With the prevalence of renewable energy sources such as wind power in the power system, analyzing the fault characteristics of systems composed of DFIGs is becoming increasingly important. Therefore, this article analyzes, at first theoretically, the fault characteristics of a doubly fed induction generator (DFIG) during fault periods. It was found that the fault current of the DFIG exhibited the frequency offset phenomenon, which is affected by the depth of voltage dips and can negatively impact traditional distance protection. Furthermore, a method using a dynamic voltage restorer (DVR) based on superconducting magnetic energy storage (SMES) was adopted to compensate for the fault voltage of DFIG, which can mitigate the voltage dips of the DFIG. This method can not only achieve the fault ride through for DFIG but also significantly improve the frequency offset of the fault current during fault periods. Finally, a model composed of a 2.5 MW DFIG-based wind turbine and a 2.5 MW DVR-based SMES was built using a real-time digital simulator (RTDS) platform, and the simulation results showed that the fault stator voltage of DFIG can be compensated at a rated value of 0.69 kV, and the frequency of fault current can be maintained at 50 Hz These results validate the excellent performance of the method in achieving the fault ride through of DFIG and improving the frequency offset of the fault current by comparing multiple type faults while employing different protection methods.

With high penetration of renewable energy, hydrogen, as a form of long-term energy storage has become more important in power systems. However, challenges remain in coordinating hybrid electrolyzers due to their different operating ranges and power rate limits. Thus, this work proposes a two-layer adaptive virtual impedance-based power management scheme for a DC hybrid photovoltaic (PV), battery energy storage system (BESS), alkaline electrolyzer (AEL), and proton exchange membrane electrolyzer (PEMEL) microgrid. In this proposed strategy, the first layer implements basic virtual impedance control to achieve power allocation among BESS, AEL, and PEMEL. The second layer is employed to optimize the virtual impedance of the first layer. The virtual resistances of BESS, AEL and PEMEL are designed to coordinate different operating ranges of hybrid electrolyzers, while reserving more adjustment margin for PEMEL to flexibly respond to system changes. In addition, the virtual inductance is utilized in AEL control, and optimized in real time to smooth the power fluctuation from the PV system by introducing the virtual power rate. Meanwhile, the virtual resistance of BESS is designed for state-of-charge management. Compared with centralized methods, the proposed strategy reduces the communication burden by 60% and requires only 27% controller utilization, demonstrating enhanced reliability and computational efficiency. Moreover, the DC bus voltage variation is reduced by 0.2%. The effectiveness is validated through simulations and experiments.

Doubly fed induction generators (DFIG) have become widely applied in wind power generation, but they suffer from high sensitivity to grid disturbances, particularly grid faults. In this paper, a fault ride-through method based on the dynamic impedance for DFIG-based wind turbines. The dynamic impedance, integrated at the rotor side of DFIG, can effectively limit the rotor fault current to help DFIG ride through faults at grid side. And the dynamic impedance can be flexibly adjusted to precisely limit different levels of fault current. The inductance of the dynamic impedance can effectively reduce the peak value of the rotor fault current at the initial stage of the fault, which is beneficial to the stability of the rotor current; The resistance of the dynamic impedance can ensure that the rotor current is within the allowable range and consume excess power to prevent the DC bus voltage of wind turbine from suddenly increasing.