AC and DC Power Transmission ( ACDC 2018 ) Analysis of DC voltage oscillation mechanism in AC / DC hybrid distribution

DC voltage oscillation occurs more and more frequently in voltage source converter (VSC)-based AC/DC hybrid distribution system (AC/DC HDS). However, currently, the explanation of the oscillation mechanism is not clear enough. One reason is the traditional state-space model of the system is too complicated, and the information in the model is too dispersed. This research aims at researching the small-signal stability of AC/DC HDS and revealing the mechanism of DC voltage oscillation. Firstly, a method to reduce the order of the state-space model is proposed. A three-order VSC model with definitude physical meaning is obtained by utilising instantaneous power theory. According to this model, the potential instability factors in VSC are identified. Then, based on this, the interaction behaviour between VSC and DC network is researched by frequency domain analysis. The instability mechanism is ultimately revealed. The analysis result shows that the VSC's phase roll-off characteristic is the root cause of instability, the inherent oscillation characteristics of DC network has negative interactions with VSC and will narrow the system stability boundary. Finally, the correctness of the theory is verified by electromagnetic transient simulation.