Stabilizing Condition of Grid-Connected VSC as Affected by Phase Locked Loop (PLL)

The small signal stability of the system that one single VSC is connected to an infinite source via the line is investigated in this letter. An analytical stabilizing condition for such system is derived based on the linearized model. It builds up the relationship between controller parameters, electrical distance, and system operating points for ensuring system stability. It is founded that both large and small proportional gains of PLL may lead to the system instability. Moreover, the system is unstable once the integral gain of PLL exceeds one critical value.