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The doubly fed induction generator (DFIG) has insufficient capacity to respond to the frequency inertia of the power grid due to the isolation between the rotor kinetic energy and the power grid by the generator-side converter. At the same time, the 'source' of the DFIG cannot be as stable as the thermal generator, so sudden changes in the wind speed on the ultra-short time scale can affect the system inertia. In this study, the inertia of a doubly fed wind generator with a super capacitor energy storage system (SCESS-DFIG) is defined from the standpoint of a single machine. Based on fuzzy logic controller, an inertia response control strategy of multi energy coordinated control is proposed. Then the SCESS-DFIG inertia response control strategy under variable wind speed is proposed at the wind field level. Based on the establishment of DFIG, DC bidirectional buck–boost converter, and supercapacitor energy storage unit model, a wind farm with three SCESS-DFIGs is simulated. When the grid frequency is dropped, the effectiveness of the proposed control strategy is verified by simulating their operation under the same rated parameters, different wind speed curves, and different initial voltages of the supercapacitors. [ABSTRACT FROM AUTHOR]

The transient response of the single‐phase enhanced PLL (EPLL) under the step change of amplitude, phase angle, and frequency is derived. It is revealed that there are transient periodic ripples in the estimation results of the standard EPLL. Furthermore, a virtual quadrature‐coordinate EPLL combining a second‐order generalised integrator with a quadrature‐coordinate EPLL is proposed and the transient response is derived. The proposed EPLL possesses a better dynamic performance than the standard EPLL. The detailed experimental results are shown and compared with each other to validate the theoretical analysis and the advantages of the proposed EPLL. [ABSTRACT FROM AUTHOR]

In this study, the transient process when the single‐phase open‐line fault (SPOLF) occurs in the three‐phase AC–DC pulse width modulation rectifier with LCL filter (LCL‐PWMR) is theoretically analysed for the first time and the corresponding diagnosis scheme is proposed. First, the equivalent circuit of the LCL‐PWMR is built and the detected phase voltage (DPV) detected by the three‐phase grid voltage sensors under fault conditions are derived in s‐domain. The result shows that the DPV in the open‐line phase is not equal to zero, while consists of a fundamental component plus a resonant component with specific frequency. Furthermore, a diagnosis scheme for the SPOLF based on 2‐order band‐pass filter (BPF) connected in series with enhanced phase‐locked loop (EPLL) is proposed. BPF is used to eliminate the fundamental component of the sum of three‐phase DPVs and EPLL to estimate the amplitude of resonant component. Then the SPOLF can be detected when the amplitude of resonant component is beyond the allowed threshold. Furthermore, the adaptability of the proposed diagnosis scheme for the system with different rated power is verified theoretically. Detailed simulations tests based on Matlab/Simulink software and experimental tests are carried out to demonstrate the validity of the theoretical analysis and feasibility of the proposed diagnosis scheme. [ABSTRACT FROM AUTHOR]

The article presents a study on In:Fe:Cu:LiNbO3 crystals grown by the Czochralski method in air. It discusses details of the experiment to look into the effects of reduction/oxidation treatments on the blue photorefractive properties and the two-wave coupling gain coefficients of the crystals. It explores the significance of the research for its applications in blue photorefraction.