Weak AC grid operation is known to challenge the stability of VSCs. This paper provides an interpretation for the cause of such instability in view of a particular modal resonance within VSC's control, i.e., the resonance between DC-link voltage control (DVC) and synchronizing control-phase-locked loop (PLL). To characterize this modal interaction, a two degrees-of-freedom mass-spring-damper model is first proposed. Subsequently, based on a multi-modal decomposition approach, the amount of interaction is quantified by the incremental damping and frequency drift superimposed on each individual mode. Analytical results indicate that when the natural frequencies of PLL and DVC mode are close, strong interaction will push the lower-frequency mode to move towards the decreased frequency and damping direction, while causing the higher-frequency mode to go the opposite. Moreover, weaker AC grid operation will amplify such frequency/damping excursion, thus will render the lower-frequency mode unstable. Further, when the AC voltage control (AVC) is disregarded, a dynamic stability margin is analytically derived, and operation beyond the margin will result in monotonic drift. With the inclusion of AVC, owing to its introduced additional negative damping, by contrast, instability will occur in presence of oscillations. Both eigenvalue analysis and simulations are conducted to verify the results.

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.

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.

For power converter-interfaced wind-generation systems, low-frequency instability issues arising from the interactions among converter outer power flow control (PFC) loops have got increased attention recently. This paper explores the possible origin of such instability in grid-tied doubly fed induction generator (DFIG) systems. To comprehensively analyze this multi-loop interacted system, a reduced transfer matrix model is first constructed allowing for the PFC dynamics. Then the individual channel analysis and design (ICAD) approach is employed to decompose such closed-loop multivariable system into components such as local subsystem and loop interactions. Subsequently, frequency-response behavior analyses of these components are individually conducted at various operating points to study their relative contribution to overall stability. Results indicate that local active power control loop will induce a right-half-plane zero (RHPZ) for certain high-power operating conditions, and the resulting inverse response leads system unstable. Meanwhile, the critical operating point at which the RHPZ arises is analytically derived, and demonstrated to be close to the actual dynamic stability boundary.

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.

In the conventional power systems, the synchronous generators (SGs) are known to suffer stability issues under weak grid conditions. Recently, similar problems are observed in the inverter interfaced systems, including the full voltage source converter (VSC)‐based high‐voltage DC, and partial VSC interfaced doubly fed induction generator (DFIG) systems, but the root causes of these instabilities in reality are not completely understood yet. This study aims to provide a possible unified explanation for the origin of these instability issues. With the experiences of SG's instability mechanism, for the VSC and DFIG systems, the dynamics of outer power flow controls are mainly focused, and the reduced‐order models are parallelly presented first. Then, centralised around the active power control loops of these systems for analysis, it is found that an open‐loop zero will move to the right half plane with grid strength decrease. The borders for the occurrence of right‐half‐plane zeros (RHPZs) are analytically derived. Further, a general interpretation regarding these identified RHPZs’ limitations, including the adverse phase lag effect on stability and the attraction effect on the stability boundary, is illustrated. In addition, a uniform explanation for the origin of the RHPZs in these systems is also provided. Time‐domain simulations are conducted to validate the analysis.