| Second-harmonic restraining technique and gap-detecting technique are widely used to distinguish magnetizing inrush current and internal fault current. However, recently more bigger the transformer capability is, more longer the transmission lines are and CT's saturation, more bigger second-harmonic content of internal faults is even sometimes more than that of magnetizing inrush current. While, due to modern advanced manufacture arts and material of transformer, the content of second-harmonic in inrush current decreases. Relay with Second-harmonic restraining technique may maloperate and delay the operate time. The cost of relay protection with gap-detecting technique is very high and this technique may be affected by CT's saturation.Therefore,some new transformer protection projects are proposed resently.This paper introduces some techniques such as Fuzzy Logic algorithm,Expert System, Artificial Neural Network,Wavelet Transformation and their advantages and disadvantages.Especially,kinds of Wavelet Transformation application in transformer protection are compared and analyzed,only to find that Wavelet Transformation is a most promising tool in transformer protection.This paper presents a new approach of transformer protection based on Wavelet Transformation.Firstly,system model is constructed by RTDS(Real— Time Digital Simulator).RTDS is used to simulate kinds of magnetizing inrush current at any switched moment and kinds style of fault,fault currentsunder CT's saturation and inrush current when energized transformer without loads.Then,the output data from RTDS is analyzed by WT of Matlab to analyze sample current signals of both sides of transformer and get the product of fault-jump in order to distinguish between inrush current and internal fault current only to get the new arithmetic.Finally,the arithmetic is tested by RTDS and Matlab.The arithmetic proved to be fast ,reliable,accurate and simple ,what's more.it is not affected by CT's saturation and the worst faults.
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