An improved grid-forming control method with adaptive virtual impedance combining current limitation and transient stability improvement

• Established large-signal model of grid forming converters with virtual impedance. • Provided effect analysis of grid and virtual impedance on transient stability. • Proposed an improved GFM control for current limitation and transient stability. • Designed corresponding coefficients of the control method quantitatively. • Validated the effectiveness through experiment prototype under various faults. The virtual impedance (VI) control method is widely used to limit fault current for grid-forming converter (GFMC). However, in this article, it is found that the VI deteriorates the transient synchronization stability of the GFMC under grid faults. To solve this problem, an improved grid-forming control method combining current limitation and transient stability improvement is proposed. By compensating the transient electromagnetic power, the power feedback of the active power control (APC) loop is changed from the real active power at the point of common coupling to the virtual active power on the converter-side during the fault period. Then, the detrimental impact of virtual resistance on transient stability can be reversed into a beneficial effect, thereby solving the contradiction between current limitation and transient stability. Besides, the transient damping power is feedbacked to adjust the active power reference, ensuring the transient stability under faulty weak grid scenario. Both of them are used as two extra degrees of freedom to eliminate the active power imbalance and jointly enhance the transient stability. Furthermore, an adaptive VI control is adopted to automatically tune the VI so that the fault current can be kept at maximum allowable value to support faulty grid. More importantly, the proposed method is not affected by the fault degree and grid strength. Simulations and experimental results validate the effectiveness of the proposed method.