This paper mainly focuses on transient stability issue of phase-locked-loop synchronization (PLL-Syn) in a weak-grid connected VSC (WG-VSC) system. First, the linear matrix inequality (LMI) optimization method has been applied to construct the largest estimated domain of attraction (LEDA) of the WG-VSC system for the first time. Compared with the frequently-used methods such as direct energy function method, Takagi-Sugeno (TS) method and Sum-of-Square Programming (SOS) method, the proposed method has the lowest conservation in constructing LEDA. Second, to ensure transient stability of PLL-Syn in severe faults, the concept of the feasible region of active/reactive current reference is proposed. Furthermore, based on the LEDA obtained by LMI optimization method, a simple algorithm to obtain this feasible region is presented. And, the influence of typical parameters on this feasible region is analyzed. Third, for transient instability scenarios, a simple variable step size iterative algorithm has been proposed to search the estimated critical action time with the derived LEDA. Moreover, two associated emergency control methods including adjusting the current reference values back to the feasible region and clearing grid fault have been considered to verify the estimated critical action time. Time-domain simulation results based on detailed switching model in PSCAD/EMTDC have been provided to verify the above theoretical analysis.

During asymmetric faults, the coupling between positive and negative (PN) sequences significantly affects the phase-locked-loop (PLL) synchronization transient stability (PSTS) of a weak-grid connected voltage source converter (WG-VSC). However, little literature is on analyzing the PSTS while considering PN sequence coupling. This article has analyzed the PSTS of the WG-VSC system considering PN sequence coupling under asymmetric faults for the first time. First, the Takagi–Sugeno method is adopted to construct the largest estimated domain of attraction (LEDA) at an equilibrium point of the system during asymmetric faults. This LEDA enables quantitative analysis of PLL synchronization transient instability phenomena. Furthermore, from the point of view of ensuring the PSTS, the feasible region of PN currents (FR-PNC) of the system during asymmetric faults has been obtained. In addition, an algorithm is proposed to visualize this 4-D FR-PNC in 2-D space. Finally, the theoretical analysis is validated by time domain experiments conducted on the RT-Box hardware-in-the-loop experimental platform.

During fault recovery stage, multiparallel grid-forming (MP-GFM) converters may retrigger the current limiter, leading to transient synchronization instability. While most existing studies focus on the transient synchronization stability (TSS) of single converters, and the well-established TSS studies designed for synchronous generators are not directly applicable to MP-GFM converters due to two key differences: active and reactive power control loop (ARPCL) coupling and current limiting. This article aims to address the TSS issue of MP-GFM converters during fault recovery stage, considering these two factors. First, a mathematical model of a system with MP-GFM converters is developed, considering ARPCL coupling and current limiting. Then, for the first time, the current limiting constrained largest estimated domain of attraction (CL-LEDA) is defined and constructed. Specifically, unlike traditional energy function (EF) that only considers active power control dynamics, an analytical EF that incorporates ARPCL coupling is proposed in this article. Furthermore, considering current limiting constraint, a new idea to calculate the critical energy value is proposed based on LaSalle's invariance principle. Then, the CL-LEDA can be constructed by the obtained EF and critical energy value. Provided that the operating state is located within this CL-LEDA at fault clearance moment, all MP-GFM converters will achieve transient stability, without triggering current limiting during faults recovery stage. Finally, experimental results based on RT-LAB hardware-in-the-loop and complete hardware-based experimental verify the analysis.

This article investigates the phase-locked loop (PLL) synchronization transient stability (PLL-STS) issue of grid-tied multiple voltage source converters (MVSCs) systems, specifically focusing on transient stability evaluation and transient interaction analysis. First, a novel PLL-STS assessment method for MVSCs systems has been proposed, which considers angular frequency jumps, nonlinear damping, and transient interactions among VSCs. Compared with previous studies, this method can accurately evaluate PLL-STS of MVSCs system since the acceleration and deceleration areas of VSCs can be accurately determined by combining equal-area criterion and numerical integration method. Moreover, it can identify the dominant converters which lead to transient instability. Second, by introducing static and dynamic interaction coefficients, influence of static and dynamic interaction on the transient stability of individual converters is quantitatively analyzed. Finally, the theoretical analysis results are verified by PSCAD/ EMTDC time-domain simulation results and RT-LAB hardware-in-the-loop experimental results.

For phase-locked-loop synchronization (PLL-Syn) based Weak-Grid connected Voltage Source Converter (WG-VSC) system under asymmetric grid faults, published works mainly focus on existing of equilibrium point and small signal stability, which cannot address PLL-Syn transient stability problems. For the first time, we have proposed an analytical solution to the question of “ Whether the WG-VSC system can reach to a new available equilibrium point (EP) of fault-on state from original EP of pre-fault state without loss of PLL-Syn transient stability under asymmetric faults? ”. Dual-sequence current injections feasible region (CIFR) of WG-VSCs in positive and negative sequence current injection space is defined firstly. Then, by considering PLL-Syn transient stability constraint, Maximum available current constraint, and PCC Voltage constraint , an analytical expression of CIFR has been obtained. Finally, experiments results based on detailed switching model in RT-BOX hardware-in-loop platform have been provided to verify the CIFR constructed in this paper. It should be noted that the work of this paper can not only address PLL-Syn transient stability problems of WG-VSC system under asymmetric faults, but also can pave the way for asymmetric fault ride-through control considering transient stability constraints.

Large power fluctuation may cause transient instability in renewable power generations (RPGs) via voltage source converter based high voltage direct current transmission (VSC-HVDC) system. First, this article establishes a reduced-order model of a system including grid-following RPGs and grid-forming VSC-HVDC, derives equivalent motion equations, and analyzes the dynamic coupling mechanism among different types of control units and their impacts on the system's transient stability. Then, a transient stability assessment method based on equivalent acceleration and deceleration areas is proposed. Furthermore, the power feasible region of such a system is constructed, and the influence of main circuit and control parameters on system stability is analyzed. Finally, the effectiveness of the proposed transient stability analysis method is validated through experimental results conducted on the real-time laboratory (RT-LAB) platform.