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The increase of wind power permeability in modern power grid has turned rapid and accurate transient stability (TS) prediction into a more challenging issue. To accurately and promptly perform online TS prediction for power system with doubly fed induction generator (DFIG)-based wind farms, an integrated model-driven and data-driven method is proposed in this paper. The influence of DFIGs is considered in the transformation to guarantee the accuracy of the equivalent one machine infinite bus (OMIB) model transformed from the target system. The P-δ trajectory of the OMIB is fitted with the generator-terminal information to predict TS. To improve the prediction speed, an extreme learning machine (ELM)-based method is utilized to process the other DFIG and system information and evaluate the system status immediately after failure. The simulation results verify that the proposed method can reduce the dependence of the data-driven method on the data sample size and improve the speed and accuracy of online prediction.INDEX TERMS Extreme learning machine (ELM), doubly fed induction generators (DFIG), transient stability, trajectory fitting.

Permanent magnet is the key component of nuclear magnetic resonance (NMR) instrument. Based on the principle of Halbach array, this study designs and optimizes a multilayer Halbach permanent magnet for human finger NMR detection. The quality factor (QF) is introduced to represent the performance of magnetic field, firstly. The impact on magnetic performance, caused by various magnet heights, is analyzed using Finite Element Method(FEM). In order to compensate the end effect, the radial adjustment method is proposed, combining the adjustments of the radial size and the radial position of the magnet blocks. Comparing the effect on magnetic field of three different plans, the results show that combining the adjustments of inner radius and outer radius shims the field dramatically. Finally the designed construction is stacked by 11 layers of magic rings and each layer is composed of 8 trapezoid-shaped magnets. The magnetic field of the center cylindrical region (ϕ20 × 20 mm) of the structure is calculated, whose final strength achieves up to 447 mT and homogeneity is within 500 ppm. The designed magnet structure is manufactured and the measurement is finished in the end. The proposed construction has the advantages of small size and high uniformity, which has an extensive application prospect in portable NMR system.

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