Research On Fault Analysis And Protection Of High-Voltage Direct Current Transmission Lines

Transmission line fault is one of the most common and rerious faults of HVDC transmission system, and must be considered when designing a DC power system. It has an important impact on the equipment parameters, control strategies and protection configuration, which is the common problem for the traditional LCC-HVDC and new MMC-HVDC transmission. On the one hand, for the traditional LCC-HVDC, due to the complex system network topology and related to high-frequency transients, there is a lack of effective fault analysis and protection evaluation method. It is contradictory between accuracy and efficiency to rely solely on the numerical simulations for actual HVDC projects. On the other hand, for the new flexible MMC-HVDC, it usually adopts AC circuit breakers to cut off fault currents, severely reducing the availability of system. There is lack of more perfect protection schemes for the existing projects to process DC-line faults. To meet the great demand of HVDC projects in China, in this paper, the fast calculation method of travelling waves, global sensitivity analysis of key factors, reliability assessment of protection action in the DC line fault are studied and an improved DC-protection scheme of MMC-HVDC is proposed, which is not only valuable in the academic area but also utility in the engineer field.The work is summarized as following:1. To alleviate the sharp contradiction between the accuracy and efficiency of traditional digital simulations, a travelling-wave fast calculation method is proposed in this paper. Based on the actual HVDC project topology and the frequency-dependent nature, the discrete frequency responses of DC-line faults are extracted firstly, then are computed section-by-section to otabin the approximants in s-domain by fitting method according to the principle of descending order, which would reduce greatly the accumulated errors in the fitting process. In order to handle with the random problem caused by different fault conditions, a related parameters adaptive selection scheme for the fitting technique was also presented. Numerical examples demonstrated the high efficiency and accuracy of the proposed method.2. With the great demand of practical engineering, the principle of actual HVDC line main protection scheme, the setting value and protection logic are investigated first. Then considering the influence of several uncertain factors including fault type, fault distance, fault resistance and fault time, a Latin-hypercube-sampling-technique-based action probability calculation and analysis method for Traveling-Wave Protection(TWP) against dc-line-short-circuit faults is presented.Examples with actual TWP scheme of Yun Guang UHVDC project as the research object, and the effect mechanism of control system to TWP is investigated, the influences of different fault condition factors and the adverse effects caused by the change of operating mode to TWP action probability are analyzed.3. The fault resistance is the key factor to affect the travelling waves and the performance of travelling-wave-based protections on HVDC transmission lines. In order to explain accurately the impacts of the fault resistance to travelling waves in theory and improve the computational efficiency, a global-sensitivity-based method is proposed to fast predict the new travelling waves with any other fault resistance. This method was built on the actual HVDC project topology and the frequency-dependent nature of DC-lines, and derived by infinite-Taylor-series and global-sensitivity in phase and frequency domain, and then realized by a rational fitting technique with a Thiele-type continued-fraction-based order estimation scheme for the conversion to the time domain. Numerical examples demonstrated the high efficiency and accuracy of the proposed method.4. Aimed at the problem that the existing half-bridge MMC-HVDC cannot rapidly and economicly handle the DC-link fault, a new scheme of protection and fast recovery is proposed.In the proposed scheme of protection, the bidirectional thyristors are configured on the AC-side, and controlled to cooperate with the appropriate designed breaker-type zero-loss fault current limiter. In addition, the DC-fault detection and recovery strategy are presented, which can achieve the fast current-limiting ability, to reduce the long short-circuit-current rush to ac system during the clearance of the DC-link fault. Simulation results have verified the validity of the proposed protection scheme. The proposed scheme does not increase the system steady-state operation loss, which can be used to reconstruct the existing project directly.