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This paper aims at the present difficulties to regulate the frequency difference and voltage difference automatically when doing power grid parallel operation in substation, and applies the back-to-back voltage source converter to the power grid synchronization parallel based on the theory of active power frequency and reactive power voltage in power system. The core of the power grid synchronization parallel method based on power transfer is carrying through the fast independent control of active power and reactive power between the awaiting-parallel bilateral systems via the back-to-back voltage source converter, so as to achieve the purpose of adjusting the frequency difference, voltage difference and phase angle difference between bilateral systems. In this paper, two methods in power transfer via the back-to-back voltage source converter, the disposable full power transfer and the successive increasing power transfer, are firstly analyzed, while their own characteristics and impact on power grids are introduced as well; then such a control strategy is proposed that to transfer a relatively small active power and a relatively small reactive power in advance, until the system has stabilized, we get the change rates of active power versus frequency difference, reactive power versus voltage difference and active power versus phase angle difference respectively according to the changes of frequency difference, voltage difference and phase angle difference, and finally calculate the required active power and reactive power to meet the parallel conditions based on the change rates. This paper ultimately establishes the control strategy of applying the back-to-back voltage source converter to the power grids synchronization parallel, presents the control flow chart of power grid parallel process, and makes a detailed analysis of the key technologies in the chart, including the converter start control, the calculation of required power, the control strategy of bilateral- - converters, the closing control of breakers at parallel points and so on. This proposed control strategy has been simulated in PSCAD/EMTDC, and the simulation results shown that the control strategy for power grid synchronism parallel in this paper is effective.