This paper investigates the effect of control-loops interactions on power stability limits of the voltage-source converter (VSC) as connected to an ac system. The focus is put on the physical mechanism of the control-loops interactions in the VSC, revealing that interactions among the control loops result in the production of an additional loop. The open-loop gain of the additional loop is employed to quantify the severity of the control-loop interactions. Furthermore, the power current sensitivity, closely related to control-loops interactions, is applied to estimate the maximum transferrable power of the VSC connected to an ac grid. On that basis, stability analysis results show that interactions between dc-link voltage control and phase-locked loop restrict the power angle to about 51° for stable operation with no dynamic reactive power supported. Conversely, the system is capable of approaching the ac-side maximum power transfer limits with alternating voltage control included. Simulations in MATLAB/Simulink are conducted to validate the stability analysis.

With increasing application of voltage source converters (VSCs), electromagnetic stability, characterized by fast dynamics and controls of electromagnetic elements such as inductors and capacitors, becomes challenging for power system stability. This paper investigates modeling and stability analysis of VSC internal voltage in dc-link voltage timescale. Concepts of VSC internal voltage inertia, damping power, and synchronizing power are introduced to provide a physical insight into VSC stability issue. Internal voltage stability can be identified by analyzing damping and synchronizing powers. Insufficient of either damping power or synchronizing power will result in loss of stability. Furthermore, internal voltage stability affected by short circuit ratio (SCR), operating point, and phase-locked loop (PLL) bandwidth is analyzed by investigating their effects on damping and synchronizing powers, respectively. It is found that internal voltage stability becomes deteriorated due to the insufficient damping power with low SCR values, high operating powers, and PLL bandwidth getting close to dc-link voltage control bandwidth in a weak grid. Simulation results and experimental results verify the analysis.

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.

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.

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.

This study investigates modelling of doubly‐fed induction generator (DFIG)‐based wind turbines for low‐frequency oscillation analysis of power systems with high penetration of distributed energy. A reduced order model of wind turbine with DFIG is proposed for small signal stability analysis in electromechanical time scale for low‐frequency oscillation analysis. Furthermore, a Thévenin equivalent model of DFIG with internal voltage is also presented. Base on the derived model, the effect of phase‐locked loop is emphasised, and explained by using the internal voltage derived. The model proposed is well verified by modal analysis and time‐domain simulations.