Research On Differential Protection Of Converter Transformer Under AC/DC Hybrid System

The AC-DC hybrid system has been gradually formed in China with the completion and development of high-voltage direct current transmission engineering,but the influence of DC system on the relay protection of the AC system is becoming more and more prominent.This paper focus on the converter transformer differential protection under the single-phase grounding short-circuit fault with the highest probability of occurrence on the valve side of the converter transformer.The fault characteristics and the action characteristics of the differential protection between inverter side and rectifier side converter transformers are studied.Firstly,this paper analyzes the mechanism and variation of the new inrush current(which defined as fault inrush current)caused by one-way electric conduction of the converter valve when the single phase ground fault occurs on the valve side of the converter transformer in rectifier.On this basis,the influence of fault inrush current on the operation characteristics of the differential protection is analyzed.The results show that the fault inrush current is easy to cause miss-operation of converter transformer differential protection in the event of internal single-phase ground fault on valve side.Then,based on the change of operating conditions of converter valves after the grounding fault occurred on the valve side of converter transformer in inverter,this paper discusses the differences on operating conditions of the converter valve at different fault locations and different fault times.And the differences on fault current and fault inrush current characteristics are studied.On this basis,the influence of fault inrush current on the operation characteristics of the differential protection is analyzed.The results show that the fault inrush current is easy to cause miss-operation of converter transformer differential protection in the event of transferring fault on valve side.In view of this problem,further analysis shows that the three-phase current of the valve side contains a certain amplitude of the DC component and the DC component is positive after the single-phase ground fault of the transformer valve side.Based on the second harmonic braking principle,the improved logic criterion of converter transformer differential protection is put forward by using this feature.The simulation results verify that the improved blocking scheme can effectively identify the protection miss-operation caused by fault inrush current.