Meng Zhan

Previous literature shows that the second-order phase-locked loop dynamics dominate the transient stability of high voltage direct current system based on modular multilevel converter (MMC-HVDC) in the during-fault stage. The effects of DC-link dynamics and transient behavior after fault recovery have rarely been studied. Focusing on the post-fault stage, this paper first develops a transient model in each fault stage based on the typical fault ride-through strategy, considering the DC-link voltage dynamics. Although the during-fault stage is stable, the MMC-HVDC system still faces the risk of transient instability in the post-fault stage under receiving-end AC weak grid faults because of DC voltage deviation. The angular frequency jumps that occur in both the during-fault stage and post-fault stage are uncovered. High-dimensional basin-of-attraction analysis reveals that the angular frequency jump generated by the DC-link voltage dynamics can weaken the resynchronization capability. Moreover, based on the proposed transient stability margin index, the parameter feasible region under different ramp coefficients, DC voltage droop coefficients, and short circuit ratios is obtained. Finally, hardware-in-the-loop experiments are conducted to validate the results of the theoretical analysis.

To enhance inertia-level and support capacity of renewable energy equipment, grid-forming (GFM) technology has been widely applied in new-type power systems, but it also brings new stability issues. With this in mind, this paper studies the instability mechanism of the single-machine, infinite-bus system (SMIB) of a GFM converter under strong and weak grid conditions and proposes corresponding oscillation suppression methods based on the power system stabilizer (PSS) technology. First, a small-disturbance model for the SMIB of a GFM converter is constructed, and based on the root locus graph, it is found that the system exhibits instability phenomena under both strong and weak grid conditions. Furthermore, based on complex torque analysis, it is found that the oscillation instability under a strong grid is dominated by terminal voltage control (TVC), while under a weak grid, it is dominated by DC voltage control (DVC). Then, for these two different instability phenomena, corresponding oscillation suppression strategies based on the PSS method are proposed, which greatly improves the small-disturbance stability. In addition, parameter sensitivity analyses are conducted. Finally, the theoretical analysis and control strategies are verified by MATLAB/Simulink simulation.

As renewable energy integration continues to accelerate, transient stability challenges in modern power systems have become increasingly pronounced, particularly due to the systems’ inherent high-dimensional nonlinear dynamics. Developing a reliable and efficient method for assessing transient stability and the associated basin of attraction remains a critical yet unresolved challenge. This study focuses on the precise estimation of the basin-of-attraction boundary in phase-locked loop based voltage source converter systems, which can be described by a generalized swing equation. To address this problem, a higher-order Taylor expansion renormalization approach is proposed. This method enables the construction of a highly accurate approximation of the stable manifold that serves as the basin-of-attraction boundary, offering substantial improvements over conventional linear and second-order local approximations. The reliability and effectiveness of the proposed approach are rigorously validated through simulations under various fault scenarios in the transient stability assessment. The proposed method provides a robust tool for analyzing transient stability in modern power systems and offers new insights into the stability characterization of other nonlinear dynamical 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.

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 (DFIG) system as an objective, this article divides its transient processes into four different stages, including the prefault, during-fault, early postfault, and late postfault 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 postfault stage dynamics, it can be treated as a series of quasisteady 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 postfault 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-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.

With rapid development of renewable energy, voltage source converter (VSC) has been widely used in modern power grid for energy conversion. Compared to synchronous generator, the transient stability analysis of VSC grid-tied systems becomes more difficult. It has been found that unstable limit cycle (ULC) is very common, while most existing transient stability assessment methods for the VSC grid-tied systems are based on unstable equilibrium point. For high-dimensional VSC systems, the basin boundary of a stable equilibrium point surrounded by the ULC should be the whole stable manifold of the ULC, which is still high-dimensional and usually hard to obtain. In this article, by using the center manifold method, a transient stability assessment method for the ULC-dominated stability region boundary of high-dimensional VSC systems is proposed, by mapping the high-dimensional stability boundary determined by the ULC into an ULC on the reduced-order center-manifold plane. The detailed algorithm is presented and error analyses for different parameters and different faults are carried out. In addition, the validity is supplemented by a modified 3-generator-9-bus system. Therefore, the center manifold method is efficient and able to solve the transient stability assessment for ULC-dominated stability boundary in VSC systems.