The result of single-ended traveling wave fault location method based on Wavelet Transform (WT) will be affected by false Wavelet Transform Modulus Maxima (WTMM) points. In this paper, fault distance/impedance information contained in the wave front of the traveling wave is analyzed first. Then the fitting method based on Generalized Logistic function is proposed to extract the fault information. Based on the fitting results, a Predictor-Corrector fault location method is proposed, which includes the following content: (i) The fitting coefficients of the zero-mode fault current wave front are used to estimate fault distance and fault impedance interval. (ii) The arrival time of the line-mode fault current reflected wave can be calibrated and the distance calculation formula can be selected according to the fault distance estimation interval. (iii) The fault distance will be corrected to be within the fault distance estimation interval if the first reflected wave is undetected. A model of ±400kV DC transmission project is built in PSCAD/EMTDC. The simulation results show that the fitting coefficients can directly correspond to fault distance/impedance. The proposed method can effectively eliminate the influence of the false WTMM points, and give the accurate fault location results as well as fault impedance interval.

As Type-3 wind turbine (WT) has already become a typical type of highly penetrated power sources in modern power systems, characterizing its properties during grid fault is a basic requirement for system analysis. However, many significant factors have not been considered in the existing analytical methods and results, such as various switching of internal control and protection circuits in low voltage ride through (LVRT) solutions. In this paper, performance of Type-3 WT during LVRT is first illustrated and summarized as the sequential switching characteristic in multi time scales, viz., instant control, AC current control, DC voltage control and rotor speed control time scales. Thus, for magnitude drops in stator voltage, a mathematical model is proposed to understand this characteristic and fault current analysis's features. Then, an analytical method, based on the operational inductance, is proposed so that fault current components, with the sequential switching considered, can be depicted with a unified and simple analytical expression. By identifying magnitude of symmetrical current, the transient and steady-state equivalent circuits of Type-3 WT are proposed to estimate momentary current and electrical power approximately. Finally, analytical results are verified by comparisons with real-time simulation results of a detailed 1.5MW WT model in RT-LAB.

Hybrid AC/DC transmission grids require high-speed operation and high reliability for relay protection. Traditional derivative-based traveling wave protection methods have exposed the problems of low sensitivity and low reliability. In this paper, the fault traveling wave is analyzed considering the attenuation, distortion, and boundary effect of the DC transmission line as well as three converter types. The expressions of the fault initial traveling wave are derived and the conclusions show that, 1) the expressions of the fault initial traveling wave at the end of the transmission line have the formation of a(1-exp(-bt)); 2) the index of the exponential function in the fault initial traveling wave expression is related to the fault location, and is not affected by the fault impedance and line attenuation. Then a Levenberg-Marquart fitting method is proposed to fit the measured zero-mode fault current initial traveling wave and extract the index coefficients. Finally, a novel traveling wave protection method for DC transmission lines is proposed using the obtained index coefficients. The simulation results show that, 1) the proposed protection method can complete fault identification within 1 ms, even with high fault impedance; 2) the proposed protection method has high robustness to the influence of the noise.

The performance of the traditional derivative-based non-unit traveling wave protection (TWP) in high voltage dc (HVDC) grids shows the problem of low-sensitivity when high-impedance faults occur. In this paper, the characteristics of the fault initial traveling wave are analyzed first, two conclusions can be drawn: i) the distortion degree of the fault initial traveling wave is related to the fault location, ii) the amplitude of the fault initial traveling wave is related to the fault impedance. Then a zero-sequence current fitting method is proposed to obtain parameters respectively reflecting fault impedance and fault distance. When a fault occurs, a fault information extraction method based on the fitting parameters will give the estimation interval of the fault impedance, which provide reference for the adaptive threshold value selection of TWP. In this way, the influence of the fault impedance on TWP can be eliminated. A ±400 kV modular multilevel converter (MMC) HVDC grid is built in PSCAD/EMTDC. The verification of the proposed method considers different installation positions of the current limit inductor. The simulation results show that the proposed method can fast and correctly identify the dc line faults and effectively improves the protection sensitivity in detecting high-impedance faults.

After the isolation of a single pole-to-ground fault, the adaptive auto-reclosing concept in high voltage dc (HVDC) systems is difficult to be applied due to little coupling between the poles. With fault ride through capability, the hybrid modular multilevel converter (MMC) can be advantageous for future HVDC projects. This research proposes an adaptive auto-reclosing operation for hybrid MMC based bipolar HVDC systems, based on fault identification by characteristic signal injection. The proposed method is based on voltage controllability of healthy pole MMC to inject dc voltage perturbations. Healthy line perturbations result in the induced characteristic signals to the faulty line, which contain information about the fault property. An additional control strategy for hybrid MMC, to inject signals with suitable amplitude and frequency, is also discussed in detail. Then, fault identification is realized by wavelet transform of travelling waves caused by the signal injection. Finally, a dc adaptive single-pole auto-reclosing scheme is proposed to shorten the reclose time and to distinguish between temporary faults and permanent faults. Simulation results verify the effectiveness of the proposed method in PSCAD/EMTDC software.