Yunhui Huang

This paper investigates the stability and improvement mechanisms of a grid‐forming (GFM) converter connected to a low‐impedance grid. Initially, a state‐space model of the GFM converter integrated into an AC grid is developed for stability analysis. Subsequently, a dynamic model of the GFM converter, derived from Newton’s theory of classical mechanics, is presented to characterise the damping and synchronising power components. The stability mechanism of the GFM converter is elucidated by analysing the evolution of damping and synchronising power components in relation to varying grid strengths or control bandwidths. It is observed that the primary cause of GFM converter instability stems from negative damping components, which are influenced by both external factors inherent to the low‐impedance grid and internal factors inherent to terminal voltage control (TVC). For the external factor, virtual impedance compensation control is proposed to mitigate the negative impacts induced by the low‐impedance grid. Moreover, for the internal factor, q ‐axis terminal voltage compensation control is proposed to decrease the negative impacts induced by TVC. Finally, time‐domain simulations and experiments are both conducted to corroborate the aforementioned results.

Ancillary frequency controls of wind turbines (WTs) bridge the dynamic interplay between turbine mechanics and the electrical grid, enabling WTs to influence low frequency range dynamics in power systems. This work examines the impact of frequency support by grid-forming (GFM) doubly-fed induction generator (DFIG) based WT on power system ultra-low frequency oscillations (ULFOs). Accounting for different wind speed operating regions, a reduced-order small-signal model is developed to characterize GFM-DFIG’s terminal power-frequency response behavior within ULFO frequency band. This model is then integrated into the system frequency response framework. Utilizing complex torque coefficients method, the effects of various control strategies—including virtual inertia, primary frequency control, and turbine blade pitching—on the ULFO mode’s frequency and damping characteristics are analytically derived and examined. The results confirm that while the frequency support improves system frequency response, it simultaneously deteriorates ULFO damping, with high primary frequency droop gain posing a risk of instability. Lastly, simulation studies with a modified IEEE 10-machine 39-bus system are conducted to validate the theoretical findings.

Stable operation of multi grid-forming converters will suffer from integration into low-impedance grid, because of strong coupling between voltage sources which include the multi grid-forming converters and AC grid. This paper contributes to giving physical insights into stability of multi grid-forming converters with connected to low-impedance grid. To start with, dynamical model of multi grid-forming converters is presented to depict damping and synchronizing components for quantitative analysis of system stability. On the basis, self-stability and en-stability components are proposed to characterize the interactions among grid-forming converters. Stability mechanism of multi grid-forming converters is revealed by studying the evolution law of self-stabilizing and en-stabilizing damping components with variation of grid impedance or converter control parameters. It is indicated that interactions among grid-forming converters will become more serious with decrease of interconnection line impedance or transmission line impedance, worsening stability of system. Finally, theoretical analysis is verified with time-domain simulation and experimental results.

The deployment of inertia control in wind turbines (WTs) is required by grid operators to support system frequency stability, while its addition also strengthens the dynamic coupling of WT's mechanical system with the electrical gird, potentially exacerbating torsional vibrations in drivetrains. Such side effect has been reported for the supplementary inertia control in the current grid-following (GFL) control frame, while for its alternative, i.e., the so-called virtual inertia control, this effect has rarely been concerned. To fill this gap, this article conducts a comprehensive study on the torsional stability for virtual-synchronous-controlled (VSynC) DFIG-based WTs. A reduced-order small-signal model considering different wind speed operating regions is firstly proposed. Then, how the paths of different controls, such as virtual inertia control, pitching control, etc., affecting the torsional mechanical/electrical torque are sorted out, and correlated loop shaping on torsional damping are investigated in detail. Through a comparative analysis with GFL scheme, we confirm that VSynC-DFIG suffers more sever torsional stability challenges due to its curtailed electrical damping contribution. Furthermore, we identify the operating conditions that are prone to instability and reveal the mechanisms by which critical control factors impact stability. Finally, hardware-in-the-loop simulations are conducted to validate the correctness of the analyses.

This paper investigates the impact of phase-locked loop (PLL) dynamic on the stability of DC-link voltage control (DVC) in voltage source converter (VSC) attached to weak grid. To start with, a small signal model of VSC attached to weak grid in DVC timescale is presented for stability analysis. On the basis, the dynamic characteristics of VSC are analyzed from the perspective of damping and restoring components which can determine the stability. With considering PLL dynamic, it is found that PLL performs as a phase-lag regulation for DVC and generates additional negative damping component in weak grid, thus weakening the stability of VSC. Furthermore, impacts of PLL’s phase-lag characteristics on additional damping and restoring coefficients are evaluated with variation of grid strengths, operating points and PLL bandwidths to reveal the impact mechanism of PLL dynamic on stability of DVC in VSC. Finally, time-domain simulation and StarSim hardware-in-the-loop (HIL) experimental results are conducted to validate the aforesaid stability analysis.

This paper studies voltage/reactive power coordination control between energy storage system and clean energy plant connected to AC/DC hybrid system. As energy storage power stations are widely integrated to grid, they pose larger influence on clean energy. It occurs that voltage/reactive power characteristic of energy storage plant and clean energy plant becomes complicated to control and analyze. For the sake of studying the energy storage system and clean energy plant’s voltage/reactive power characteristics, a model of energy storage and clean energy plant connected to AC/DC hybrid system is proposed, with a proper coordination control strategy of PCC voltage/reactive power. Then, the coordination control strategy is conducted under different conditions in time domain simulation to analyze voltage/reactive power dynamics of system. Lastly, the experiment on hardware-in-the-loop(HIL) system is finished to validate proposed voltage/reactive power coordination control above.

The DC-link voltage control (DVC) timescale (i.e., the frequency dynamics covering converter outer controls) instabilities in wind generation have gained increased attention recently. This paper presents DVC timescale modeling and stability analysis for multi doubly-fed induction generators (DFIGs) connected to weak AC grids. A reduced-order, small-signal model of a grid-tied multi-DFIG system, designed for DVC dynamics analysis, is firstly proposed. The model allows for the dynamic interactions among the DC-link voltage control, active power control (APC), terminal voltage control (TVC) and phase-locked loop (PLL). Eigenvalue and participation factor analyses are conducted to explore the potential instabilities and correlated critical factors for such a multi-machine system. The sensitivity studies find that instability can occur at high levels of power generations or low short-circuit ratio (SCR) conditions. In addition, the dominant mode is identified to be highly related to the PLL, and its modal damping is decreased when the bandwidths of PLLs in different generators are close. Detailed model-based time domain simulations verified the analysis above.

Recent studies show that the loss of stability for a voltage-source converter (VSC) in weak-grid connection is largely related to its synchronization unit, i.e., the phase-locked loop (PLL). This paper studies the synchronization stability of a system comprised by two VSCs in parallel connection to a weak grid. A reduced transfer function based small-signal model, which can allow for the interactions between PLL and converter outer power controls, is first proposed. Then, an improved net damping criterion is used to analyze the damping and stability characters of such system under various operating conditions and different controller configurations. Compared to the conventional net damping criterion, the used criterion has wider applicability in terms of stability judgment. Case studies show that the studied system tends to be unstable at weak-grid or heavy-loading conditions. The instability can be in the form of oscillations or monotonic divergence, in which, the latter is more likely to occur for the converters without grid voltage regulation capabilities. Besides, the net damping-based sensitivity studies can provide guidance on control tuning or design for stability enhancement. Detailed model-based time domain simulations are conducted to verify the analysis results.

Recent works have shown that phase-locked loop (PLL) synchronized wind turbines (WTs) suffer stability issues when integrated into weak grids. However, most of the current studies are limited to a single machine case, the interactions among the WTs are usually overlooked. This paper studies the stability of multiple doubly-fed induction generators (DFIGs) that are connected in parallel to a weak AC grid. A state space model of a two-DFIG system is firstly presented. Subsequently, eigenvalue sensitivity analysis shows that instability can occur at low short-circuit ratio (SCR) or heavy loading conditions. Meanwhile, participation factor analysis implies that the unstable mode is primarily induced by the interactions between the PLLs of the two WTs. Further, to make out how the PLLs interact to cause instability, a reduced-order model is proposed for analysis simplicity, and an explanation in terms of transfer function residue is given for illustration. Detailed model-based time domain simulations are conducted to validate the analyses’ results.