Abstract: The data-driven dynamic security assessment under the concept of security region urgently needs a systematic fault location feature representation method. Based on the concept of the electrical coordinate system, the identification of blind areas for fault location feature representation and an expansion method of electrical coordinate system are proposed, which further improve the representation accuracy of fault location in partial areas. First, the reasons why the representation accuracy of the electrical coordinate system is insufficient in partial areas are analyzed. The blind point, blind line and blind area of fault location feature representation are defined. Then, by analyzing the locational relationship between two points, the judgment principle of blind lines is determined, and the rapid searching method of blind lines considering adjacent lines is proposed. Finally, the identification algorithm of initial blind area based on adjacency matrix is proposed, and the blind area is fine-tuned based on the model assessment results. The fault location representation accuracy in the blind area is improved by the expansion of reference nodes of the electrical coordinate system. Taking the transient stability assessment as an example, the applicability of the proposed method in the dynamic security assessment is verified by the example of a provincial power grid in China.