Power Transformer Differential Protection Through Gradient of the Differential Current
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Power Transformer Differential Protection Through Gradient of the Differential Current Raidson Jenner N. de Alencar · Ubiratan Holanda Bezerra
Received: 3 November 2011 / Revised: 9 March 2012 / Accepted: 18 April 2012 / Published online: 26 March 2013 © Brazilian Society for Automatics–SBA 2013
Abstract This paper presents a new methodology for identifying internal faults and inrush currents in power transformers based on the gradient of the differential current. The technique is to calculate the angle of the gradient vector along the curve of the differential current in the data window and through waveforms recognition techniques to identify the type of occurrence. The gradient vector of a function corresponds to a vector tangent to the curve at the point under consideration. The gradient vector, to move along the curve of the differential current, changes its angle to the horizontal reference. The behavior of the gradient vector angle, through statistical calculations will be used to identify the occurrence of internal faults or the presence of inrush currents. The method was tested by simulating various types of internal and external faults and also several cases of inrush currents in a power transformer which is modeled by the EMTP/ATP software and also by implementing the algorithm in MATLAB® , presenting highly satisfactory results. Keywords Differential protection · Power transformers · Inrush current · Current gradient
1 Introduction The power transformer is an essential equipment in the operation of electric power systems (EPS). Given its importance and its high cost, it is necessary to develop and implement a highly reliable protection system that is able to identify the R. J. N. de Alencar (B) GESEP, Federal Institute of Pará–IFPA, Belém, PA, Brazil e-mail: [email protected] U. H. Bezerra PPGEE, ITEC, Federal University of Pará–UFPA, Belém, PA, Brazil e-mail: [email protected]
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occurrence of internal faults and remove the equipment as fast as possible for operation. Differential protection constitutes the main method of protection for large transformers and is highly recommended for transformers with power greater than 10 MVA. During the last years, several proposals of methods have been presented in order to solve the main difficulty that appears in the differential protection: the distinction between the magnetizing currents and internal faults. Techniques based on wavelet transform (WT) have been proposed in recent years and demonstrated good results in detecting internal faults in power transformers. Wilkinson and Cox (1996), Mao and Aggarwal (2001), Mortazavi and Khorashadi-Zadeh (2004), Megahed et al. (2008), Rahmati and Sanaye-Pasand (2008), Oliveira and Bretas (2009), Saleh and Rahman (2010), and Hossam Eldin and Refaey (2011) propose algorithms based on this tool. Discrete WT tools are time-frequency analysis that breaks down the current signal into a set of approximations and details coefficients each corresponding to a frequency band. The characteristics of these approximations and
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