Research And Suppression Measures Of Fault Inrush Current Taking Into Account The Influence Of DC System Control And Protection

Our country has built the world’s largest and highest voltage AC/DC hybrid power grid.Its operation and control are complex,and its fault characteristics are different from traditional power grids.This poses a challenge to the relay protection system as the "first line of defense" for the safe operation of power grids..The converter transformer is expensive.As the key equipment for AC and DC system coupling,it works in a complex electromagnetic environment.The protection of the converter transformer has a high probability of rejection and mis-operation.This article combines the influence of the DC control and protection system to convert the transformer The faulty inrush phenomenon induced by the valve-side grounding fault is the core,and the research is carried out around the generation mechanism of the faulty inrush current,the characteristics of the faulty inrush current and its influence on the differential protection and the active suppression strategy of the faulty inrush current.The main contents are as follows:First,the fault characteristics and fault inrush current mechanism of the ground fault on the converter transformer valve side without considering the DC control and protection action are analyzed.Based on the analysis of the current flow path on the valve side under different converter valve conduction conditions during the fault period,the characteristics of the ground fault current on the converter valve side are studied,and the generation mechanism and characteristic changes of faulty inrush current caused by the unidirectional conductivity of the converter valve are analyzed.Secondly,the influence of DC control and protection action strategy on the characteristics of faulty inrush current is studied.Based on the analysis of the DC protection configuration and action area,the response to the valve side grounding fault is studied,and it is pointed out that the converter differential protection and the DC pole differential protection in the DC protection after the valve side grounding fault will be Identify the fault and respond.Furthermore,the influence of DC control and protection on the characteristics of faulty inrush current is clarified.After the converter differential protection detects a fault,the DC control and protection system will implement a fault clearing strategy to clear the fault.The forced phase shift is a key factor affecting the characteristics of the faulty inrush current on the rectifier side.Under the influence of the forced phase shift,the DC component entering the converter valve side is reduced.and the magnitude of the faulty inrush current is reduced.On this basis,the action characteristics of the converter transformer differential protection under the influence of DC control protection are analyzed,and it is pointed out that due to the large second harmonic content.the converter transformer differential protection has the risk of false blocking in the case of a ground fault in the valve side area.Finally,a risk assessment of the faulty inrush current is carried out,and an active suppression strategy for the faulty inrush current is proposed in combination with DC control and protection.Combined with the basic theory of power system risk assessment,a reliability risk assessment model for the differential protection of converter transformers under the influence of faulty inrush current is constructed,and the risk level of the differential protection after the failure is evaluated,and it is concluded that when the commutation transformer occurs When there is a ground fault in the valve side zone,the converter transformer differential protection has a greater risk of refusal to operate and false blocking.In order to solve the above problems,this paper proposes a faulty inrush current suppression strategy based on phase-selected switch-by-pass.Through the identification of the fault location and the fault phase,the fault phase bypass pair of the converter is used after the fault occurs,and the fault is suppressed from the source.The simulation results verify that the scheme can effectively suppress the faulty inrush current.