Transient synchronization stability analysis and assessment of DFIG system under severe faults

TL;DR

This paper addresses transient synchronization stability of grid-tied DFIGs under severe faults with complete LVRT processes. It introduces a four-stage LVRT mechanism (pre-fault, during-fault, early post-fault, late post-fault) and derives stage-wise reduced-order models, revealing that PLL dynamics dominate stability, particularly through stage-2 GSE behavior and the slow-evolving stage-3 driving-response. It develops two efficient TSS assessment methods—BOA-based and EAC-based—and validates them against EMT simulations and hardware-in-the-loop experiments, showing that the initial state of stage 3 decisively determines ultimate stability. The work provides a physically intuitive framework and practical guidelines for enhancing transient synchronization stability in DFIG systems facing LVRT compliance and severe faults, with broad implications for grid codes and controller design.

Abstract

In the transient stability analysis of renewable energy grid-tied systems, although a large amount of works have devoted to the detailed electromagnetic transient simulation and the stability analyses of during-fault stage, the whole low-voltage ride through (LVRT) process and relevant transient stability mechanism remain to be uncovered. Taking the doubly fed induction generator system as the objective, this paper divides the 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 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). For the early post-fault stage, it can be treated as a series of quasi-steady states and its dominant driving system dynamics can still be described by the GSE. Based on the local dynamics of unstable equilibrium point, 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 basin of attraction (BOA). Based on these observations, 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 have to be considered in most of DFIG engineering situations.

Figures (13)

• Figure 1: Schematic show of the DFIG system considering LVRT.
• Figure 2: Schematic shows of three-phase stationary $abc$ reference frame, $xy$ reference frame, $dq$ common reference frame, and rotor reference frame, where $\omega_0$$\omega _{\rm \mathit{g}}$, $\omega_0$$\omega _{\rm \mathit{pll}}$, and $\omega_0$$\omega _{\rm \mathit{r}}$ represent the rotation angular frequencies of the $xy$, $dq$, and rotor reference frames, respectively. $\omega_0$ = 2$\pi$$f_0$.
• Figure 3: Schematic shows of typical switching controls under different stages.
• Figure 4: (a)-(f) Plots of $U_g$, $U_t$, $i_{\rm\mathit{rd}}$, $i_{\rm\mathit{rq}}$, $\varphi_{{\rm{\mathit{pll}}}}$, and $\omega_r$, respectively, for comparison of the mechanism model with the detailed EMT model.
• Figure 5: (a) and (b) Plots of $U_g$ and $i\rm_{\mathit{rd}}$, and (c) EAC-based TSS analysis for permanent faults.