Research on the Mechanism of Simultaneous Instability of dq Frame Sub- and Super-Synchronous Oscillations in Grid-Tied Converter

The integral link of current loop PI controller of grid-tied converter and the line inductance form the intrinsic
dq
frame sub- and super-synchronous resonant points. This paper studies the mechanism of simultaneous instability of
dq
frame sub- and super-synchronous oscillations (SSO/SPO) by analyzing the interaction energy corresponding to the SSO and SPO components. First, based on the interaction energy model of the grid-tied converter, the analytical expressions of the aperiodic components of the derivatives of the interaction energy terms (ACDIE) are derived. The ACDIEs can reflect the damping levels of SSO and SPO. On this basis, it reveals analytically that the damping level of SSO first worsens and then improves, and the damping level of SPO changes latter than SSO but keeps worsening. The oscillation frequency turns from sub-synchronous to super-synchronous. And the instability mode turns from single-mode (SSO) to double-mode (SSO/SPO), and then back to single-mode (SPO). Simultaneous instability of SSO and SPO occurs during the transition from the SSO mode to the SPO mode. It is caused by the interaction energy that provides negative damping in super-synchronous frequency band approaches the amplitude in sub-synchronous frequency band. Moreover, how to stabilize SSO and SPO simultaneously is obtained based on the above analysis results. Finally, the theoretical analysis results are verified by the hardware-in-loop tests.