Impedance-Based Stability Analysis of a Low-Inertia AC Grid Connected to a DC Grid by a VSC With Virtual Inertia Controllers

The dynamics and performance of low-inertia ac grids with frequency support (e.g., droop and virtual inertia controls) have been investigated thoroughly. Recently, the dc grid concept has been considered a cost-effective and efficient option in modern power distribution systems, where one of its most valuable applications is to support the frequency and voltage of low-inertia and weak grids. However, the literature does not fully address a comprehensive stability analysis and performance assessment of a low-inertia system supported by a dc grid. Therefore, this paper presents a thorough yet straightforward dc-side impedance-based stability analysis of a low-inertia ac grid connected to a dc grid via a bidirectional voltage-source converter (VSC) equipped with a virtual inertia controller to improve the ac grid frequency behavior. In the proposed virtual inertia control, low-pass and high-pass filter implementations are investigated, and their performance is compared. The dc-side equivalent impedance models are developed for the 1) interlinking VSC combined with its ac grid, considering virtual inertia control and typical ac grid components: synchronous generators (with their turbine, governer, and exciter models), induction motor loads, and static loads, and 2) dc grid considering its practical components: distributed generators, constant-power loads, and resistive loads. Therefore, unlike previous methods, the proposed impedance modeling and stability analysis approach considers the impacts of the ac grid frequency/voltage dynamics and dc grid dynamics on the VSC dc-bus stability. Impedance interactions are characterized and used to assess the overall system dynamics and stability and the impacts of the system and control parameters. The analytical results are validated by detailed offline and real-time simulation studies.