Research On Differential Protection Of Transformer Based On Magnetic Saturation Characteristic

The power transformer is one of important electrical equipment in the power system.It is expensive and must be equipped with high performance and reliable relays.The principle of current differential protection is simple,reliable and optional,so it is widely used as the main protection of transformer.At present,the ratio differential protection is widely used as the main protection of transformer.However,the traditional ratio differential protection may mal-operate in the case of inrush current or current transformer saturation during external faults.In order to avoid current transformer saturation and inrush current caused by magnetic saturation,an improved method is studied in this paper,which can improve the action correctness of transformer differential protection.Firstly,the research background of transformer differential protection are briefly introduced.The advantages and disadvantages of current transformer saturation and inrush current identification methods are summarized.The inadequacies of the traditional differential protection and its causes are pointed out.The mechanism and relative factors of current transformer saturation and inrush current are studied,and the adverse effects on differential protection are briefly discussed.Based on the discussion of transformer and current transformer models,a transformer differential protection model considering current transformer saturation and inrush current is established.Secondly,based on the influence of current transformer saturation on secondary current,a new method of transformer differential protection is proposed,wh ich can resist current transformer saturation.After current waveforms on both sides are adjusted with phase and ratio,current waveforms are symmetrical during external faults.Even if one of current transformers is saturated,current waveforms are symmetrical during the linear region of transformer core.However,under internal faults,there is no symmetrical part of current waveforms.By extracting the partial samples of symmetrical current waveforms,action value and resistance value which are independent of current transformer saturation are constructed.Finally,a complete differential protection scheme is proposed.This scheme also has strong anti-abnormal data performance for differential protection with digital current transformers.Simulation results indicate that the proposed method has higher sensitivity during internal faults and higher reliability during external faults,which can effectively avoid incorrect actions caused by abnormal cuFinally,based on the influence of transformer saturation on differential current,a new method to identify inrush current using current sampling data central trend is proposed.After taking absolute value of current sampling data,the sampling data of fault current is concentrated in the upper part of the waveform,while the sampling data of the inrush current is concentrated in the lower part of the wave form.Based on the opposite central tendency of fault current and inrush current sampling data,the identification of inrush current can be realized by using the weight mean which can reflect the central trend of samp le.Theoretical analysis and simulation results show that the proposed method has the advantages of simple principle,non e of anticipated parameters,short data window and is independent of sampling frequency,current transformer saturation and noise.The detection results of recorded current waveforms show that the proposed method is practical and effective.