Stability Analysis of Photovoltaic Virtual Synchronous Generators

Abstract

Virtual Synchronous Generators (VSG), as an effective mean to improve the frequency and voltage regulation ability of the photovoltaic power generation, had aroused wide concern. The analysis on whether VSG would cause stability problem became an important issue to ensure the safety of power system. The stability analysis of photovoltaic VSG is an enormous challenge because this VSG not only retains the characteristic of power electronic devices but also emulates the essential behavior of synchronous generators. Existing studies concentrated on the stability of VSG device and the risk of oscillation when VSG applied in microgrid. The damping characteristic of distributed photovoltaic VSG has not been researched. Meanwhile the impact of each VSG control parameter on oscillation model damping has already been studied. However, the interactive influence from several parameters to system stability has not been discussed in existing researches. To address these issues, this paper developed the small signal model of photovoltaic VSG. The oscillation mode of VSG system was calculated based on this model, which was also applied to investigate the stability of whole system with different control parameters and under various system scenarios. The results obtained by small-signal model are validated by digital simulation equipped with physical controller. Brief conclusions were drawn as follows: (1) grid-connected photovoltaic VSG system has three dangerous oscillation modes of poorly damp, which are the mode caused by the power control, the mode introduced by virtual impedance control and the mode associated with LC filter; (2) unstable subsynchronous oscillation will appear if power droop gain or virtual mechanical time constant exceeds the threshold. High-frequency or synchronous oscillation may be excited by decreasing the virtual resistance. Subsynchronous oscillation can be caused by increasing the virtual resistance. Reduction of virtual inductance is likely to result in high-frequency or subsynchronous oscillation and growing of virtual inductance will contribute to a negative damping for subsynchronous and synchronous oscillation mode; (3) the damping of new oscillation mode introduced by power control of VSG is relatively small when photovoltaic VSG is applied in low voltage power grid. The influence from system condition to damping of oscillation mode associated with LC filter is the key factor to decide the stability of VSG system in high voltage grid.