This paper introduces a problem on large-signal stability of an interconnection of AC and MTDC (Multi-Terminal DC) grids. We formulate the problem as an estimation of stability region (or basin of attraction) of an asymptotically stable equilibrium point that is exhibited by coupled nonlinear differential-algebraic equations. A candidate of Lyapunov function is introduced for the interconnected AC/MTDC grid in terms of composite dynamical systems. We visualize several slices of the stability region of the interconnected grid under a practical setting of the grid's parameters. The preliminary visualization shows that the stability region can be estimated with the candidate of composite Lyapunov function.
The so-called modular modeling of interconnected AC/MTDC (Multi-Terminal DC) systems aids their synthesis and analysis by decomposing it into structural modules (e.g. voltage-source converter, synchronous generator, AC line, DC line) and by exploring the patterns of interconnection that yield the overall performance of a system. In this paper, we introduce our idea on the modular modeling of large-signal dynamics for an interconnected AC/MTDC system. Effectiveness of the idea is shown with numerical simulations for a relatively simple configuration of three-terminal DC grid.
The so-called short-term voltage stability is typical of nonlinear problems arising in dynamic performance of electric power systems. This paper reports a simulation result on load margin for short-term voltage stability of the IEEE 9-bus system connected to a multiterminal DC system based on voltage-source converter (VSC). The simulation shows that the introduction of AC voltage control in VSC increases the load margin under a step change in load on the AC system, thereby showing a clear benefit of the AC voltage control for enhancing the short-term voltage stability without harnessing the AC system.
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