Study On Transformer Differential Protection Considering DC Bias Effect

As the main protections of transformers,differential protection is related to the safe and reliable operation of substations and power grids.In recent years,HVDC transmission technology has been widely used.As a result,the current transformer and transformer DC bias magnetic phenomenon is becoming more and more serious,causing current transformer saturation and transformer excitation current distortion and increase,which seriously affects the reliable operation of transformer differential protection.In order to solve this problem,the method of current transformer saturation recognition and the identification method of transformer inrush current are studied in this paper.The influence of DC bias on current transformer and transformer differential protection is studied.Firstly,the mechanism of DC bias is analyzed,and the simulation model of DC bias is built.The influence of DC bias on current transformer and transformer is discussed through simulation research,and the common restraining methods of DC bias are summarized.A saturation identification method of current transformer based on sine wave sequence characteristic is proposed.The saturation principle of current transformer and the harm caused by saturation are introduced.Several classical saturation recognition methods are analyzed.On this basis,the full cycle and half cycle criteria are set up to identify the saturation of the current transformer by using the sine wave in any half period and a fixed value,and the effectiveness of the method is verified by simulation..An identification method of inrush current based on modulated PWM waveform characteristics is proposed.A series of PWM waveforms are modulated by signal wave and isosceles triangle carrier,and the difference between the excitation inrush current and the PWM waveform of the fault current in the transformer area is analyzed.The pulse width and the two changes of the amplitude value of a periodic modulation PWM waveform are 0 and the maximum half period data difference modulation PWM waveform is 1.Two criteria are used to identify the inrush current and the fault current in the area.The simulation results of Matlab show that the method can accurately,quickly and effectively identify the inrush current and the fault current in the transformer area caused by DC bias and other factors.The method is simple in principle and good in sensitivity.It can effectively identify the inrush current caused by DC bias magnetizing,and the reliability is high.An identification method of magnetizing inrush current based on transformer differential current skewness coefficient is proposed.The probability distribution difference between inrush current and fault current is analyzed.The fault current bias of the sinusoidal characteristic is negative and keeps constant,and the total working frequency deviation after the absolute value is equal to that of the half frequency period,and the full cycle deviation of the inrush current is positive,and it is stored with the semi periodic bias coefficient.The skew coefficient of the differential current sampling values of 1 power frequency cycles and half power frequency cycles can be calculated,and the inrush current can be identified quickly.The proposed method is simple in principle and small in computation.It can identify the inrush current caused by DC bias accurately and effectively,and has strong anti-noise and anti-interference ability.The simulation results verify the feasibility and superiority of the method.