  Transients of a grid-synchronized voltage source converter (VSC) are closely
related to over- currents and voltages occurred under large disturbances (e.g.
a grid fault). Previous analysis in evaluating these transients usually neglect
the nonlinear control effects of a VSC (e.g. phase-locked-loop, PLL).
Therefore, potential stability issues related with nonlinear dynamics cannot be
revealed properly. This work aims to move further in this respect. To better
analyze and gain more insights into the nonlinear properties, dynamical
analysis of a grid-tied VSC is conducted by parts. Specifically, the nonlinear
behaviors of VSC power control loop (PCL) are firstly analyzed, in which the
dynamics of PLL are assumed steady. Then, the nonlinear behaviors of
PLL-dominant dynamics are further explored in detail, where the PCL is assumed
steady. In this case, frequency instability and the mechanisms behind it are
revealed. At last, effects of PQ controller regulation as well as controller
bandwidth on the frequency stability are discussed. All the analysis and
conclusions are verified by time domain simulations in PSCAD/EMTDC, where a
switching model of VSC is adopted.
