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In this paper, a flux-coupling-type superconducting fault current limiter (SFCL) is suggested to protect a 15-MW class doubly fed induction generator (DFIG)-based wind farm. Detailed conceptual design and performance evaluation of the SFCL are conducted. From the technical requirements and operational parameters of the DFIG wind farm, a 35-kV/500-A flux-coupling-type SFCL is designed, taking into consideration: 1) the winding type, core selection, and inductance values of the coupling transformer (CT); and 2) the structural style, tape length, and quenching resistance of the superconducting coil (SC). By use of different simulation tools, the electrical and electromagnetic characteristics of the SFCL are evaluated. The results show that the maximum magnetic field of the CT is 2.4 T, and the ac loss of the SC is 0.51 W. Not only the electromagnetic properties and loss of the SFCL are acceptable, but also using the SFCL in the wind farm enables to suppress the fault current, compensate the voltage sag, and mitigate the wind power fluctuation. Thus, the robustness of the DFIG-based wind farm against the fault is enhanced, and the effectiveness of the proposed SFCL design is verified.

This paper suggests a small-scale superconducting magnetic energy storage (SMES) to enhance the transient behaviors of a 100 kW grid-connected photovoltaic (PV) system, and conducts the conceptual design and performance evaluation. Considering the PV system requirements, the stored energy of the SMES is 90 kJ, and the YBCO tapes are adopted to make a solenoid SMES magnet. Based on the genetic algorithm, the magnet parameters including critical current, tape length, parallel and perpendicular magnetic fields are optimized. Using simulation tools, the effects of the SMES on the PV system are assessed, and the electromagnetic properties, stress, and loss of the SMES magnet are analyzed. The results prove that 1) the SMES improves the fault-ride-through capability and realizes an effective protection for the PV system. 2) A relatively large perpendicular magnetic field is found in both ends of the SMES magnet, and the maximum field is induced in the middle of the SMES magnet. 3) The maximum stress of the SMES magnet is acceptable, and the mechanical strength is satisfied. 4) The ac loss is crucial for voltage maintenance performance of the SMES at the fault. Under a short clearance time, the heat accumulation may have a moderate controllability.

ToTo prevent the degradation of battery life and reduce switching loss in inductive power transfer (IPT) systems for electric vehicles (EVs), it is vital to realize constant current/voltage (CC/CV) charging and zero-voltage-switching (ZVS) operation. However, due to the existence of couplings between the charging and ZVS operating control loops, the controllers are neither optimal nor robust. In this paper, a model-based decoupled control method is proposed to improve the performance of the charging and ZVS operating controllers. Since the operating point of the CC and CV modes are different, two dynamic models for the corresponding charging modes are built and linearized. Besides, the couplings between the charging and ZVS operating control loops are analyzed, and in turn, decoupled by designing compensators. To optimize the system performance, we chose PI controllers, and their parameters are obtained by calculating the settling time and overshoot of the close-loop dominant poles according to the control performance requirements. In the experiment, a 500-W series-series IPT system prototype is built and tested to verify the improvement of the control performance. Compared with the coupled control method and the traditional closed-loop control feedback method, the recorded response curves and transient waveforms demonstrate that the proposed decoupled control method brings a better performance to the system.

The limited operational flexibility of combined heat and power (CHP) units is the main cause of wind power curtailment in the thermal-electrical power system of Northern China. Pumped hydrostorage (PHS), heat storage (HS), and electric boiler (EB) are investigated as three alternative options for the promotion of wind power integration. On the basis of two linear models that determine the capacities of these three facilities required for integrating the curtailed wind power, economic evaluation in terms of investment costs and environmental benefits is presented. Analysis results show that HS requires the least investment and has a good performance of coal saving when accommodating the same amount of curtailed wind power. And EB has the greatest potential for wind power integration with the huge growth of installed capacity of wind power in the future.

In regards to the cogeneration system in Northern China, mainly supported by combined heat and power (CHP) plants, it usually offers limited operation flexibility due to the joint production of electric and thermal power. For that large-scale wind farms included in the cogeneration system, a large amount of wind energy may have to be wasted. To solve this issue, the utilization of the electric energy storages and the thermal energy auxiliaries are recommended, including pumped hydro storage (PHS), compressed air energy storage (CAES), hydrogen-based energy storage (HES), heat storage (HS), electric boilers (EB), and heat pumps (HP). This paper proposes a general evaluation method to compare the performance of these six different approaches for promoting wind power integration. In consideration of saving coal consumption, reducing CO2 emissions, and increasing investment cost, the comprehensive benefit is defined as the evaluation index. Specifically, a wind-thermal conflicting expression (WTCE) is put forward to simplify the formulation of the comprehensive benefit. Further, according to the cogeneration system of the West Inner Mongolia (WIM) power grid, a test system is modelled to perform the comparison of the six different approaches. The results show that introducing the electric energy storages and the thermal energy auxiliaries can both contribute to facilitating wind power integration, and the HP can provide the best comprehensive benefit.

This paper proposes an advanced small-signal of multi-terminal MMC-HVDC for large-scale systems stability analysis based on dynamic phasors and state-space, and this advanced model. In accordance with active and passive network control strategies for multi-terminal MMC-HVDC, the matchable small-signal stability models contained higher harmonics are conducted, and related theoretical derivation is carried out. The comparison of the advanced small-signal model, electromagnetic-transient model, and traditional small-signal state-space model is performed under a typical multi-terminal MMC-HVDC network with offshore wind generation. According to simulation, the advanced small-signal model can successfully follow the electromechanical transient response with small errors and predict the damped oscillations. Consequently, the validity and applicability of the proposed model are well confirmed.

Wind power curtailment in winter of Northern China is mainly caused by the limited flexibility of combined heat and power (CHP) plants. Improving the profitability of CHP by heat storage is a promising alternative to increase the integration of wind power. Dispatching method of CHP and heat storage is proposed in this paper based on two linear models. The first model is used for the pre-judging of time intervals of heat charging and discharging while the second is for the determination of power output of the CHP units and heat storage. Wind power curtailment and operation cost are both considered in the objective function of the second model. Operation results with different amounts of wind power generation and heat storage capacity are analyzed. Case studies are presented to demonstrate the benefits of heat storage on improving the integration of wind power economically with proposed methods.

Battery Energy storage system (BESS) makes it possible for wind power to participate in pre-determined power dispatching. To deal with the variability and uncertainty of wind power, peaking constraints of BESS accounting for the effect of wind power on system dispatching are proposed in this paper. On the basis of that, an optimal capacity model considering investment costs, operational benefits, and wind curtailment saving is established with respect to the wind-storage union system, which takes both the economy of wind farm and the adaptability of scheduling into account. Finally, results based on the actual wind farm data verifies the validity of proposed method, and various sensitivity analyses are performed to assess the effect of key parameters on this optimal capacity. A foundation is laid for allocating BEES capacity of large-scale wind farm in matching with the dispatching operation.