| Because of long transmission distance of UHVDC and complex condition, the probability of line fault increases, so a set of reliable protection scheme is needed to guarantee the safe operation of transmission lines; After transmission line fault, quickly and accurately locating fault point can not only make it easier to eliminate the fault and avoids fault occurring again, but also save line inspection workload, so fault location is very important. At present, the DC protection systems in our country are mostly supplied by ABB and SEIMENS, and the main protection of the DC line is traveling wave protection, which has problem of poor reliability and failure to operating at high resistivity grounding fault. New UHVDC transmission line protections with higher reliability are needed to be studied. On the basis of the analysis of UHV DC transmission system boundary and transmission circuit frequency characteristics following work has been done in this thesis:a) The frequency characteristics of UHVDC transmission line and the "boundary" were analyzed, the conclusion was that if the length between fault point and protectors is over certain value, the UHVDC transmission line will has more attenuation to high frequency signals than the "boundary", so existing protection method can’t protect the whole line. External high frequency transient fault signals from the opposite side are attenuated twice by UHVDC transmission line and the "boundary", while internal fault signals from the UHVDC transmission line are attenuated only by transmission line, based on which the single-ended transient based voltage protection method of UHVDC transmission line distinguishing the opposite side’s internal and external fault is put forward. Internal and external faults will be distinguished by comparing high frequency transient wavelet energies; The fault line will be selected by calculating the ratio of low frequency transient voltages on positive and negative lines. Simulation results show that this approach works.b) The frequency characteristics of UHVDC transmission line and UHVDC transmission boundary were analyzed, and analytic results showed that when the length of transmission line gets over a certain length, the attenuation of transmission line will be stronger than transmission boundary’s, and the double-ended transient based voltage protection method of UHVDC transmission line which distinguishes the opposite side’s internal and external fault with Protectors was put forward. Simulation results showed this method could protect the whole line. Multi-scale wavelet transform was applied to transient voltages detected at both ends of transmission line, and by comparing the transient fault signals of two ends, the fault position was nearer to which end could be known; The internal and external faults were distinguished based on the differences between high-frequency wavelet energy and low-frequency wavelet energy; By distinguishing the differences of transient voltages on two polarities, the fault polarity was decided; PSCAD simulation results show that this approach worksc) UHVDV boundary and transmission line have attenuation to fault current, based on which a transient current protection of UHVDC transmission line is proposed. With HHT the instantaneous frequency of IMF1is calculated, and from the maximum value of instantaneous frequency the opposite internal or external fault is judged; fault current from outside the region has obvious difference in mutation direction with the one form opposite side, and the direction criterion is structured by integral method. A kind of UHVDC transmission line transient current protection scheme is put forward, the simulation results of PSCAD showed that the protection scheme works.d) When HVDC transmission lines fault, the high frequency transient signal travels along the lines, and the high frequency component attenuates during the course. In this paper, the phase-model transform is analyzed, the frequency characteristic of double-poles UHVDC transmission lines is studied, and the conclusion that the UHVDC transmission line attenuate the high frequency component, the UHVDC transmission line is longer, the attenuation is acuter is drawn. The single-ended fault location theory of UHVDC transmission line based on high frequency attenuation characteristic is studied, the distance calculation formula of fault position is deduced, and the conclusion that the single-ended fault location theory of UHVDC transmission line based on high frequency attenuation characteristic can not realize the fault location exactly is drawn. The double-ended fault location theory of UHVDC transmission line based on high frequency attenuation characteristic is studied, the distance calculation formula of fault position is deduced, the conception of frequency band attenuation is put forward, the distance calculation formula of fault position based on frequency band attenuation is deduced. The actual parameter simulation model of Yunnan-Guangdong UHVDC system is built up, and the double-ended fault location theory based on high frequency attenuation characteristic studied in this paper is simulated. |
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