Commutation failure is one of the common faults in the line commutated converter high voltage direct current (LCC-HVDC) transmission system, which will lead to a transient voltage disturbance with the characteristic of "decrease first and then increase" at the feed end. Such a transient voltage may cause severe dynamic responses on the wind generator adjacent to the converter station, resulting in the tripping of large-scale wind turbines and even cascading failure. By analyzing the principle of the transient voltage, this paper investigates the response mechanism of doubly-fed induction generator (DFIG), involving the variation of electrical parameters and fault ride through (FRT) ability. Further verifications through simulations and experiments prove that under the disturbance of commutation failure, DFIG may suffer greater transient shocks compared to a single sag or swell disturbance event. Finally, some suggestions are put forward to mitigate the transient voltage and improve the transient response of DFIG.

Performance evaluations of a MW-class dynamic voltage restorer (DVR) system are presented for mitigation of voltage quality disturbances. A presag compensation strategy is introduced to lock the instantaneous magnitudes and phase angles of real-time line voltages, and thus to compensate the improper voltage components exactly after symmetrical or asymmetrical voltage disturbances. A 0.3-H/1.76-kA superconducting magnetic energy storage (SMES) magnet is used to cooperate with conventional battery energy storage (BES) device for developing a high-performance hybrid energy storage (HES) system. In the SMES-BES HES-based DVR system, the SMES can compensate the worst voltage sag for five cycles (1 MW, 100 ms, and absorb the highest voltage swell for six cycles (1 MW, 120 ms). The accessorial high-capacity BES device is subsequently discharged or charged for lasting long-time mitigation operations. Various simulation cases with regard to combined voltage sag, swell, and harmonic disturbances are carried out. The simulations and comparisons among the SMES-based, BES-based, and HES-based DVR schemes have demonstrated that the HES-based DVR scheme integrates the merits of fast response speed and high-power density from the SMES-based scheme, and the merits of low capital cost and high-energy density from the BES-based scheme.

Commutation failures (CFs) including repetitive CFs (RCFs) are frequently occurring faults in line commutated converter based high voltage direct current (LCC-HVDC) systems. Consequently, sending end system will suffer transient voltage disturbances (TVD) with a characteristic of “first reduce then rise and may repeat”. Due to the complex interaction process of TVD, the electromagnetic process behind this phenomenon cannot be easily explained, which imposes great challenges on the development of mitigation strategies. Given this, this paper provides a deep insight into the mechanism and the impact factors of the TVD caused by CFs and RCFs. The numerical relationship between the transmitting power and the TVD is derived. Research results revealed that the transient process after fault can be segmented into five stages. In each stage, the control interactions among HVDC controllers and power interactions at the PCC are analyzed. Besides, we discussed the TVD under RCFs in detail. Several indices are further proposed to facilitate the characterization of the TVD, and main control loops of HVDC that have impacts on the characteristics of TVD are thereby studied. Finally, the mathematical relationship between wind farm power and the TVD is derived to quantify the impact of wind farms on the TVD.

Hybrid energy storage system (HESS) is an effective measure to improve the electrical performance of naval dc microgrids supplying pulsed power loads (PPLs). Coordination control scheme and capacity configuration of the HESS are two key issues to meet multiple control objectives and constraints. In response to the requirements of optimal operation for HESS under various complex scenarios, a dual model predictive control (D-MPC) strategy is proposed for the HESS integrated with the superconducting magnetic energy storage (SMES) and battery in this article. Firstly, the current reference of battery is obtained through the MPC integrated adaptive low-pass filtering (ALPF). Then, the local MPC controller of HESS converter is utilized to track the current reference of battery and SMES as well as the bus voltage reference to achieve transient power allocation of HESS and energy balancing of dc microgrid. Meanwhile, a cost function with adaptive weighting factor is designed to make tradeoff of conflict control objectives. Furthermore, the HESS capacity configuration and superconducting magnet optimal design according to its actual operation condition are presented. Comparative case studies are conducted in hardware-in-the-loop (HIL) experiment to demonstrate the superiority of the proposed scheme in terms of improving system operation performance under various PPLs.

The doubly fed induction generator (DFIG) has been required to operate continuously when confronted with faults. For improving its fault ride through capability (FRT), a novel modified bridge-resistive-type superconducting fault current limiter (MBR-SFCL) is put forward in this paper. Compared with the conventional SFCL, the steady-state operating loss is reduced, and the resistance value can be controlled. The model, operation principles, and control scheme of MBR-SFCL are described. The comparative analysis between the proposed MBR-SFCL and conventional SFCL in the DFIG system is studied. All the critical variables of the DFIG system can be kept in domain. The simulation results show that the proposed limiter can improve the voltage ride-through capability of DFIG during the fault and post-fault process.