling the new reality of modern sustainable power systems integrating high levels of renewable energy. This has led to the emerging concept of power system flexibility, to emphasize on the need to plan capacity, ramping, and energy altogether. Concurrently, there has also been an increasing interest in stochastic planning to handle the increased uncertainty. That being said, the current state of the art on operational planning remains confined to the traditional thinking, especially when defining operating reserve requirements. To that end, this dissertation proposes a modernization of the concept of operating reserve, in light of the emerging concept of power system flexibility. The concept of a flexibility requirement envelope is introduced to accurately capture the capacity, ramping, and energy requirements arising from variability and uncertainty. Equally, the concept of a flexibility envelope is proposed to capture the capacity, ramping, and energy capability of a power system resource. Adequate system flexibility is asserted when the aggregate flexibility envelope of power system resources encloses the flexibility requirement envelope while performing operational planning. The concept of envelope enclosure is formulated both deterministically and probabilistically, and its effectiveness is illustrated via several examples throughout the dissertation. Furthermore, the dissertation proposes dynamical operational planning, which is expected to open new avenues in operational planning research and practice.ii Résumé La planification opérationnelle fait partie intégrante du maintien d'un régime opérationneĺ economique et fiable des réseauxélectriques. Traditionnellement, les réseaux ontété conçus sans avoirà se soucier des phénomènes de rampes de puissance, de telle sorte que la planification opérationnelle devait uniquement traiter l'adéquation des capacités en partant du principe que les capacités de rampe deséquipements du réseauétaient suffisantes. De nos jours, l'intégration des sources d'énergie renouvelables a contribuéà augmenter la variabilité et l'incertitude dans l'équilibre de puissance entre la production et la charge. Cela nécessite d'évaluer p...
