A Novel Hybrid Integrated Grid-Forming and Grid-following Control Method to Enhance Synchronization Stability of VSC

The grid-forming (GFM) controlled voltage-source converters (VSCs) are a potential solution for future power systems with a high renewable energy penetration, thanks to their ability to support grid voltage and frequency. However, synchronous stability issues of VSCs will inevitably occur, especially when connecting to a strong power grid. To address this issue, this article reveals the synchronous instability mechanism of the GFM-VSCs from a multiinput multioutput (MIMO) impedance perspective. It is found that the voltage orientation conducted by the inner voltage loop introduces a reverse resonance peak near synchronous frequency when the power grid becomes stronger, thus resulting in synchronous instability. Consequently, a novel hybrid control that integrates GFM and grid-following (GFL) structures is proposed, which combines the power synchronization controller of GFM-VSC with the inner and outer control loops of GFL-VSC, thereby eliminating the reverse resonance peak caused by d-axis voltage orientation and significantly enhancing the synchronization stability of the system. Moreover, the VSC can adjust its output power flexibly by using outer active and reactive power (PQ) control loops or stabilize terminal voltage by using active power and AC voltage (PV) control loops, thus adapting to different operation scenarios. Finally, the effectiveness of the proposed method is validated by experimental results.