I. Introduction
High-speed railway is an indispensable part of the national economy, while the access of a large number of electrified railways has become an important factor affecting the safety and stability of power grid because of its strong impact, large capacity, nonlinear and so on [1]. However, current research focuses on building a digital simulation model that bases on the physical mechanism of locomotives mostly, then the characteristic quantities of the system under various operating conditions are simulated and analyzed. Nevertheless, this model is not practical for the simulation calculation of power grid, the equivalent modeling of the composite load of traction power supply system for high-speed railway is relatively few at present.