With the increasing scale of wind farms, the fault characteristics tend to be complex, which poses a technical challenge to establish the dynamic equivalent model of wind farms. In this paper, a dynamic equivalent method of DFIG-based wind farm based on the density peak clustering algorithm (DPCA) is presented. First, under an analysis of short-circuit current (SCC) in single doubly fed induction generator (DFIG), the clustering indexes are selected. Second, with the selected clustering indexes and DPCA, a more refined two-stage clustering of DFIGs in wind farm is carried out. Third, the units in the same cluster are equivalent to one unit, and then the dynamic equivalent model of the DFIG-based wind farm is established. Finally, the proposed method is validated through the MATLAB/SIMULINK-based simulation results, and the comparison results also show that the dynamic equivalent model proposed in this paper has a better performance than two other equivalent models. Moreover, another comparison between DPCA and K-means clustering algorithm is analyzed, and the result shows that DPCA has a better performance which provides a better choice for dynamic equivalence of wind farms. INDEX TERMS DFIG-based wind farm, short-circuit current (SCC), dynamic equivalent, density peak clustering algorithm (DPCA).

Partial discharge (PD) is an early manifestation of multiple cable faults. The traditional PD monitoring method has a weak monitoring effect and low positioning accuracy when monitoring the PD of cable. This paper proposes a distributed on-line monitoring method for cable PD based on phase-sensitive fiber-optic time domain reflection (ϕ-OTDR) principle. Firstly, the production and propagation model of the partial discharge ultrasonic wave (PDUW) of the cable was established and simulated, and the distribution of PDUW inside the cable with different conditions and different structures was obtained. Then we established a model of cable PDUW, and the backscattered Rayleigh light intensity in fiber and theoretically analyzed and experimentally verified the relationship between the cable PDUW and backscattered Rayleigh light intensity in fiber. The experimental results are consistent with the theoretical analysis. When the cable has PD, the backscattered Rayleigh light intensity change of the PD position is higher than the intact position. We analyzed the experimental data and obtained the judgment conditions of the cable PD. Finally, we calculate and obtain the sensitivity of the monitoring method to different cables and the warning value when cable PD is abnormal.INDEX TERMS Partial discharges, cable insulation, optical fiber testing, ultrasonic variables measurement, ϕ-OTDR.

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This paper presents a calculation method of short-circuit current (SCC) for doubly fed induction generator (DFIG)-based wind farm considering the voltage distribution characteristics. When a short-circuit fault occurs in a wind farm, a large SCC will be contributed by the dozens or even hundreds of DFIGs in the wind farm. And the security and stability of the wind farm will be affected by the SCC. The SCC characteristics of the DFIG are closely related to its post-fault terminal voltage (PFTV). Therefore, a practical calculation model of SCC in single DFIG is proposed in this paper, based on the electromagnetic transient analysis. And according to a correction method of the topological matrix for a post-fault wind farm proposed in this paper, the relationship model between voltages and currents under short-circuit faults is established. Then, the PFTVs and SCCs of the DFIGs in the wind farm can be obtained by solving the model equation. Finally, the proposed method is validated through MATLAB/Simulink-based simulation results, and a comparison with the national standard calculation method is analyzed. The results show that this method has better accuracy, which provides a more accurate basis for the electrical safety design of wind farms. INDEX TERMS Wind farm, doubly-fed induction generator (DFIG), short-circuit current, voltage distribution, MATLAB/Simulink.

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. INDEX TERMS Submarine cable, distributed parameter circuits, cable insulation, broadband impedance spectroscopy (BIS), sensitivity, the resonant frequency.