arXiv subjects

Hongsheng Xu

Publications and source records attributed to Hongsheng Xu.

3 recordsLinked to original sources

arXiv2026

Transient Synchronization Stability Analysis of SG-DFIG Parallel System Considering Complete LVRT Processes

Although a large amount of work has been devoted to detailed electromagnetic transient simulation in analyzing transient synchronization stability (TSS) of hybrid systems containing renewable energy equipment and synchronous generator (SG), the underlying mechanism considering complete low-voltage ride-through (LVRT) processes of renewable energy equipment remains to be studied. Taking the SG and doubly fed induction generator (SG-DFIG) parallel system as an objective, this work divides its transient processes into four different stages: pre-fault (stage 1), during-fault (stage 2), early post-fault (stage 3), and late post-fault (stage 4), based on the LVRT of the DFIG, and establishes a transient model to describe the complete 4-stage LVRT processes. By studying the condition for entering the LVRT, it is found that vast majority of faults can cause the DFIG to enter the LVRT and make the parallel system exhibit the sequential switching characteristics. Similar to the SG-SG parallel system, which can be reduced to a single SG and described by a second-order swing equation, a unified generalized swing equation (GSE) under different parameters for different stages 1, 2, and 3 is derived. Therefore, the transient stability of the parallel system can be dealt with easily, and further, an improved equal area criterion method considering two additional effects of frequency jump and nonlinear damping is proposed to evaluate the TSS. These GSE-based theoretical analysis results are all supported by extensive hardware-in-the-loop experiments and simulations. Obviously, this work provides a clearer physical picture for the TSS mechanism of the hybrid system considering complete LVRT processes, and makes a closer connection with the transient stability of traditional power systems dominated by SG.

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arXiv2025

A Unified Theory for Transient Synchronization Stability Analysis of Renewable Dominated Power Systems

The change of electric power generation - from synchronous generator (SG) to converter - is generally regarded as the second revolution of power system. Different from rotor swing of SG in traditional grids mainly described by the swing equation (SE), the converter dynamics plays an indispensable role in modern renewable dominated power systems (RDPS). The high complexity of the RDPS, including spatial large-scale, nonlinearity, multi-time-scale, and even sequential switching, prevents us from fully understanding its dynamics and assessing its transient stability under large disturbance. Here, a variety of transient switching mechanism models of renewable devices relying on wind or solar energies under low-voltage ride-through are established and unified, which can be perfectly described by a generalized swing equation (GSE) under parameter changes for switching dynamics. The GSE focusing on the dominant phase-locking loop dynamics is similar to the SE. Mainly relying on the mechanical equivalence and the energy conservative principle, a substantially improved equal-area criterion method is proposed. Based on this method, even for large-scale renewable fields, the calculation errors for the critical clearing time are only about 1%. This elegant nonlinear-dynamics-based approach establishes a unified theory including modelling and analysis for the RDPS transient dynamics.

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arXiv2025

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 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.

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