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.

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.

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.

Voltage sag can result in extensive losses of sensitive users. However, such losses have yet to be quantified based on power quality monitoring data. The load loss rate (LLR) is a practical index for evaluating the consequences of voltage sag, providing vital information for mitigation decisions. This study proposes a data-driven LLR evaluation method of sensitive users under voltage sag, which can be used as a basis for configuring mitigation devices. First, the LLR calculation index is proposed, based on the sensitive load response characteristics and the active power trajectory under voltage sags. Second, a feature indices system is established for the active power trajectory, based on the basic attributes and load response characteristics of sensitive load, to characterize the change of active power under voltage sag. Furthermore, a voltage sag data augmentation model is constructed to generate sample data, considering the characteristic calculation model of voltage sag and the probability density distribution of characteristics. Finally, LLR evaluation model is proposed, based on random forest regression for sensitive users under voltage sags. The effectiveness and accuracy of the proposed method are verified through a case study.

Voltage sag occurrence frequency (VSOF) estimation includes the prediction of residual voltage and duration. The former has been well studied, whereas the latter presents many challenges, and is the main focus of this study. A method for estimating the VSOF based on the calculation of the protection operating time characteristics is proposed. First, this article adopts a recognition method for protection types based on monitored historical data. The protection type is recognized by dividing the fault electrical quantity (FEQ) into several intervals and calculating their mean standard deviation. Second, this article proposes a calculation method for the protection operating time characteristics. The FEQ and operating time samples are clustered based on the proposed algorithm. The density-based spatial clustering of applications with noise (DBSCAN) algorithm is enhanced by the Mahalanobis distance and K-nearest-neighbor (KNN) method. The least-square fitting and segmentation calculation of the operating time curve is performed in accordance with the clustering results. Finally, this study proposes a method to predict VSOF by considering operating time curves, different fault resistances, and generator scheduling, whereas the IEEE-30 test system is used to verify this method.

This paper presents a coordination scheme of superconducting magnetic energy storage (SMES) and photovoltaic (PV) generation system for regulating the DC bus voltage in the low voltage direct current (LVDC) distribution systems. Firstly, the effect of AC grid faults on LVDC distribution system is analyzed. Then, the PV is designed to operate as the backup power of SMES by adjusting the control mode automatically according to the residual energy of SMES. The proposed scheme can regulate DC bus voltage in milliseconds and maintain the bus voltage during the whole fault process by taking use of both SMES and PV. A LVDC distribution system is modelled in PSCAD/EMTDC. Simulation results have demonstrated the feasibility and effectiveness of the proposed scheme.