Study On Reducing The Unbalance Current Of Transformer Longitudinal Differential Protective Relay

Transformer is the most important primary equipment in power system,it is necessary to protect it reliably and quickly,thus transformer longitudinal differential protective relay is widely applied.At the same time,the veracity and transformer rapidity of longitudinal differential protective relay are affected by the unbalance current in engineering application,so methods to reduce the unbalance current are widely researched.Firstly,this paper reviews the concepts and the components of the transformer longitudinal differential protective relay,and then the reasons for causing unbalance current are revealed.The selection of current transformer's rated ratio,the difference of phases between the primary and the secondary currents and taps of transformers would cause the steady unbalance current.Meanwhile,the excitation inrush current caused by energization of unloaded transformer or voltage recovery when the fault is clear would also lead to the transient unbalance current.The veracity and transformer rapidity of longitudinal differential protective relay are affected by the above-mentioned unbalance current.As for the unbalance current caused by the difference of phase in the three-phase transformer,the method of transforming the Y connection to detla connection or transforming the detla connection to Y connection in the secondary side of current transformer is introduced,and the unbalance current can decrease efficiently.Then,the excitation inrush current is studied,the reason for it is revealed by theoretical analyzed,and simulations based on MATLAB are conducted to compute the inrush current in different transformers.The characteristics of inrush current are revealed by those analyses,and those research would help a better understanding of how to reducing the transient unbalance currentAt last,the applications of different kinds of relay are introduced to reducing the unbalance current considering different reasons.The quick magnetic saturated current converter is introduced to reduce the unbalance current caused by the selection of current transformer's rated ratio.The BCH-1 quick magnetic saturated current converter which is equipped with the brake winding additionally is introduced to reduce the aperiodic components of the transient unbalance current.The DCD-2enhanced quick magnetic saturated current converter is introduced to reduce the the transient unbalance current caused by inrush current.The calculation cases are also provided.