Study On The Impact Of Grid Fault On D-STATCOM Based On Direct-current Control And Its Countermeasures

Distribution static synchronous compensator(D-STATCOM) is widely used in power distribution network as its superior performance. But in power system, grid fault occurs frequently. The operation aspect under grid fault of D-STATCOM determines the corresponding measures should be taken. Therefore, this thesis mainly aims at the impact of grid fault on D-STATCOM based on direct-current control and its corresponding countermeasures.This paper first introduces the working principle of D-STATCOM based on direct-current control, and analyses the advantages of direct-current control than indirect-current control, additionally points out that this paper uses the ip-iq detection method to measure instruction current. Then, it mainly analyzes the impact of grid fault on D-STATCOM based on direct-current control. The analysis result indicates that when fault occurs at the source side of mutual inductor, the compensator can be able to continue running in the grid, while it must be separated from the grid when fault occurs at the load side of mutual inductor, which is determined by the nature of direct-current control. Besides, if the fault before mutual inductor is asymmetrical fault, the compensation effect will decline, which is caused by the detection error of ip-iq detection method under asymmetrical voltage. Therefore, it is necessary to propose the improved method to improve the performance of the device. In order to get accurate instruction current when uses ip-iq detection method, the a-phase positive sequence voltage must be obtained accurately. For this purpose, three extraction methods of the positive sequence voltage are proposed. They are extraction method based on positive sequence voltage filter, extraction method based on the definition of positive sequence voltage and the extraction method based on low pass filter. These three methods are analyzed and simulated in this thesis. The simulation results indicate that the extraction method based on low pass filter can obtain the accurate a-phase fundamental positive sequence voltage under unbalanced voltage. Because of this, this thesis apply the extraction method based on low pass filter to ip-iq detection method, and use it as the countermeasure against the impact of asymmetrical fault before mutual inductor. Finally, the improved method is simulated in this paper. The simulation results show that when the asymmetric fault occurs in front of the load current mutual inductor, the improved method can effectively suppress the impact of fault on D-STATCOM based on direct-current control. In other words, the simulation results verify the effectiveness of the improved method.