Studies Of Power Transformer Magnetizing Inrush Identification Based On Mathematical Morphological Theory

The differential protection has been the main protection of transformer for a long time. However, the correct operation rate of transformer differential protection is relatively low compared to its application in transmission line. The main reason for that is the existence of transformer magnetizing inrush current, which has become the most difficult and pivotal problem of transformer differential protection. Many researchers have been worked on this subject and lots of methods of identifying magnetizing inrush current were brought out.Mathematical morphology (MM) is developed from set theory and integral geometry, and is widely used in geometrical analysis and description. Its application in one dimension signal processing has been a long time, and has become one of the latest outcomes in advanced signal processing field. Morphological filters are nonlinear signal transformation. Compared to Fourier transform and Wavelet transform, MM totally works in time domain rather frequency domain. Its calculation needs only the comparison and addition operations without multiplication; the computational complexity is much smaller than conventional digital filter algorithms. Because of its so many advantages, many researchers have brought MM into electrical power system study and take advantage of it to pick up the distortion characteristics of electrical transient signals.The following contents are included in this paper:1. Typical methods of discriminating between internal fault and magnetizing inrush current are summarized, as well as their advantages and weaknesses.2. The mechanism and characteristic of magnetizing inrush are studied, and simulating model is proposed in PSCAD/EMTDC to simulate the inrush current and internal fault current. The waveform date of inrush current is analyzed.3. The definition and basic operation law of mathematical morphology are represented, as well as its explanation in set. Based on the principle of mathematical morphology filter, a new MM filter and peak-valley detector are proposed. According to the characteristic that inrush current wave form distorts while internal fault current mainly retains sine wave form, an algorithm is programmed in MATLAB platform to discriminate between magnetizing inrush and internal fault current.The research of this paper is funded by special fund of Key laboratory of Guangdong Regular Higher Education Institutions (No. ZDSYS200701) and key projects of Education Department of Guangdong Province (No.070043)...