Energy storage equipment is an important part of integrated energy systems, but the construction and operational costs of it are great. Therefore, it’s difficult to apply energy storage equipment on a large scale. As an alternative measure, adjusting controllable loads including electric vehicles (EVs) and air conditioning loads jointly can transfer loads across time periods and reduce variation difference between heat and electric loads. These controllable loads can be regarded as “virtual energy storage system”. Managing the charging of EVs and heat storage of buildings, a joint virtual energy storage system including electric energy storage and thermal energy storage is proposed in this paper. Then, these models are integrated into a scheduling model for regional integrated energy system (RIES). The charging/discharging power management of joint virtual energy storage systems can be realized by arranging the charging of EVs based on vehicle-driven rules and by adjusting building indoor temperatures within the temperature comfort range. Finally, simulations are carried out to demonstrate the effectiveness of the scheduling method. Simulation results show that the RIES optimal scheduling model considering joint virtual energy storage can reduce operational costs. The stability of RIES is improved with the combination of virtual electrical and thermal energy storage.

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.

The inertia level of microgrids decreases with the access of inverter-based distributed generations. Uncertainty of source loads in islanded microgrids can lead to frequency instability. Considering electric vehicles (EVs) as a typical regulable load, a charging station (CS) virtual synchronous generator (VSG) frequency control (FC) strategy that considers the user charging and grid FC demand is proposed. First, the control architecture of CS based on the VSG is designed. Second, considering the impact of VSG FC on EV user charging demand, the charging power response model considering the charging power FC cost is designed. Furthermore, multiobjective model predictive control algorithm is designed to improve the VSG control strategy, and the FC strategy is obtained. Simulation results show that the proposed method can provide inertial support and frequency regulation capability by the CS with VSG control strategy while satisfying EV charging demand. In addition, the FC cost can be decreased.

For that wind plants are connected to an electric power grid through voltage source converter-based high-voltage direct-current (VSC-HVDC) transmission, the improvement of the VSC-HVDC system's robustness against a dc short-circuit fault is critical. This paper suggests the inductive and resistive superconducting fault current limiter (SFCL) to alleviate the fault current and enhance the system's transient behaviors. Each of the two SFCLs is installed in series with the dc transmission line, and once the fault is detected, the current-limiting inductance or resistance will be activated to affect the system characteristics. Herein, wind plants are based on doubly-fed induction generators, and related theoretical derivation and transient simulation analysis are done. From the results, both of the two SFCLs can limit the dc fault current, improve the dc-voltage sags and suppress the power fluctuations. By comparing the specific performance indexes including current-limiting ratio and restraining capability to voltage/power fluctuations, the resistive SFCL is more preferable than the inductive SFCL. At the end, the SFCL optimization technique considering current-limitation, voltage stability, and device cost are discussed, so as to further promote the SFCL's application in the VSC-HVDC system.

Doubly fed induction generators (DFIGs) have attracted a wide interest for wind power generation, but they suffer from high sensitivity to grid disturbances, particularly grid faults. In this paper, a modified flux-coupling-type superconducting fault current limiter (SFCL) is suggested to improve the fault ride-through (FRT) capability of DFIGs. The SFCL's structure and principle is first presented. Then, considering that the SFCL can be installed at a DFIG's different locations, its influence mechanism to the DFIG's FRT capability is analyzed, and some technical discussions on the design of the SFCL are carried out. Furthermore, the simulation model of a 1.5-MW/690-V DFIG integrated with the SFCL is built, and the performance analysis is conducted. From the results, introducing the SFCL can effectively limit the fault currents across the DFIG's stator and rotor sides, and when the stator side is selected as the installation site, the terminal-voltage sag can be also improved, which helps prevent the disconnection of the DFIG from the power grid.

Concerning that a short-circuit fault happens inside a microgrid system, parts of distributed generation (DG) units may not meet the fault ride-through (FRT) requirements and will be enforced to disconnect. This paper suggests a modified flux-coupling-type superconducting fault current limiter (SFCL) to improve the DG's FRT capability, and investigates the relay protection coordination in the microgrid. The SFCL's structural principle is introduced, and according to the fault characteristic of the microgrid under its grid-connected and islanded statuses, the modified SFCL's application fields are conducted. Furthermore, the directional overcurrent protection and differential protection are proposed for the microgrid's two statuses, respectively. Based on the transient simulations in the MATLAB, the SFCL's positive effects on enhancing the DG's FRT capability can be confirmed, and also it will not affect the relay protection coordination. A comparison of the conventional distance relay protection and the proposed protection scheme is performed, and the latter has a better applicability than the former to match the SFCL. Finally, regarding the impacts of fault type and fault resistance on the FRT operation and the proposed protection, technical discussions are performed preliminarily, and the application value of the SFCL integrated with the proposed protection is clarified further.