Xuanwei Qi

Study on the Unusual Misoperation of Differential Protection During Transformer Energization and its Countermeasure

The differential protection of the transformer, generator, or transmission line under normal operating conditions may misoperate when the adjacent transformer is switched-on. To research the cause of the misoperations, impacting factors, such as magnetizing inrush, sympathetic inrush, and current-transformer saturation are analyzed based on the field-recorded fault data and the simulation results in this paper. It is disclosed that the CT transient or CT local transient saturation, which are caused by the long-lasting dc component from the magnetizing inrush current, will lead to misoperation. To solve this problem, a new CT saturation detection algorithm based on the dc component in secondary current is proposed to prevent this sort of misoperation, and the effectiveness of the proposed algorithm is proved by the field-recorded fault data and simulation results obtained under different conditions.

A simulation platform for the analysis of the current transformer transient characteristic

The analysis of the current transformer transient saturation characteristic and its impact on differential current protection is an urgent problem to be solved. A simulation platform for analyzing the transient characteristic of the current transformer is introduced in this paper. The actual operating scene can be constructed in this platform in order to perform the complex transient process of power system. A variety of complex transient process in the power system can be simulated. The class P, PR and TPY current transformer model can be selected for analyzing the transient characteristic of the current transformer. This platform can achievement the analysis on the current transformer transient characteristic and its impact on the differential protection. The dynamic test and practical application prove the accuracy and the effectiveness of this platform.

Study on complex inrush caused by transformer energization and its influence on current differential protection

Several cases of unusual mal-operation of the current differential protection for transformer, generator and transmission line already in service, due to the adjacent transformer switching-on, have been reported. The complex inrush current, including the magnetizing inrush current and the sympathetic inrush current, is considered to be relevant with this sort of mal-operation. However, the detail reason still means further verification. In this paper, the main factors, including magnetizing in-rush, sympathetic inrush, and CT saturation are analyzed in detail. It is disclosed that the CT transient saturation or CT local transient saturation, caused by the long-lasting aperiodic component from the magnetizing inrush current, leads to the mal-operation. To solve this problem, the countermeasure is proposed, including applying the harmonic restraint criterion and the improved the ratio restraint characteristic. The field recorded fault data and the simulation results prove the correctness of analysis conclusion and the effectiveness of the proposed improving methods.

Identification and coping approaches of sympathetic inrush current based on the station-area information of smart substation

Due to the absence of the effective identification and coping approaches of the sympathetic inrush current, the mal-operation of differential protection may be occasionally caused by sympathetic inrush current. In order to address this issue, the features of the sympathetic inrush current and the influence of the sympathetic inrush current on the differential protection are analyzed in detail in this paper. It is discovered that the second harmonic phase angle difference between the inrush current and the sympathetic inrush current are nearly 180°. Based on this, an identification and coping approaches for the sympathetic inrush current utilizing the information sharing between the smart substations is proposed in this paper. The effectiveness of this method is validated by the digital simulation and dynamic model experiment.

A sympathetic inrush detection method based on on-line magnet flux calculation

The sympathetic inrush is a hot issue in the field of differential protection research, due to its characteristic of concealment, diversity, and complexity. The mal-operation of differential protection probably caused by sympathetic inrush is still a difficult problem to be solved. This paper proposed a new sympathetic inrush detection method based on the on-line magnet flux calculating by integrating the measuring voltages on the tips of transformer. This method can completely overcome the negative influence of CT saturation on the differential protection from principle. The EMTDC-based simulation results prove the effectiveness of the proposed method and also verify that this method would not decrease the sensitivity of differential protection under the condition of internal fault.