Foreword for the Special Section on Enabling Very High Penetration Renewable Energy Integration into Future Power Systems R

enewable energy such as wind power, photovoltaic (PV) is regarded as one of the main solutions to achieve power system sustainability. In recent years, renewable energy has been experiencing fast development. In 2016, 24.5% of the total power demand in the world was supplied by renewable energy, where 5.5% was produced by wind power and PV [1]. It is estimated that by the end of 2017 the global wind power and PV capacity had reached about 565GW and 335 GW respectively. Some power systems have already entered the era of high penetration of intermittent renewable energy integration. E.g., over 22.4% of the total power demand was supplied by intermittent renewables in Spain in 2015. It is reported that on July 11, 2015, the wind power generation reached 140% of the whole country's demand in Denmark [2]. In several provinces of China such as Gansu, Qinghai, and Xinjiang, the installed capacity of intermittent renewable energy have already occupied over 30% of the generation capacity [3]. Countries around the world have set aggressive goals for high penetration of renewable energy in future power systems. The integration of high penetration of renewable energy will fundamentally change the configuration of power system [4]. The way that power system balances the generation and load will change in both planning in long-time scale and operation in short-time scale, and in both transmission and distribution networks. The intermittency of renewables will raise power system security concern, ancillary services adequacy concern and economic issues. Hence, high penetration of renewable energy integration will bring significant challenges to future power systems. Failing to address these challenges may result in large amounts of wind power and PV curtailment and jeopardize the economics of renewable energy. Different countries have funded more and more projects for high penetration renewable energy integration research through certain departments, such as National Science Foundation (NSF) in the US, the biggest research and innovation programme H2020 in European Union, and National Science Foundation of China (NSFC). Take China as an example, in 2016 the National Key Research and Development Program of China approved a project entitled Fundamental Theory of Planning and Operation for Power Systems with High Share of Renewable Energy Generations. The overall objective of this project is to study the basic theory and method of planning and operation of high penetration renewable energy power systems and to provide standardized systems for a high share of renewable energy integration. This Special Section is dedicated to providing innovative insights on future power systems accommodating high penetration of renewable energy. We follow a two-tier review process. First, about 300 two-page extended abstracts were received and reviewed by the guest editorial board. Around 100 papers were invited for eight-page full paper submission. Finally, we accept 24 papers in this special section. Since the challenges brought by high penetration of renewable energy are multifaceted, we group these papers into seven schemes: 1) stochastic dependencies modeling; 2) power system forecasting; 3) power system flexibility; 4) transmission system related issues; 5) distribution system & microgrid operation; 6) power system frequency control; 7) power system oscillation analysis. A brief summary of these paper is presented in the following.