Study Effect Of Commutation Failure On AC Protection And The Countermeasure

The AC/DC (Alernative current/Direct current) hybrid system has been formed gradually with the completion of many HVDC (High Voltage Direct Current) projects. In addtion, the HVDC tecnology is a necessarty conditon for Global Energy Internet. Thus, more and more attention is paid to the effect caused by HVDC. In general, the AC/DC hybrid brings greater challenge for the reliability, sensitivity, selectivity and speedability of relay protection. This paper mainly focuses on the study effect of commutation failure on AC relay protection and countermeasures. The main research and achievements of this paper are as follows.(1)A thought that the amplitude differential protection can be applied for the AC/DC interconnected network is presented, and a novel restraint scheme for differential protection based on correlation coefficient is also proposed in this paper. Protection failure may occur during internal fault by using the conventional current differential protection. However, the amplitudes of primary currents that located in two sides of transmission line vary greatly during internal fault. Thus, the amplitude differential protection has a better adaptation of AC/DC hybrid system than that of conventional one. The mechnism of transformer differential protection with the second harmonic restraint may block the protection is analyzed. Then, a novel restraint scheme by using the change trend of two sides currents of the converter transformer is proposed. The novel one can discriminate the internal fault from the external fault correctly. The simulation results prove the scheme reasonable.(2) A novel analysis model for AC/DC hybrid system by considering the zero-sequence current characteristic is proposed in this paper. The effect of commutation failure on AC system zero-sequence component is analyzed in detail based on the introduction of the conventional model, and it is found that the DC system has little effect on the zero-sequence component. Then, a novel equivalent model is presented. The novel model lays a foundation for studying the effect of commutation failure on distance protection that considers the connection mode.(3) A novel commutation failure detection scheme based on mathematical morphology is proposed in this paper. The transient characteristic of DC (direct current) in inverter side is first analyzed, and its morphological gradient value is calculated by using self-defined morphological gradient. If the morphological gradient value is larger than a threshold, it means commutation failure is caused by AC fault; otherwise, it means commutation failure dose not occur. The novel detection scheme is better than these existing methods, as it combines the measurement and prediction.(4) A nvoel blocking method, based on’three-time’commutation failure, is proposed in this paper. The effect of reclosing on the communication failure is analyzed in detail. Then, A new blocking method based’three-time commutation failure’ is proposed by coordation with the reclosing time. The novel scheme can avoid some unneceesary block actions in the greatest extent, and it can block the DC system reliably during continuous commtation failure.(5) The zero sequence directional component and R-L algorithm can be applied into the distance protection. The mechanism that the distance protection may fail to trip triotrip over commutation failure is presented, especially when the fault location trip is presented, especially when the fault is near the relay. Then, some counter-measures are proposed, such as the second harmonic amplitude, zero-sequence directional component, et al. Inaddition, it indicates that the R-L algorithm is more suitable for AC/DC interconnected network than the Fourier algorithm, as it has a better immunity against the aperiodic component.