Adaptive differential protection of three-phase power transformers based on transient signal analysis Mario Orlando Oliveira ⇑ , Arturo Suman Bretas, Gustavo Dornelles Ferreira Federal University of Rio Grande do Sul, Electric Power Systems Laboratory, Av. Osvaldo Aranha, 103, Porto Alegre, 90035-190 RS, Brazil article info Article history: Received 4 December 2012 Received in revised form 3 December 2013 Accepted 10 December 2013 Keywords: Adaptive protection Discrete wavelet transform Transformer differential protection Transient analysis abstract This paper presents a three-phase power transformer percentage differential protection formulation based on transient signal analysis. The proposed formulation uses discrete wavelet transforms (DWT) to extract transitory features of non-stationary signals with fast transition, mapping the signal in time–frequency representation. The proposed formulation was implemented on MATLAB Ò environment and evaluated through a case study using BPA’S ATP/EMTP software. Comparative test results are pre- sented showing that the proposed formulation is highly reliable, fast, accurate and easy for real-life construction. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Power Transformers (PT) play an extremely important role on the reliability and energy supply continuity of electric power sys- tems (EPS). The inherent characteristics of power transformers introduce a number of unique problems that are not present in transmission lines, generators and motors protection [1]. When internal faults occur in PT, immediate disconnection of the equip- ment is necessary to avoid extensive damage and/or preserve power system stability and power quality. Furthermore, the replacement of a faulted transformer is very expensive and time consuming. Therefore, PT protection can prevent great economic losses and also avoid long power outages [2]. Currently, percentage differential protection is a common prac- tice for power transformer protection. However, nonlinearities in the transformer core or in the currents transformers (CTs) core, can generate a substantial differential current causing a percentage differential relay miss-trip [1]. Thus, the differential relays are equipped with harmonic restraint, where magnitudes of the sec- ond and fifth harmonic components are compared with the funda- mental frequency component magnitude to discriminate internal faults from magnetizing inrush currents and transformer overexci- tation, respectively [3]. Aiming to improve the efficiency of PT differential protection a significant number of relaying formulations have been proposed [4–24]. These formulations are based on finite elements, artificial neural networks, fuzzy systems, dynamical principal components analysis, wavelet transforms (WTs), hybrid systems, symmetrical components, space vector and power-based protection methods. In [25] is presented a review on computational intelligence tech- niques applied on oil-immersed PT conditions evaluation for oper- ating costs reduction, to enhance operational reliability and to improve power supply and customer service. However, all men- tioned relaying formulations have hard to design parameters, which make real life construction difficult. Ref. [26] present a Dis- crete Wavelet Transform (DWT) application for PT differential pro- tection, however the proposed scheme does not have adaptive characteristics. In [27] is proposed an adaptive differential protec- tion scheme, however not all possible PT operational conditions were tested and evaluated. In this paper, a simple to implement percentage differential relaying algorithm for three-phase power transformers protection based on DWT is proposed. The proposed algorithm formulation uses logical decision criteria based on wavelets coefficient spectral energy variation to identify and discriminate correctly internal and external faults, inrush currents and incipient internal faults all un- der or not current transformer saturation. In order to analyze the proposed algorithms efficiency, the formulation was built in MAT- LAB Ò platform [28] and tested with simulated fault cases under BPA’S ATP/EMTP software [29]. Comparative test results with a tra- ditional percentage differential relaying with harmonic restraint formulation [1] shows the proposed algorithms efficiency and its easiness for real life construction. The remaining of this paper is divided as follows. Section 2 de- scribes the percentage differential protection formulation. Section 3 describes the DWT used. The proposed algorithm is presented on Section 4, while Section 5 presents the case study. Section 6 0142-0615/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijepes.2013.12.013 ⇑ Tel.: +55 51 3308 4437; fax: +55 51 3308 3293. E-mail address: moliveira@ece.ufrgs.br (M.O. Oliveira). Electrical Power and Energy Systems 57 (2014) 366–374 Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes