Research On Novel Travelling Wave Protection Principles For HVDC Transmission Lines

In recent years, HVDC transmission technology is developing rapidly because of its advantages in long distance and large capacity transmission. But long transmission distance and complex working environment of DC transmission lines lead to high rates of DC transmission line fault, which seriously affect the safe and stable operation of power grids, and put forward higher requirements for travelling wave transient analysis, fast, accurate fault identification and location. At present, the sensitivity and reliability HVDC transmission line protection is poor and the protection cannot account for the impact of line attenuation. And with the emergence of Ultra high voltage DC transmission(UHVDC) technology and Double-circuit on the same tower HVDC transmission technology, Operation modes of HVDC will be more diversified, the change of line arrangement will lead to transient electromagnetic coupling characteristics change, and all of these have given a great challenge to the transient travelling wave calculation, relay protection and fault location. Therefore, the calculation method of DC transmission line transient traveling wave propagation is studied, single / double circuit HVDC transmission line protection principle, the novel location algorithm for HVDC line is proposed. The main work is summarized as following:(1) A time-domain calculation method of traveling wave transient propagation for HVDC transmission lines is presented, aiming at the problem that the traditional traveling wave calculation method cannot considere the line frequency-dependent characteristic. In the method, the line frequency-dependent wave impedance, attenuation coefficient and decoupling matrix are equal to the form of the time sequence matrices with fixed elements, using vector fitting method and superposition theorem, then the proposed matrices are applied to the traveling wave calculation. The method has thses advantages: 1) consider the influence of frequency-dependent parameters; 2) solve the unbalance line decoupling problem; 3) the proposed method is only calculated in the time-domain, which avoids the complex time and frequency domain switching process. It can be real-time applied to protection and control system of HVDC. The simulation results verify the accuracy and applicability of the proposed method.(2) In view of the problem of the low sensitivity and poor reliability of the traveling wave protection on(U)HVDC transmission line, a novel travelling wave protection scheme for(U)HVDC transmission line is proposed, which can eliminate long-line attenuation effect. The new protection principle can better distinguish between internal and external faults, using the traveling wave calculation method to eliminate the influence of line attenuation and the normalized voltage change rate to reduce the influence of the transition resistance. Based on PSCAD/EMTDC UHVDC transmission system simulation model, which is used for analyzing Yun-Guang UHVDC project in the process of operation and maintenance, and the fault recorded data from an actual project, the test results prove that the protection scheme, is applicable to different operation modes, with very high sensitivity, high reliability and the possibility of engineering application.(3) A traveling wave protection scheme for double circuit on the same tower DC transmission lines is proposed. The double circuit DC lines on the same tower is still using the traditional travelling wave protection for single circuit DC transmission lines. The change of fault feature, caused by the line arrangement change, is bound to affect the traveling wave protection performance. In view of the above problems, this paper based on the decoupling of the line electrical quantities, uses the differential of the calculation and measurement results of current traveling waves in different fault conditions to detect faults. Simulation results show that the proposed protection action is clear which high sensitivity and low sampling frequency. Even when the multipole fault with different transition resistances, the protection still can accurately select line, greatly improve the reliability of the protection.(4) A novel fault location method for single circuit HVDC transmission line is proposed considering the line frequency-dependent characteristic. The traditional fault analysis location method did not take into account the effect of line frequency-dependent characteristic, which will lead to errors in the calculation, and may become the key factor of hindering the location accuracy is further improved. In view of the above problems, this paper uses the time sequence wave impedance matrix and the traveling wave waveform fitting, which can accurately calculate the influence of the frequency-dependent characteristics on fault traveling wave. A large number of electromagnetic transient simulation results based on PSCAD/EMTDC show that the proposed fault location method is not affected by the transition resistance, and the accuracy is better than the other location method based on the Bergeron line model.