The study on CT saturation caused by the circulating current in the delta-winding of converter transformer and its impact on the protection in HVDC system

摘要

In the typical HVDC power system, the current transformers (CTs) between the Y-delta converter transformer and the converter bridge is located in the delta-winding of the transformer. Under such condition, the measuring current of these CTs includes the zero-sequence circulating current of the delta-winding. In the case of the adjacent transformer energization or the fault of AC system, the zero-sequence circulating current in the delta-winding maybe very large containing a great amount of dc component. Thus the CTs will be saturated, resulting in the misoperation of the relevant protection in HVDC system, such as the bridge current differential protection. This sort of misoperation seriously threatens the safe and sound operation of the HVDC power system. In this paper, the misoperation cause is analysed in detail via the PSCAD/EMTDC-based simulation. Based on this, a method is proposed to prevent this sort of misoperation by utilizing the zero-sequence circulating current as the restraint quantity.