Protection For HVDC Transmission Line And Its Filter In AC/DC Hybrid Grid

The distribution of energy resources and load centers is reverse and unbalanced in China,the contradiction between energy demand and supply needs to be solved.HVDC transmission technology has the advantages of large transmission capacity,long transmission distance,low loss and flexible control,which provides an effective way to alleviate the above contradiction.At present,the largest AC/DC hybrid system with the highest voltage level in the world has been built in China.The length of HVDC transmission line is generally long,which leads to a high probability of failure.At the same time,the related accidents of DC filter directly lead to over 15% of the pole outage in HVDC system.Therefore,protection for HVDC transmission line and its filter which can clear the fault quickly and reliably is very important for the safety and stable operation of the whole AC/DC hybrid system.However,in the actual operation of HVDC project,for HVDC transmission lines,traditional traveling-wave protection has poor ability to respond to fault resistance,the operation delay of pilot differential protection is too long and there is no protection to distinguish line fault for a long time after fault;for DC filter,differential protection has poor ability to respond to fault resistance,and it is easily affected by the inconsistent transient characteristics of current transformer.In order to solve the above problems,this thesis focuses on the protection for HVDC transmission line and its filter to establish a full-time protection scheme after fault.The main contributions of this thesis are follows:(1)The thesis proposes a pilot protection scheme for HVDC transmission lines based on Pearson correlation coefficient of current derivatives,to solve the problem that the delay of traditional backup protection is too long.Theoretical analysis shows that the correlation between DC line current derivative and DC filter current derivative is not affected by transition resistance.Pearson correlation coefficient is utilized to detect the correlation characteristics of current derivatives.The simulation results show that the protection scheme can quickly identify internal and external faults with high reliability,and it is insensitive to high fault resistance and does not require data synchronization.(2)Aiming at the problem that the sensitivity of Pearson correlation coefficient of current derivatives based protection is affected by the saturation of current measuring element,the thesis proposes a pilot protection scheme for HVDC transmission lines based on smoothing-reactor voltage.The superimposed network of HVDC system is used to analyze that the transient characteristics of smoothing-reactor voltage under internal and external faults.Theoretical analysis shows that the polarity of smoothing-reactor voltage is not affected by DC line capacitance and transient resistance.A fault pole selection criterion based on smoothing-reactor voltage is proposed.The simulation results prove that the protection scheme has high reliability,fast operation speed,a strong ability to select fault pole and respond to transient resistance.(3)According to the harmonic equivalent circuit and control strategy of HVDC system,specific frequency current characteristics of DC filter are theoretically analyzed for internal and external faults in the fault steady period.Based on specific frequency current of DC filter,a pilot protection scheme for HVDC transmission lines is proposed.The simulation results show that compared with traditional backup protection,the proposed protection needs shorter time delay,has higher reliability and sensitivity and does not require data synchronization,which can effectively use the steady-state electric quantity after fault to realize the internal and external fault identification of HVDC line.(4)DC filter is an important electrical apparatus in HVDC system,of which ground fault can not only damage itself directly,but also even cause emergency shutdown of HVDC system.According to the equivalent circuit in complex frequency domain and time domain,virtual capacitance characteristics are analyzed under different ground faults of DC-filter high voltage capacitors.Based on virtual capacitance,a ground fault protection for DC-filter high voltage capacitors is proposed.The simulations show that the proposed protection can reliably identify a ground fault of DC-filter high voltage capacitor during the whole fault period and is immune to fault resistance,which effectively solves the problem that the DC filter differential protection can not locate the fault of high voltage capacitor.The research work in this thesis is helpful to improve the protection performance of HVDC transmission line and its filter,meet the security needs of HVDC system,and it is of great significance to ensure the reliability and safety of AC/DC hybrid system.