Fault Analysis Of Double-circuit HVDC Transmission Lines And The Novel Control And Protection Principles

In recent years,HVDC transmission develops rapidly owing to its advantages of long distance and great capacity transmission.With continuously increasing power transmission dimands and the scarcity of transmission corridor,the technology of double-circuit HVDC transmission lines with shared towers has been an effective method and has been applied in the HVDC transmission project.However,double-circuit HVDC transmission lines,with four poles placed in a trapezoidal shape,are unbalanced lines due to the pole arrangement scheme employed in actual projects,which leads to the complicated coupling between each two poles.The electromagnetic coupling between four poles would bring great challenges to the DC line protection,or even worse,lead to the DC line faults caused commutation failures.Therefore,the fault analysis of travelling waves in double-circuit HVDC transmission lines are given,the performance of traditional travelling-wave-based protection scheme are analyzed,a new protection scheme for double-circuit HVDC only using the information of each single-circuit transmission lines is proposed,and finally,the commutation failures mechanism due to DC line faults are researched.The main contributions of this work are summarized in the following:(1)Aiming at the complicated coupling relationship caused by the asymmetrical pole arragement and its impact on traveling wave protection,the decoupling method first decouples the two circuits using symmetric decoupling matrix,and then decouple the two matrix blocks separately.In view of the fact that the traditional DC line protection allocated in single circuit is still adopted for each circuit of double-circuit DC lines,a new phase-modal transformation method based on the voltage quantity of each single circuit is proposed,in order to prepare for new protection scheme research.Further,the propagation characteristics of each independent modal component of traveling waves in double-circuit DC lines,the law of the catadioptric reflection at the boundary points and the influence on the calculation of the transient traveling waves are analyzed.(2)The solution of each modal component for double-circuit HVDC transmission lines under different fault types is given,including single-pole-to-ground faults,line-to-line faults of the same circuit and cross-circuit faults.The fault characteristic difference of ideally transposed and untransposed DC lines is analyzed and verified.Owing to the fact that the dc line protection scheme allocated on each single circuit of double-circuit transmission lines are still those from traditional single circuit on the same tower,the performance of the protection scheme is further investigated.It shows that there are some disadvantages such as incomplete decoupling,unclear physical meaning,more complicated threshold value setting,and poor performance for high-resistance faults existing in the original protection schemes.(3)Based on the DC voltages and DC currents of each single circuit,the linear combinations of modal components rather than each independent modal component can be achieved and investigated.Finally,an integrated traveling-wave-based protection scheme is proposed.The efficiency of the protection scheme is validated through extensive simulations.It shows the proposed protection scheme can detect faults under different fault types and different operation modes even with high resistance.It employs only the DC voltages and DC currents from the circuit itself and the sampling frequency required is only 10 kHz.The measured quantities are all available based on the existing hardware platform in practical engineering.Therefore,this approach can be implemented in existing hardware platforms without other special requirements.The efficiency of the protection is verified by the RTDS recording data from Xiluodu-Guangdong double-circuit HVDC transmission project.(4)The mechanism of commutation failure caused by DC line faults on the double-circuit DC lines with its countermeasures were studied.Theoretical analysis of the change of DC line currents at inverter side during DC line faults and DC line restarts is given,considering the impact of fault location,power change and control system response.It suggests that the electromagnetic coupling interaction between the four poles and the interaction of the transient traveling wave and the control system response,will finally lead to the commutation failures.Finally,a commutation failure control strategy based on DC current prediction at the end of the commutation time is proposed.The simulation results based on PSCAD/EMTDC verify the effectiveness of the control strategy.