Transformer Differential Protection Based On Aperiodic Component Characteristics

This paper explores the dynamic attenuation rules of the aperiodic components of the transformer inrush current and the short circuit fault current when the transformer is under different conditions.The dynamic characteristics of aperiodic component of inrush current are used for recognition basis.The concept of dynamic time constant to distinguish between inrush current and fault current is put forward.A novel method of transformer differential protection with the identification function of inrush current based on the direction and attenuation characteristics of aperiodic components is proposed in this paper.The large criterion coefficient when considering the aperiodic component could reduce.The speed,sensitivity and reliability of transformer protection are increased.The four power system circuit diagrams are established according to the different power supply modes and the different fault locations.The voltage equation between the Bus and the short circuit fault is written according to Kirchhoff's law.The differential equation is solved.The instantaneous value of current on both sides of the transformer is obtained.By integrating the instantaneous current value,the aperiodic component of short circuit current is obtained.Then,the symbol characters and variation characters of the aperiodic components are analyzed.Based on the study of the direction feature of the aperiodic component,the internal and external fault criterion based on the direction feature of the aperiodic component is given.The criterions of internal and external faults for unilateral power supply and bilateral power supply are unified by referring to the power frequency transformer differential protection with the brake characteristics.The transformer inrush current circuit diagram is established.The inrush current expressions are found according to the internal electromagnetic relationship in transformer.The analytical formulas of the aperiodic component of the inrush current in different intervals are derived by using the piecewise integral method.The change characteristics of aperiodic components are analyzed when the inrush current occurs.The qualitative and quantitative comparisons between the short circuit current when the internal short circuit fault occurs with unilateral power supply and the inrush current are explained.The different attenuation characteristics of inrush current and short circuit current are revealed.A novel method based on the changes of the dynamic time constant to identify inrush current is proposed.According to the short circuit fault current and inrush current direction characteristics and attenuation characteristics,a novel criterion of transformer differential protection with inrush current identification function based on the characteristics of aperiodic components is proposed.The flow chart and the corresponding simulation analysis are presented.The extraction algorithm of aperiodic component is given and the measurement is proposed when the aperiodic component nearly equals zero.Extending the single phase transformer to the three phase transformer,the electromagnetic relationships of the symmetrical fault conditions and asymmetric fault conditions of three phase system are revealed.The criteria which got from single phase transformers also could meet the three phase system needs no matter the symmetrical fault or the asymmetric fault.