Crystalline iron oxides/hydroxides are generally preferred as supercapacitor electrode materials instead of the low-crystalline structure, despite the fact that an amorphous phase could have a comprehensive electrochemical performance owing to its structural disorder. Herein, we present a facile and scalable method for preparing amorphous FeOOH [email protected] carbon nanotubes (FeOOH [email protected]) composites. The resulting hybrid nanoflowers hold a distinctive heterostructure composed of a self-assembled amorphous FeOOH nanofilm on the MWCNTs surface. The low-crystalline 1FeOOH [email protected] composites at pH 8 exhibit a high comprehensive capacitive performance, which may be attributed to the advantageous structural features. In a −0.85 to 0 V vs Ag/AgCl potential window, the prepared hybrid electrode delivers a high specific capacitance of 345 F g^–1 at a current density of 1 A g^–1, good cycling stability (76.4% capacity retention over 5000 consecutive cycles), and outstanding rate performance (167 F g^–1 at 11.4 A g^–1). This work may trigger the possibilities of these nanomaterials for further application in supercapacitor electrodes, specifically low-crystalline oxide/hydroxide-based electrode materials.

The main merit of grid-connected adjustable-speed pumped storage unit (ASPSU) is the capability to control the pumping power flexibly, which contributes to active power balance in power system operation. Physics-based dynamic modeling of ASPSU in pumping mode under power regulation is the foundation of the quantitative analysis on its transient performance, which has been studied inadequately. This study aims to derive a dynamic model of ASPSU in pumping mode which embodies reversible pump-turbine characteristics, and to conduct eigen analysis on ASPSU in pumping mode which facilitates a better understanding of its intrinsic dynamics. Firstly, the dynamic model of reversible pump-turbine is revealed to depict its regulation properties that the pumping power is adjusted by rotational speed rather than guide vane opening. Then, the state-space representation of ASPSU in pumping mode is established to be utilized with small disturbances (including indicial and ramp tests) and validated by comparing simulated results with actual records of a commissioned ASPSU in Japan. It is also effective for compensating the wind power fluctuations based on a timescale of seconds in simulation. Finally, the eigenvalues, damping ratios, and participation factors of the state variables are investigated based on the small-signal-stability model of ASPSU in pumping mode under power regulation. The results of eigen analysis indicate that the small-signal behavior of the system is characterized by electromechanical mode and electromagnetic mode, and the latter is dominant. Moreover, the damping ratio of electromagnetic mode which is sensitive to system stability needs to be increased.

Broadband impedance spectroscopy (BIS) has become a new method for detecting cable insulation defects, but the sensitivity of this method to local defects in cables has not been fully studied. Taking long submarine cable as an example, firstly, the intrinsic correlations between the impedance spectroscopy characteristics and the cable parameters are studied, and the influence of local defects on the peak of impedance spectrum is studied by establishing a simplified model of long cables with insulation defects. Secondly, the formula for calculating the sensitivity of input impedance to the insulation defects is proposed and the characteristics of sensitivity are studied. Finally, the sensitivity of the input impedance to different positions and degrees of defects at the first resonant frequency is analyzed, which can be verified by simulation and experimental. The Results show that, for long cable with length l, under the condition of open-circuit, the sensitivity trends to zero when the defects occur at 25% or 75% of the cable length l, and under the condition of short-circuit, the sensitivity trends to zero when the defects occur to the end of the cable. Therefore, in the application of BIS, the above positions should be paid attention to, and the cable length can be changed and measured again to confirm its insulation status.

Crowbar protection and chopper protection (CCP) are commonly used hardware protection measures in the process of low voltage ride through (LVRT) of doubly-fed induction generator (DFIG). The protections operating states during fault have an important influence on the electromagnetic transient (EMT) process of DFIG. Therefore, different from only considering the influence of single protection, the coordinated control of crowbar protection and chopper protection are further considered in this paper, and a novel analysis method of CCP operation criterion is proposed by analyzing the relationship between rotor current, DC-link voltage and post-fault terminal voltage (PFTV) of DFIG. On this basis, the states of CCP under different voltage drop depths are analyzed. And an EMT analysis model of DFIG considering CCP is studied. The proposed method and EMT analysis model for DFIG are later validated through PSCAD/EMTDC-based simulation results. The results show that the method and model proposed in this paper have good accuracy and practicability, which provides more references for further study of the transient characteristics of DFIG.

Integrating large amounts of wind power into power systems brings a large influence on the dynamic frequency response characteristic (DFRC). The traditional low-order system frequency response (SFR) model is no longer applicable at the current time. Based on the small signal analysis theory, a set of novel low-order SFR models for doubly-fed induction generator (DFIG) distributed wind power generation systems (DWPGS) are derived under low, medium, and high wind speed conditions, respectively. Time-domain simulations have been conducted on PSCAD/EMTDC, and the novel SFR model is tested and evaluated on a real system. The simulation results from the novel model agree with those from the detailed model. The novel SFR model can also directly show the impact of the initial wind speed and auxiliary frequency controller (AFC) parameters on DFRC, but not on the detailed model.

Having great performance on active power regulation, grid-connected adjustable-speed pumped storage unit (ASPSU) has attracted world-wide concern. This superior transient response is achieved by both its excitation system and speed governing system, but dynamic characteristics study of ASPSU taking into account the optimal control of speed governor has seldom been explored. This paper first presents small-signal-stability model of on-grid ASPSU. Furthermore, instead of the dynamic-performance-indexes rule, a stability-degree criteria is proposed to optimize parameters for the speed regulator of ASPSU, which aims at improving the dynamic behavior of rotational speed and guide vane opening. Firstly, a mathematical model of on-grid ASPSU with power priority control is derived and it is validated by comparing simulated performance with on-site measurements of a Japanese commissioned ASPSU. Secondly, the small-signal-stability model is showcased, and the influence of control parameters on the stability degree which builds the bridge between eigenvalues and transient performance is investigated in details. Thirdly, instead of improved integral of time-weighted-absolute-error criteria, a novel stability-degree criteria is utilised in particle swarm optimization algorithm to optimise the regulator’s parameters of ASPSU. Finally, with the optimised parameters, the indicial response of a 400MW on-grid ASPSU built with PSCAD/EMTDC is significantly enhanced.