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The small impedance branch is a primary factor that results in the divergence of the power flow methods. The study of the methods of power calculation in Newton-Raphson (N-R) method and backward/forward sweep (BFS) method indicates that the method of power calculation together with small impedances may result in the convergence problem. The impact of small impedance branches on the convergence of the two power flow methods is different because the two methods are different in mechanism. So the ways to improve the convergence performance of the power flow methods are also different. Based on theoretical analysis, a branch dividing method is presented which divides a transformer with small impedance into a transformer with normal impedance and a capacitive line. The proposed method can conveniently and effectively deal with the convergence problem in the BFS method caused by small impedance transformer branches, and experimental results illustrate the effectiveness of the method.