Stability Of Voltage Source Inverters Research Articles

Virtual Inductance for Stable Operation of Grid-Interactive Voltage Source Inverters

In this paper, a feedforward virtual inductance control strategy is developed for the stability of voltage source inverters (VSIs) in weak grids. A weak grid is characterized by a low short-circuit ratio and a low inertia constant, in which VSIs become susceptible to voltage distortions and instability. The proposed feedforward virtual inductance term is integrated into the current control loops of grid-tied VSIs. In this paper, the effectiveness of the virtual inductance control strategy in weak grids is first proved through closed-loop transfer function analysis and then the system's root locus analysis to determine the stability region of the system versus the grid impedance. The developed virtual inductance controller is also verified through laboratory experiments for different weak grid scenarios.

On Stability of Voltage Source Inverters in Weak Grids

As the number of inverters increases in the power grid, the stability of grid-tied inverters becomes an important concern for the power industry. In particular, a weak grid can lead to voltage fluctuations at the inverter terminals and consequently cause inverter instability. In this paper, impacts of circuit and control parameters on the stability of voltage source inverters are studied using a small-signal state-space model in the synchronously rotating $dq$ -frame of reference. The full-order state-space model developed in this paper is directly extracted from the pulsewidth modulation switching pattern and enables the stability analysis of concurrent variations in the three-phase circuit and control parameters. This paper demonstrates that the full-order model of a grid-tied active (P) and reactive (Q) power (PQ)-controlled voltage source inverter (VSI) can be significantly reduced to a second-order model, preserving the overall system stability in the case of grid impedance variations. This paper also shows that a decrease in the grid inductance does not necessarily improve the stability of grid-tied VSIs. The system stability is a function of both the grid R/X ratio and grid inductance. Despite the grid-side inductor of the LCL filter is in series with the grid impedance, they have different impacts on the stability of a grid-tied PQ-controlled VSI, i.e., an increase in the filter inductance may improve the system stability in a weak grid. These findings are verified through simulated and experimentally obtained data.