LAUSR

The mechanism of voltage sourced converter's (VSC) instability caused by phase-locked-loop (PLL) in weak system is revealed in a simplified and efficient way. Based on the key-variables small signals model, the PLL's impact to VSC stability is described by the defined low frequency torque and high frequency torque. The low frequency torque decides the characteristic of VSC in low frequency domain, and supplies synchronous ability and low frequency damping to VSC, while the high frequency torque decides VSC's high frequency characteristic and supply high frequency damping. The two torques impact factors are researched and their influences are revealed. It is found that to keep system stable, sufficient torques are needed. Because weakening system strength will decrease both torques, the mechanism of the instability caused by PLL in weak system is actually lack of sufficient torques. The simulation based on PSCAD software is implemented to validate the theory and matches well with the proposed theory.