Traditional equivalent methods of the detailed system in the transient stability program (TSP)mainly consist of current source

Norton equivalents and power source equivalents. The error principle of power source equivalent of electromagnetic side system under single-port interface condition is analyzed

the information restrictively required by TSP for accurate calculation after fault occurs in the detailed system is discussed. It is pointed out that transmitting the power information alone cannot ensure correct calculation of TSP program when a fault occurs in the electromagnetic side system. The information needed by TSP program calculation for single port interface and multi-ports is given

and a new equivalent method is proposed that is different from the power source method. The method transfers the iteration process of current calculation of interface into the process of solving linear equations

thus improving the accuracy of TSP side calculation. The method is applicable to the situation where the detailed system is simulated by real-time digital system (RTDS)

and can be extended to the situation where there are several interfaces between the detailed and TSP systems.This work is supported by Fundamental Research Funds for the Central Universities (No. 2015XS22).