Commutation Failure Research In Multi-infeed HVDC Systems

The Multi-infeed direct current transmission system has been already formed in the East China Power System and China Southern Grid. The interaction property of Multi-infeed system has great influence on the AC/DC hybrid transmission system for its safety and steady operation. Commutation failure (CF) is the inevitable problem in Multi-infeed system, and the fault of AC system may cause commutation failure in one DC system or multiple DC systems. If the fault is serious, the DC power transmission may be interrupt, and the safety as well as steady operation of power grid suffers the impact. Thus, it is necessary to take deep analysis for the CF mechanism and property of Multi-infeed system. Moreover, the feasible solution should be put forward to avoid the concurrent CF problem.Firstly, the theory of CF in direct current transmission system was analyzed, and the abnormal CF phenomenon, which could potentially exist during the concurrent or successive CF in Multi-infeed HVDC transmission system, is pointed out. By use of continuous simulation method, the possibility of CF occurred in remote converter station due to different fault levels of local commutation bus as well as the abnormal CF situation occurred under various AC system strength and different coupling impedance are analyzed. Through contrastive analysis on voltage waveforms at commutation bus under the three typical fault levels and the Fourier analysis on voltage of phase B within one cycle after the occurrence of the fault, the DC components and harmonics under the three fault levels are achieved, and then it is proposed that the root cause leading to the abnormal CF lies in the bigger DC component and low-order harmonics in the post-fault voltage waveform. By means of analyzing such features as the DC components, the harmonic contents and the drop of voltage amplitude, the reference quantities of voltage and harmonics are put forward to judge the severity of abnormal CF.Secondly, to solve the problem of concurrent, successive and the abnormal CF, an installation strategy of fault current limiters (FCL) based on the graph theory is presented. For this strategy, the ac power grid with double dc landing is abstracted into a graph G,in which the ac buses and tie lines correspond to its vertices and edges. Two weights ω1 and ω2 are defined, which represent the relative value of modulus of load flow and the number of edge. Install FCL on the edges whose ω1 is more than 1 after the sort of ω1, and then group the FCL. By using continuous simulation method and the criteria of inverter extinction angle, the least FCL allocation plan can be obtained by Stepwise Optimization. This installation strategy is applied to IEEE14-bus system, which achieves fault isolation and grid segmentation after fault. And the Multi-infeed system does not encounter concurrent CF, and then the effectiveness of this strategy is verified.