As superconducting magnetic energy storage (SMES) and battery are complementary in their technical properties of power capacity, energy density, response speed, etc., this paper proposes an SMES-battery energy storage system to stabilize a photovoltaic-based microgrid under different faults. The related theoretical modeling is stated, and the control and coordination methods of the SMES-battery are put forward. Using MATLAB, a comparison of with the SMES-battery and only with the battery is carried out. From the main specifications of the SMES magnet, the ac-loss calculation is also performed. The results show that i) the SMES-battery is better than the battery to timely deal with the transient faults of the microgrid; ii) the SMES-battery enables to ensure a seamless mode-transition for the microgrid under the external fault, and reduce the fault current in the point of common coupling to avoid an unnecessary off-grid under the internal fault; and iii) the SMES magnet's ac-loss power has a maximum value of 380 W, and it is acceptable for the future design of conduction-cooled structure and cryogenic system.

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.

In this paper, through AC and DC overcurrent tests on second generation high temperature superconducting tape (2G HTS tape), we respectively summarize the typical types of quenching resistance and corresponding quenching degree, in which there are three types under AC overcurrent and two types under DC overcurrent. According to experimental results, a rule was found that, when 2G HTS tape quenches to normal state, the relationship between quenching resistance and joule heat generated from 2G HTS tape presents a fixed trend line, and the influence of liquid nitrogen can be ignored. Then, the characteristics and rules of quenching resistance found in experiments are well explained and confirmed by a detailed 3D finite element model of 2G HTS tape including electromagnetic field and thermal field. Finally, based on above works, our group proposes a new equivalent method to estimate the quenching resistance, where the results of AC and DC overcurrent experiments can be equivalent to each other within a certain range. Compared with FEM, the method has the following advantages: (i) The method is simple and easy to implement. (ii) This method combines precision and computational efficiency. (iii) With superconducting tape quenching to normal state, this method presents a good consistency with experimental results.

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.