(Key Laboratory of Smart Grid of Ministry of Education, Tianjin University, Tianjin 300072, China)

Integrated energy system (IES) has become an important strategic research direction in international energy field for its great significance in improving the energy efficiency, the utilization efficiency of social infrastructure and the energy supply security, as well as promoting the large-scale development of renewable energy and achieving energy conservation and emissions reduction in China. Research on IES technology is in conformity with the major national strategic needs of China. Focusing on the vital theory of planning and optimal operation of IES, six fundamental problems in the study of IES are proposed systematically, including the common modeling technology for IES, the integrated simulation of IES, the planning theory and method of IES, the security theory and method of IES, optimal operation and control of IES, benefit assessment and operational mechanisms of IES, and the status of domestic and foreign research directions involved in the scientific problems are surveyed and projected. This work is supported by National High Technology Research and Development Program of China (863 Program) (No. 2014AA051901), National Natural Science Foundation of China (No. 51377117, No. 51407125), Post-doctoral Science Foundation of China (No. 2013M540207) and State Grid Corporation of China.