Mechanism Analysis of Instability in Grid-Connected PV Systems With Volt-Var Control

Utilizing the flexible reactive power regulation capability of PV inverters for voltage support is proved to be an effective solution to voltage fluctuation and violation, which however can induce control of instability. To address this issue, this paper focuses on the dq-frame admittance modeling of three-phase grid-tied inverters with volt-var control, which behaves as a multi-input-multi-output (MIMO) system. It is found that the volt-var control can reshape the q-d channel output admittance in the low-frequency range and increase the risk of oscillation instability. To explore its mechanism, the MIMO model is transformed into a single-input-single (SISO) model from I
qd
to U
d
. Applying the impedance-based stability criterion, this paper analyzes in detail the effect of volt-var control, grid-following (GFL) control and the grid impedance on the system stability. Finally, simulations and semi-physical experiments results validate the above analysis.