With the strengthening of interconnection power grid construction, the connection between the regional networks is becoming increasingly close, and it’s becoming a trend to consider the influence of external power grid in the internal network state estimation. The existing state estimation method based on the extended Ward equivalence of external grid, needs both the impedance parameters and state parameters of the external network equivalent model, and requires that the equivalence state and the internal network state remain consistent in the sampling time. In the actual power system, the calculation cycles of interconnected power grid and internal network independent state estimation are difficult to maintain consistency, and it’s easy to lead to large errors due to the mismatch of the state information of the internal network and external network. Based on the existing extended voltage-source-branch Ward-equivalent-circuit model, this paper presents the calculation method of equivalent impedance parameters and state parameters. Compared with the extended Ward equivalent model, the same is the structure and parameters of the equivalent impedance branch, the difference is the equivalent injection power. The latter is at the boundary node and the former is equivalent to the power node. And the equivalent state information of the proposed model only has the voltage and power of the equivalent power node, which can be calculated from the voltage and power information of the boundary nodes based on the node 基金项目:国家自然科学基金项目(51477017) power balance. Accordingly, the state information of the proposed model is completely matched with the state information of the internal network, and there is no state mismatching problem of the extended Ward equivalence, which can greatly improve the accuracy of the independent internal network state estimation with the external network equivalent. At the same time, the proposed model inherits the impedance branch structure and parameters of extended Ward equivalence, so that when the whole network information is known, it has the similar equivalent accuracy with the extended Ward equivalent model. The simulation result on IEEE 39 bus system verifies the correctness of the proposed method. This work is supported by National Natural Science Foundation of China (No. 51477017).