Power hardware-in-the-loop simulation（PHILS） is an advanced simulation technology as a new development of traditional hardware-in-the-loop simulation（HILS）. PHILS can be used to test power devices， which can also make full use of the advantages of HILS. PHILS can not only meet the test and research demand of a variety of new electrical devices during the new energy revolution， but also extend the traditional application range of HILS in a power system， giving PHILS important research value and broad application prospects. Proceeding from the basic concepts and the structure of PHILS and by addressing the interaction between numerical and physical subsystems， an analytic framework， namedframe-step” timing， is proposed. Based on this timing， the coordination of virtual and natural time is analyzed in modeling the interface. Then a discrete dynamic model applicable to a general PHILS system is developed.This work is supported by State Grid Corporation of China.