Impedance Reshaping Method Based on Compensating PLL Dynamic to Improve Active Power Transfer Capability of VSC Under Weak Grid

The vector current control scheme based on the phase-locked loop (PLL) is widely used in grid-connected voltage-source converters (VSCs). However, the power transfer capability of the VSC will first be limited by dynamic power limitation (DPL) and then by static power limitation (SPL). The SPL is determined by the steady-state operation of the system, such as steady-state algebraic equations. Simultaneously, the DPL mainly relies on control strategy and controller parameters, which is also called the small-signal stability boundary and will not exceed SPL. In this paper, the SPL and DPL of the VSC with different outer control loops are investigated, i.e., active power and reactive power (PQ) outer control loops, or active power and AC voltage (PV) outer control loops. It is found that extra reactive power injected into the grid is beneficial for improving the SPL and DPL of the VSC, especially under weak grid conditions. To maximize the SPL within the power constraint of VSC, the optimal design method for power references is presented. Meanwhile, a comparative analysis for the SPL of VSC with PQ or PV control loops is given, and the application scenario of both control strategies is clarified. Furthermore, aiming at the main factor restricting the DPL, i.e., PLL influence, an impedance reshaping method based on compensating PLL dynamics is proposed to increase the DPL of VSC, resulting in the DPL of the VSC almost extended to SPL. Finally, simulation and experimental tests are also carried out, and the results validate the effectiveness of the proposed methods.