Increased LVRT capability for VSG-based grid-tied converters

Grid-Tied Converters (GTCs) serve a vital role in integrating renewable energy generation into power networks with high penetration rates. During a grid voltage sag or recovery, the existing LVRT technologies still suffer from overcurrent caused by fault detection delay under the control of the Virtual Synchronous Generator (VSG). This vulnerability may result in trip-off occurrences or damage to the power electronic components. At present, VSG-related research mainly focuses on control and stability analysis, etc. There is a relative lack of work on the fault current-voltage (I-V) characteristics of VSG during grid voltage dips, which is urgently needed. Due to this indistinct nonlinear characteristics, the grid-forming GTC either loses the voltage source characteristics of the VSG or deteriorates the transient stability against deep asymmetrical grid dips. As a result, the conventional solutions provide limited assurance of transient stability and inadequate management of asymmetric grid disturbances. The analysis in this paper begins with a dimensional examination of the I-V characteristics of the VSG (IV-VSG) during grid contingencies. Subsequently, this research presents an improved control strategy for increasing the LVRT capability of GTCs under different grid-fault circumstances. Additionally, a rigorous analysis of transient stability is performed to uphold the synchronization between the VSG and the grid during severe grid disturbances. Both the simulation and experimental verification demonstrate that fault currents are restricted at a rapid rate, power synchronization is maintained and the need for an enlarged GTC and current limiting strategies are eliminated. In addition, the transient stability of the VSG is validated under a variety of fault conditions.