Frequency Domain Admittance Model of Line-Commutated Converter Based on Single-Side Modulated State Function

The small-signal admittance model is an important basis for stability analysis of power electronics-dominated system. In contrast to the extensively explored voltage-sourced converter (VSC) modeling in recent years, the accuracy of the admittance model for the line-commutated converter (LCC) is compromised due to inadequate representation of its switching dynamics. In this article, the state function based on single-side modulation is introduced to characterize the conduction state and switching characteristics of the LCC. Combining the linear dynamics of AC and DC variables in each conduction state, the AC/DC admittance matrix considering multi-frequency coupling is further obtained by applying multi-harmonic linearization method. Based on the defined matrix gain, the truncation order of the matrix is determined, and a detailed discussion is conducted on the influence of different modeling methods and factors on the precision of the model. Then, the equivalent admittance considering the coupling between LCC and grid impedance is derived and the influences of operation conditions on which is analyzed. Finally, the equivalent admittance is applied to the stability analysis of a practical project in China Southern Power Grid (CSG), and the predicted resonance shows high accuracy compared with the simulation results.