Regarding a DC microgrid integrated with wind, PV generators and energy storage, this paper suggests a resistive type superconducting fault current limiter (SFCL) to protect it from short-circuit fault. The modeling and control of the DC microgrid are firstly stated, and then the resistive SFCL's impacts on the DC microgrid's fault current and bus voltage sag are analyzed in brief. Using MATLAB, simulations on different SFCL parameters are done, and the results confirm the SFCL's contributions for the DC microgrid include that: i) The DC fault current is limited quickly and effectively. ii) The bus voltage drop is compensated enough to ensure that the renewable sources achieve the fault ride-through operation. iii) The power transfer efficiency of the DC microgrid under the fault is appropriately enhanced. Finally, the resistive SFCL's conceptual design is done. Not only the magnetic field/current density distribution of the HTS tape is given, but also the Ampere Force and AC loss are estimated. In summary, the feasibility of the resistive SFCL for the DC microgrid is verified.

To solve the low-voltage ride-through (LVRT) issue of a virtual synchronous generator (VSG) under severe grid fault, our research group proposes to use a modified flux-coupling-type superconducting fault current limiter (SFCL). In this study, the configuration structure of the VSG with the SFCL at the point of common coupling (PCC) is designed. The fault analytical circuit of the VSG is presented, and the functions of the SFCL are stated from the aspects of current restraint, voltage compensation, and energy dissipation. Using MATLAB/Simulink, symmetrical and asymmetrical faults are both simulated to assess the transient behaviors of the VSG, and the SFCL parameters are changed to clarify the quantitative effects. From the comparison results, the SFCL parameters are properly determined to provide favorable performance. The SFCL can visibly decrease the fault current of each phase within the permissible level, and facilitate the VSG to have a preferable voltage profile. Furthermore, the SFCL can positively stabilize the VSG by enhancing the power delivery, mitigating the frequency fluctuation and consuming the excess energy. A satisfactory LVRT operation is gained, and the efficacy of the SFCL on reinforcing the VSG robustness is validated.

Virtual synchronous generator (VSG) strategy is an effective means for renewable energy sources to be connected to power grids. Meanwhile, a greater focus should be on the multi-VSG system's (MVS) transient stability issue. Regarding the MVS with resistive superconducting fault current limiters (R-SFCLs), this paper proposes a control method based on deep reinforcement learning (DRL) agent for increasing the transient stability. Firstly, the theoretical model of the MVS incorporating R-SFCLs is established, and R-SFCLs are used to limit the fault current in VSGs and keep the fault ride-through (FRT) operation. Then, the working mechanism of the proposed method is elaborated, by designing the Markov decision process (MDP) model of transient control, and applying the improved softmax deep deterministic policy gradients (SD2) algorithm to train the DRL agent. The advanced time series feature extraction network (TSFEN) based on the convolutional neural network (CNN) and gate recurrent unit (GRU) is suggested to enhance the actor network and critic network of the SD2. A detailed simulation model is created using MATLAB, and a comparison with the traditional VSG control, only R-SFCL, deep deterministic policy gradients (DDPG) algorithm, and improved power loop scheme is conducted. From multiple cases, the proposed approach can satisfactorily boost the transient stability of the MVS, and the generalization ability of the DRL agent under different untrained scenarios is validated. The proposed method's validity and suitability are well-confirmed.

Directing at the robustness increase of a high voltage direct current (HVDC) system with line commutated converter (LCC) and voltage source converter (VSC), this paper proposes to coordinate resistive superconducting fault current limiters (SFCLs) and hybrid-type DC circuit breakers (HDCBs) for dealing with the DC line fault. The theoretical modeling of the SFCLs is presented, and the DC fault current characteristics are analyzed. Considering that the fault interruption involves multi-stages, the coordination of the SFCL and the HDCB in the LCC/VSC station is elaborated. Using PSCAD/EMTDC, the simulation model of a 320 kV HVDC system is built. Changing the SFCL size and the HDCB operation delay is simulated, and the performance indexes such as the fault current level, breaking time and dissipated energy are compared. The proposed coordination is verified to be very effective in handling the fault. Especially for the VSC station, it enables to visibly limit the fault current rising, shorten the transient voltage duration, and facilitate a significant decrease in the dissipated energy of the HDCB. For the LCC station, the proposed coordination can serve as a competitive backup to ensure a timely fault breaking. Consequently, the DC line fault is removed more rapidly and reliably.

Regarding a paralleled synchronous generator (SG) and virtual synchronous generator (VSG) system connected to a weak grid, the transient stability should be given more concern. This paper proposes applying resistive superconducting fault current limiters (R-SFCLs) to boost the paralleled SG-VSG system's transient stability while meeting the current limitation requirement (CLR). Firstly, the theoretical modeling is carried out, and the influencing mechanism of the grid strength on the transient stability is clarified. Then, the functions of the R-SFCLs to the paralleled system are analyzed, and the parameter design guidelines for the R-SFCLs are stated. Time-domain simulations are implemented in MATLAB, and a detailed comparison of using the R-SFCLs and the additional torque control for the paralleled system is made. From the simulations, the proposed approach offers a more significant stabilization effect. It outperforms the additional torque control to decrease the unbalanced power, eliminate the power angle oscillation, and handle the short-circuit fault with a longer duration for the paralleled system. The viability and appropriateness of the proposed approach are well confirmed.