AS THE WORLD FACED THE g r ip s of t he COV I D -19 pa ndemic, the mere idea of potential lockdowns sent packages of toilet paper flying off the shelves. That has been followed by the loss of jobs and income that staggered economies globally. Fortunately, power and energy system operation has remained largely stable. But imagine the further hoarding and pandemonium that would ensue should the electrified world's societies confront shortages in access to the energy on which they have become so

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... pendent. Think of the economic collapse that would transform the orderly operation of businesses and governments. Anxiety at the thought of the reactions to such situations does more than lead to lack of sleep. Many of us in this business have jobs that directly consider the short-and long-term availability of uninterrupted energy access. Most of the rest of us work to see that energy is delivered safely at an affordable price. These two aspects form the foundation for exploring energy security. A host of global, regional, and local issues inspires socioeconomical trends and policies that drive changes in the energy supply chain. Our energy systems are evolving in response to these trends and policies as spurred on by seemingly faster leaps in technology-based capabilities. The responses to climate change, and the factors that may encourage it, have given rise to advancement in renewable resources being produced at cost-effective rates. This leads to an associated reduction in more traditional fossil supply systems characterized by an abundance of spinning mass to which we have applied decades of engineering advancement in design and paradigms for system operation. Innovation in batteries, sensing, communications, and information technology gives rise to economic drivers for cooperative operating arrangements that harness greater efficiencies and offer future promises of robustness and resilience in response to system operation perturbations. But this new type of system is not so familiar. Now, as in the past, our industry is responding to change to better understand the complex interplay of these emerging shifts so that we may attain some level of proficiency in managing these evolving power and energy systems. May our success in this endeavor afford us a moment to rest as we summon new strength for the next set of energy security challenges that are sure to follow. In This Issue The articles in this issue explore the operational security of the world's power systems in the face of change and associated system challenges as seen by the higher penetration levels of renewable energy generation, increased adoption of new grid technologies, and market developments for grid services to address an expanding suite of operational issues. Guest Editors Antonio Conejo and Federico Milano have assembled an international contingent of experts in various specialized areas of operational security to provide a truly comprehensive perspective of the wide range of challenges and solutions evolving in this space. In total, the seven feature articles include perspectives from six countries and seven separate power systems. The feature articles discuss the following: ✔ the operational challenges being managed in Texas, given sustained growth in renewable energy generation capacity ✔ an in-depth analysis of the interdependency of California's natural gas and electric power system to ensure the secure operation of the electric power system as it is modernized ✔ the challenges to and impacts on power system short-term stability being experienced and addressed in Ireland as higher levels of renewable energy operate within the system ✔ a view of how the Danish power system can meet operational security and decarbonization goals harmoniously ✔ the operational security issues realized in the strongly interconnected Swiss power system as the power systems of both Switzerland