Research On AC/DC Hybrid Power Grid Fault Simulation And Convertor Station Protection

In recent years, China has been energetically developing DC high-voltage power transmission to meet the requirements of sustainable development strategy and energy structure adjustment. The newly launched DC high-voltage power transmission projects are taking domestic power grid into an era of high-speed development of AC/DC hybrid electric grid. The converter equipments, along with the AC grid and the DC transmission lines, are the major components of the AC/DC hybrid electric grid, and play a fundamental role for maintaining safe and stable operation of the AC/DC hybrid electric grid. However, some problems have been exposed during the operation of the relay protection of the converter station. Therefore, it is of great significance to do researches on the improvement of the relay protection performance of the converter station. Based on the realities of Hubei grid, the paper mainly involves researches concerning simulation model of AC/DC hybrid grid and relay protection of converter station.The analyzing requirement of the fault characteristics of AC/DC hybrid electric grid and relay protection performance of converter station could be achieved by construction of a detailed fault transient simulating model, reasonable simplification of complex AC grid structures, and establishment of a local electric power simulating system with DC transmission system as its core. Based on Hubei grid, the paper has constructed the Longquan-Zhengping DC transmission system for the purpose of investigating the converter station and related system failures. And then through equivalent simplification to the AC grid, an equivalent electro-magnetic transient simulation network model for the Hubei AC/DC grid was constructed. Meanwhile, whether the constructed equivalent electro-magnetic transient simulation model meets the requirements of engineering analysis or not is verified through comparative analysis with the results obtained on the basis of the whole grid fault network.Based on the electro-magnetic transient simulation model of AC/DC hybrid electric grid mentioned above, the relay protection performance of the converter station was analyzed. It's found through simulated analysis that, during specific operation and disturbance of the AC/DC system, misoperations might happen to the converter transformer and the converter valve due to some problems—for example, complex magnetizing rush current such as the sympathetic inrush current might occur to the converter transformer and zero-sequence circulating current might cause transformer saturation.Concerning problems mentioned above, the research shows that the identification method of sympathetic inrush current based on information sharing between stations could prevent the sympathetic inrush current from causing misoperation of differential protection of the converter transformer. Meanwhile, the paper has proposed a method to lock the differential protection of the converter transformer set with the second harmonics of the differential current based on a single converter transformer, which is a solution to the misoperation of magnetizing rush current in differential protection. Moreover, the paper has also put forward a method to prevent misoperation of bridge differential protection caused by transformer saturation of the triangle winding transformer located at converter transformer. The method introduces zero sequence voltage to broaden the time delay of bridge differential protection, and applies zero sequence circuiting current as the restraint portion of bridge differential protection, which improves the reliability of bridge differential current protection. Besides, the validity of the protection improvement solutions mentioned above has been verified through simulated analysis.