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In China, a structure in which multiple line-commutated converter based high-voltage direct current (LCC-HVDC) systems feed into the same AC system has been formed. For the multiple LCC-HVDC infeed system, the electrical strength and real operating condition evaluation of the receiving AC system are of great significance for system operation. The multi-infeed short circuit ratio (MISCR) is generally utilized to assess the strength based on the rated parameters; hence, it cannot analyze the real operating condition under the non-rated status. In this paper, an LCC-HVDC system is equivalent to a grounded impedance without changing the external characteristics. The multi-infeed operating short circuit ratio (MIOSCR) is then deduced from the equivalent impedance and equivalent nodal analysis. Relationships between the MIOSCR and electrical strength indices of AC and AC/DC systems are analyzed, and the MISCR is deduced as a special case of the MIOSCR under the rated status. The multi-infeed operating margin coefficient (MIOMC) is finally proposed, which can be utilized to evaluate the operating margin of the LCC-HVDC system and provide guidance for designing operating strategies. The proposed MIOSCR and MIOMC overcome the previous shortcomings of inaccuracy about operating status assessment by the MISCR. The results of the case study demonstrate the effectiveness of the MIOSCR and the MIOMC for assessing the operating condition and margin of converter buses.

Analysis of the Operating Margin Evaluation of Multi-Infeed LCC-HVDC Systems Based on the Equivalent Impedance