Identification And Analysis Of Transformer Inrush Current And Fault Current

With the development of Chinese economy for these years , there has been more and more requirements of power energy .Since the power system is enlarged continually, large transformers in power system is getting more and more important, and some new requirements for the transformer protection are needed . Differential relay is the master protection for transformers all along , but magnetizing inrush current is still the main reason that causes transformer relay' s miss-operating, so distinguishing between magnetizing inrush current and fault current correctly is the key to ensure transformer differential relay to operate credibly .Wavelet transform is a method for data analyzing and processing that has been developed fast in recent years ; EMTP is a kind of well developed tool for Electric-Magnetic Transient simulation ; MATLAB has many toolboxes that can be used essily . In this paper , transformer is first simulated in EMTP and data is available , then we process the data in MATLAB with wavelet toolbox . Thus we make a research into transformer and get some of its important characteristics .Magnetizing inrush current has obvious singularity. Wavelet transform does well in multi-scale and time-frequency domain , and it is particularly well adapted to extract the characteristic of signal' s edge and peak value, and can check up the singularity well. This thesis chooses DB wavelet that adapts to the analysis of power system signal , and uses the wavelet transform to identify the magnetizing inrush current and inner fault current . Some elementary microcomputer transformer relay designing is released in this thesis, and on the base of intermission angle principle . A new criterion is raised , which pierces through the traditional method with accurate measurement of intermission angle , and analyses the wave by wavelet transform. The criterion raised in this thesisis requires higher in hardware, it can be realized in transformer relay by DSP technology .Emulated in EMTP and MATLAB , the new criterion and its arithmeticdesign in this thesis has been proved that it can distinguish between magnetizing inrush current and fault current . While the criterion based on intermission angle principle may vanish in TA saturation, the new one can distinguish correctly without module to recover the vanishing intermission angle . In conclusion, this criterion has applied value in transformer relaying design .