Available online at www.sciencedirect.com Electric Power Systems Research 78 (2008) 547–558 Wavelet based transformer protection using high frequency power directional signals Simi P. Valsan, K.S. Swarup ∗ Department of Electrical Engineering, Indian Institute of Technology Madras, India Received 19 October 2006; received in revised form 15 March 2007; accepted 29 April 2007 Available online 6 June 2007 Abstract This paper proposes a novel wavelet transform based relaying scheme for power transformer protection. The relay logic consists of two parts: disturbance detection based on first level high frequency details of the voltage signals only and fault discrimination using a power based directional signal derived from the first level high frequency details of both voltage and current signals. The logic is deterministic, computationally efficient, fast, secure and highly reliable. The operating time is 6 ms, about 1/3rd of power frequency cycle (20 ms). The scheme uses only the sign of the directional signals, rather than the difference in their magnitudes, hence it can work reliably in the presence of transformer tap variation, fault resistance and CT saturation. The validity of the proposed logic was exhaustively tested by simulating various types of internal and external faults, energization conditions and load variations on a 132kV system modeled in ATP/EMTP with a 31.5 MVA, 132/33kV, Y– transformer. The proposed logic was able to correctly discriminate between internal faults, external faults and non-fault disturbances for all the 880 test cases. © 2007 Elsevier B.V. All rights reserved. Keywords: Transformer protection; Wavelet transform; Signal decomposition; High frequency details; Disturbance detection; Fault discrimination 1. Introduction The power transformer is an important component of power system. The reliable operation of a transformer is of vital impor- tance in maintaining the continuity of power supply. When transformer internal faults occur, immediate disconnection of the faulted transformer is necessary to avoid extensive damage and/or preserve power system stability and power quality. Any unscheduled outage, especially replacement of a faulty trans- former, is very expensive and time consuming [1]. Apart from this, the unique problems associated with transformers make the design of a protection system very challenging. Some of these problems are listed below: (1) Magnetizing inrush currents created by transformer tran- sients during energization. (2) Variable transformation ratio caused by a tap changer. (3) Phase shift between the power transformer primary and secondary currents resulting in the need for different cur- ∗ Corresponding author. E-mail address: swarup@ee.iitm.ac.in (K.S. Swarup). rent transformer (CT) connections for Y–Y, – and Y– configurations. Traditionally, a differential scheme is employed for trans- former protection. The performance of simple differential relays is seriously affected by CT saturation. Consequently, a percent- age differential scheme was introduced which could avoid the effect of CT saturation to some extend. But these relays are subjected to false tripping due to the inrush current that flows during transformer energization. Since the inrush current is rich in second harmonics, a restraint quantity based on second har- monics, was derived and employed to restrain the relay during energization. But over the years, the advances in transformer construction and improvements in core materials has brought down the level of second harmonics whereas its level during an internal fault can be quite high in certain cases. Recently, modern protection algorithms based on wavelet transforms (WT), artificial neural networks (ANN), fuzzy logic and multi-channel filtering have been proposed by some authors. Bo et al. [2] proposed a fault detection technique based on fault generated high frequency transients extracted using multi- channel filters. Peilin et al. [3] and Okan [4] used wavelet transform to decompose the differential current signals into 0378-7796/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2007.04.008