Transient Synchronization Stability Analysis and Assessment of DFIG System Under Severe Faults

In the transient stability analysis of renewable energy grid-tied systems, although a large amount of works have been devoted to the detailed electromagnetic transient simulation and the stability analyses of during-fault stage, the whole low-voltage ride through (LVRT) process and transient stability mechanism remain to be uncovered. Taking the doubly fed induction generator system as an objective, this paper divides its transient processes into four different stages, including the pre-fault, during-fault, early post-fault, and late post-fault ones, establishes the full mechanism models for each stage, and studies the sequential switching dynamics in detail. It is found that the during-fault dynamics can be determined by the phase-lock loop second-order equation within the framework of the generalized swing equation (GSE), with additive scaled parameters. In particular, for the early post-fault stage dynamics, it can be treated as a series of quasi-steady states and its dominant driving system dynamics can still be described by the GSE. Thus the system transient stability can be completely determined by whether the initial state of the early post-fault stage is within or out of its corresponding basin of attraction (BOA). Based on these unique dynamical characteristics, the BOA-based and equal area criterion (EAC)-based transient stability assessment methods are developed, which are supported by broad numerical simulations and hardware-in-the-loop experiments. This work provides a clear physical picture and perfectly solves the difficult stability analysis problem when severe faults and LVRT showing discontinuous switching dynamics have to be considered in most of DFIG engineering situations.