[Objective] With the increasing penetration of renewable energy sources， traditional rotating machines have been replaced with power electronic converters， causing a considerable decline in the overall inertia of power systems. This reduction in inertia poses serious challenges to the frequency stability and system disturbance rejection. To overcome these challenges， grid-connected converters that can actively provide synthetic inertial support are required. [Methods] First， based on a power-sharing control strategy， this study analyzes the pole-zero distribution characteristics of conventional phase-locked loop （PLL）-based control structures and highlights the associated stability degradation under weak grid conditions. Next， an inertia enhancement control scheme based on an improved frequency-locked loop （FLL） structure is proposed to address these problems. By proportionally integrating the frequency-derivative signal with the reference active power， the proposed method effectively mitigates power oscillations and enhances the inertial response of the converter. Furthermore， a modified model predictive control strategy is employed to replace the conventional inner current loop， significantly improving the transient performance of the system. Further， the effectiveness of the proposed control strategy is validated through hardware-in-the-loop simulations.Comparative studies of frequency step responses demonstrated the superiority of the proposed FLL-based structure in maintaining frequency stability under weak grid conditions. The active power control strategy under the improved FLL framework is detailed， along with the principle of inertia emulation and tuning of the associated parameters. [Results] The frequency response of the system is substantially improved and synthetic inertia enhancement is achieved. [Conclusions] The proposed method prevents high-frequency noise issues caused by the direct differentiation of frequency signals and eliminates stability problems typically introduced by PLL.

[Objective] With the increase in transmission capacity of DC grids， issues such as rapid rise of fault current， high amplitude， difficulty in breaking， and high cost arise in hybrid DC circuit breakers based on fully controlled power electronic devices. To address these issues， this paper proposes a hybrid DC circuit breaker topology with coupled inductors for current limiting. [Methods] In the proposed topology， the primary side of the coupled inductors in the main branch limits the fault current for the first time， and the current-limiting branch limits the fault current for the second time， thereby effectively reducing the fault current rise rate. The secondary side of the coupled inductors in the main branch pre-charges the self-charging capacitor； this remarkably reduces the equipment complexity. Traditional IGBTs and parallel capacitors are replaced by thyristors to switch surge arresters， improving the economy. [Results] The working principle and parameters of each main circuit in the proposed topology were analyzed. On this basis， a four-terminal DC transmission grid system model was established in PSCAD/EMTDC. The proposed topology was then compared with existing topologies via simulations. When a short-circuit fault occurs， the proposed topology further reduces the peak value of the fault-current limit， shortens the breaking time， reduces the energy absorption of the surge arrester， and provides economic benefits. [Conclusions] The analysis shows that the proposed topology affords good breaking performance and certain economic benefits. These findings can provide a theoretical basis for the future application of high-voltage DC circuit breakers in DC transmission grid systems.