Small Signal Modeling and Stability Analysis of AC/DC Distribution Network: Case Studies on a Low Frequency Oscillation in Tongli Region

Abstract

The integration of large number of power electronic equipment makes AC/DC distribution networks cause new dynamic issues and brings great difficulty to the theoretical analysis of system stability. Recently, in a practical distribution network in Tongli Region in Jiangsu Province, a 4Hz low-frequency oscillation phenomenon occurred, which involved various types of distribution equipment, such as power electronic transformer (PET) main valve tower (i.e., cascaded H-bridge (CHB), and dual active bridge (DAB)), buck converter and three-phase voltage-source converter (VSC). In this paper, the practical distribution network and the oscillation phenomenon in Tongli Region are first introduced. Then, the mathematical models of the involved distribution equipment, i.e., CHB, DAB, buck converter, and VSC, are first derived, respectively. On this basis, their small-signal models are established in detail, and the small-signal stability is analyzed based on the eigenvalue analysis method. With the mathematical model, the 4Hz low-frequency oscillation phenomenon that occurred at the practical grid in Tongli Region is reproduced. Moreover, with the small-signal stability analysis, the dominated low-frequency mode and its dominated participation variables are identified, and the critical influence parameters are located, and their impacts are presented quantitatively. Finally, the time-domain simulations are carried out to verify these theoretical analysis results.