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A dynamic equivalent approach based on coherency is researched for dynamic simulation test in ±800 kV DC transmission project from Yunnan to Guangdong in China. To reduce the full system, the dynamic equivalent model of large-scale power system based on coherency is established, in which the identification of coherent generator groups, coherent buses reduction, aggregation coherent generator parameters and power network reduction are included. Coherent generators are identified by means of swing-curve method, identical power technique presented to coherency buses reduction, CSR method used for network reduction and fitting the transfer function in frequency domain applied to aggregation the coherent generator parameters. The equivalent system of South China power grid is compared with the original system in size, accuracy and performance, respectively. Calculation and simulation results show that the equivalent system can preserve the main dynamic characteristics of the original system well.