Identification and Small-Signal Analysis of Interaction Modes in VSC MTDC Systems

In this paper, a methodology is presented to identify and analyze interaction modes between converters in voltage-source converter multiterminal high-voltage direct current (VSC MTDC) systems. The absence of a substantial level of energy stored in such power-electronics-based systems results in fast system dynamics, governed by electromagnetic phenomena. Moreover, interactions between converters are largely influenced by the control parameters and, in general, an a priori identification of interaction modes based on associated time constants is less straightforward than in ac systems. Furthermore, the extent to which converters interact not only depends on the controller parameters, but is also influenced by the physical characteristics of the HVDC system. The methodology introduced in this paper is based on aggregated participation factors to distinguish between local modes, primarily associated with one terminal, and interaction modes involving multiple terminals. To illustrate the proposed methodology, the influence of droop control parameters, as well as dc breaker inductors, on the system dynamics and the participation of the terminals in system interactions are investigated for a three-terminal MTDC system.