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.

The isolated bidirectional DC/DC converter (IBDC) plays an important role in DC buses connection, voltage conversion, electrical isolation and bidirectional power transmission for DC distribution networks. In this paper, a multiple modular IBDC scheme based on hybrid- dual-active-bridge (HDAB) for medium-voltage (MV) and high-voltage DC (HVDC) distribution network is proposed. In the proposed HDAB, the switches in diagonal position of full-bridge are replaced by half-bridge sub-modules (SMs), the proposed HDAB maintains advantages of conventional DAB and achieves bidirectional fault handling capability, avoiding the application of large DC-link capacitors and DC circuit breakers, reducing the volume and construction cost. Besides, high-frequency-link (HFL) voltage regulation strategy is also proposed in this paper, adjusting the HFL voltages in HDAB to match the HFL transformer voltage adaptively, thus improving the efficiency and applicability of IBDC under the fluctuation of power flow and DC bus voltage. The topology, operation principle, control structure and corresponding bidirectional fault handling and HFL voltage regulation strategy of proposed IBDC scheme are investigated. The experimental results verify correctness and effectiveness of proposed scheme and control strategy, promoting the adaptability and practicality for both IBDC and DC distribution network.

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.

This article systematically focuses on the transient stability problem of droop-based multiconverters dc (MCdc) system caused by large power disturbances. Therefore, a new and effective transient stability analysis method is proposed. In the present method, it starts with reduced-order nonlinear mathematical modeling, followed by Lyapunov energy function building, and then the inscription of the largest estimated domain of attraction (LEDA) and the power feasible region (PFR). First, a state-variable equivalent transformation method is proposed. With this method, the droop-based MCdc system can be modeled as an equivalent single-converter (ESC) model without ignoring any coupling effects between the control state-variables. Second, a Takagi–Sugeno fuzzy model of the ESC is developed, which is able to construct the LEDA. Third, to ensure the transient stability of the MCdc system under large power disturbance, the concept of PFR is proposed. Furthermore, a PFR inscription algorithm is given based on the LEDA. In addition, the influence of control parameters on the PFR is analyzed, which will provide guidelines for the optimization of the PFR. Finally, the time domain experiments are carried out on the RT-BOX hardware-in-the-loop experimental platform to verify the effectiveness of the above theoretical analysis.

The increasing penetration of converter-interfaced renewable generation and the declining share of synchronous generation reduce voltage and frequency support. This paper proposes a grid-forming (GFM) converter capacity ratio estimation method and a capability assessment framework for hybrid grid-following (GFL) and GFM converter systems at fixed total installed capacity. First, a model incorporating the principal control loops of both converter types is developed. The critical short-circuit ratio and the GFM converter terminal impedance are used to derive an analytical relationship between the GFM capacity ratio and the generalized short-circuit ratio. This relationship defines a continuous lower bound for the capacity ratio subject to small-signal stability and target grid strength constraints. The assessment framework uses normalized margins and a bottleneck index to evaluate basic support conditions, control parameter effects, and disturbance response performance. Generalized Nyquist analysis and time-domain simulations verify the method. Under the studied conditions, increasing the GFM capacity ratio increases equivalent grid strength and oscillation damping, reduces the peak power deviation, and shortens the settling time. Phase-locked loop parameters mainly affect synchronization dynamics and power oscillations, whereas virtual inertia and the damping coefficient primarily affect frequency support and power recovery. The method supports GFM capacity planning and control coordination.