Averaged-value Model of Low Frequency AC Transmission System and Stability Analysis

This paper begins by analyzing the fundamental principles of low-frequency power transmission, establishing mathematical models for both the low-frequency and power-frequency sides of the M3C. Based on an in-depth examination of M3C bridge arm operation, as well as differential and common-mode circuits on the low-frequency side, along with the bridge arms' power balance principle, a decoupling control strategy is proposed to enable input on both the power-frequency and low-frequency sides of the M3C. This approach allows for complete, independent control of the nine bridge arms in the dq coordinate system. Additionally, using the Thevenin equivalent circuit of the M3C bridge arm, an innovative averaged value model of the M3C bridge arm is introduced, leveraging the concept of energy equalization. Finally, a new energy low-frequency collection and transmission system model is developed on the PSCAD platform. By analyzing voltage and current waveforms of each bridge arm under steady-state conditions, the proposed model’s effectiveness and accuracy are verified, and the system’s voltage stability under transient conditions is also assessed.