Study Of Line Protection And Fault Location For UHVDC Transmission Line Based On Distributed Parameter Line Model

As the juncture for large area network and transmission line for high power, highvoltage direct current （HVDC） transmission lines commonly with long transportationdistance, its safety and reliability will greatly affect the system itself and its connectionregional power grid, even cause instability for the whole power grid. HVDCtransmission line as the weakest part in DC power system, mostly ground fault, however,just with almost50%of correct operation rate for line protection. Therefore, how toensure the HVDC lines act accurately, rapidly and pinpoint the fault position is aneffective way to reduce instability of power grid after the monopole ground fault. Basedon its character of long transportation distance and with none stability power frequencyparameters after fault, a method which can make use of its transient fault signals andsuit for field of the transmission line protection, fault location is greatly necessary.Firstly, the ultra-high voltage direct current (UHVDC) system model is proposed,the steady and mainly faults for UHVDC are discussed, results indicate that theproposed model can present the steady and transient operation characteristics of HVDCtransmission lines. Aiming at the weakness of the traditional line model, in this paper,distributed parameter mathematical model considering multi-stage distance infinitesimalis taken account, with adequate considering the distributed characteristics of DCtransmission lines. By making most use of the instantaneous value produced in faulttransient process, the protection and fault location algorithm can be analyzed.Based on the above distributed parameter model, a new protection algorithm forHVDC transmission lines is proposed. For normal condition which inlcude steady stateand external fault, by applying the electric parameter which gets from one side of linesfor the high order differential equations, corresponding derivation current value for theother side will accord with itself measured value. If internal faults occurs, the derivationvalue will differs that of measured value, hence, the external and internal faults can bedistingushed easily. After comparing with the traditional DC differential protection andBergron model, results indicate that the proposed algorithm not only considers effects ofthe distributed capacity,but also with advantages of high withstand resistence and goodability to choose a failure pole, and improves the protection sensitivity greatly.Finally, based on the above distributed parameters model, a new method for faultlocation under monopole ground fault condition is proposed. After phase-mode transformation for synchronous voltage and current of both sides, the mode voltagedistributions are calculated by the voltage and current measured in each terminalrespectively, and find out the corresponding distance which is the actual fault positionfor the least difference of mode voltage. By setting different sampling frequency,transition resistance and fault location. The comparisons between Bergeron model andproposed model are conducted. Results indicate that the algorithm needs a shortsampling rate and could lower the effect of noise on fault location. It also shows that theresults are not affected by transition resistance and fault location, thus results with highaccuracy are achieved.