| As a major technology area of grid technology innovation pushed by global energy internet, UHVDC has the advantages of large capacity transmission, high transmission efficiency, strong self protection ability, flexible control mode, long length of transmission line and so on, which plays an important part in developing clean energies, governance haze, and constructing a strong smart grid. Recently, UHVDC technology in China is developing rapidly, and until now,6 ±800 kV UHVDC transmission projects have been put into operation. As for UHVDC transmission lines have longer length, which can be influenced by lightning stroke, short circuit, broken line and so on, so it is much more important to improve the performance of DC line protection to ensure safety and stability of the DC line. For HVDC projects put into operation presently, traveling wave protection as the main protection can move fast but reliability and sensitivity can be easily influenced by transition resistance and disturbances and current differential protection as the backup line protection, which needs data synchronization of both ends, has slow operation speed and can be influenced by signal path. Therefore, it is of great theoretical significance to research protection methods with fast moving, high reliability and sensitivity. The work and achievements of this paper are as follows:This paper proposes a complete main and backup line protection scheme using S transform to analyze transient signals for HVDC transmission line. The main protection is a S-transform based non-unit protection method. Known from the analysis on magnitude characteristics of the DC line boundary, which is made of the smoothing reactor and the DC filter, high and low frequency components of the transient voltage traveling waves can suffer from different degrees of attenuation when they go through the DC line boundary, so this paper calculates the specific frequency component accurately using S transform and constructs the protection criterion, and proposes a new method based on the ratio differences of S-transformed transient energies for high and low frequency components to identify internal and external faults. By comparing the ratio differences of S-transformed transient energies for high and low frequency components from lightning disturbance, lightning faults and short-circuit faults, this paper constructs a S transform transient energy ratio criterion to identify lightning disturbance. Besides, for a pole-earth fault in a bipolar system, the faulted line can be identified by the ratio differences of the S transform 0 Hz transient energies of the fault component on two lines.In order to reduce the effects of fault resistance for protection sensitivity and ensure the high selectivity for protection, this paper puts forward a S-transform based pilot protection. Known from the analysis, when a fault occurs on the forward direction of the protection, the polarities of the fault components of voltage and current detected are opposite while for a backward direction fault, they are the same. According to such characteristics, a new method based on S transform angle differences to identify the polarities of the fault components of voltage and current is proposed to distinguish fault direction and lightning disturbance.According to practical parameters of Xiluodu-Zhexi ±800kV UHVDC transmission projects, which is put into operation in 2014, this paper builds a simulation model using PSCAD, which has the structure of two lines and double pulse 12 converters, and rated power is 7500MW, rated current is 4.7kA, and the length of the DC line is 1728km, to do integrated and systematic simulation tests for the main and backup line protection scheme and the simulation results prove the effectiveness and feasibility of the proposed methods. |
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