Experimental test results for a parallel transmission lines protection scheme using wavelet transform A.H. Osman and O.P. Malik Abstract: A scheme for the protection of parallel transmission lines using wavelet transform (WT) has been implemented and results of experimental tests on a physical power system model are described. The relaying scheme depends on the three line voltages and the six line currents of the two parallel lines at each end. Fault detection, fault discrimination and calculation of the phasors of the measured signals are done using WT. The performance of the proposed scheme has been investigated by a number of online tests. The results show that all types of shortcircuit faults can be correctly identified in less than one cycle of the fundamental frequency. 1 Introduction Performance of the conventional distance relays is affected by the mutual coupling between parallel lines. Because of the mutual inductance between conductors of corresponding phases in the two lines, voltages are induced in the phase of one circuit by the current in the corresponding phase of the other circuit. As a result, the apparent input impedance of a line on which a fault is present is affected by the load currents or currents being fed to the fault by a parallel healthy line and the operation of the distance relay can be affected. A number of solutions have been proposed to solve the problems associated with the protection of parallel lines [1– 5] . None of these solutions completely solved the problem as they might fail either in differentiating between internal and external faults or in classifying same-phase faults on both circuits. A new scheme to overcome the drawbacks of the schemes in [1–5] is proposed. The proposed scheme is based on using two relays, one at each end of the parallel lines, and uses the six current signals and three voltage signals on each end. It is easy to detect any disturbance or fault on the lines and to estimate the phasors of the measured signals that are needed to execute the proposed protection algorithm using wavelet transform (WT) [6]. The algorithm has three stages. The first stage is the fast fault detection and phasor estimation using WT. The second stage is the comparison of the estimated magnitudes of line currents in the corresponding phases of the two circuits. At this stage most of the common internal fault types can be discriminated between. The third stage is the backup distance protection for some types of faults that current comparison cannot discriminate between, such as same-phase fault on both circuits and external faults. Experimental tests of this scheme have been investigated in this paper. The proposed algorithm has been implemented on a digital signal processor (DSP) mounted on a Pentium PC and tested in the laboratory on a physical power system model. The online studies illustrate the effectiveness of the proposed scheme. 2 Wavelet transform Wavelet transform was introduced at the beginning of the 1980s and has attracted much interest in the fields of speech and image processing since then. Its potential applications to power industry have been discussed recently [6–13] .A brief introduction to the WT is given in Appendix 8.1. In this approach, any function f(t) can be expanded in terms of a class of orthogonal basis functions as described in the Appendix Section 8.1. In wavelet applications, different basis functions have been proposed and selected. Each basis function has its feasibility depending on the application requirements. Daubechies family (db) is one of the most suitable wavelet families in analysing power system transients [8–13]. Although there are no definite criteria for the selection of wavelets, the best choice is a wavelet that most strikingly exhibits the phenomena to be studied [9] . WT has shown the ability to detect power system disturbances and to accurately and rapidly extract the fundamental frequency phasors. The advantages of using WT in fault detection and phasor estimation over the widely used signal analysing tool, discrete fourier transform (DFT), has been investigated in [6] . In the present work, the db1 wavelet (with two filter coefficients) has been used as the wavelet basis function for disturbance detection. db1 is a short wavelet and therefore it can efficiently detect transients. db4 wavelet (with eight filter coefficients) has been chosen, after extensive examination, for different mother wavelets to be used as the wavelet basis function for estimating the fundamental frequency phasors of the measured voltage and current signals. 3 Proposed algorithm The measured three voltages and six current signals (at each end) are filtered using pre-band-pass filters with a centre frequency of 60 Hz to attenuate the DC component. These nine signals are sampled at a sampling frequency of 960 Hz. The proposed protection algorithm is divided into three operating conditions. The first one is the pre-fault operation until a disturbance is detected. The second operating condition starts immediately after the disturbance detection r IEE, 2004 IEE Proceedings online no. 20041071 doi:10.1049/ip-gtd:20041071 Paper first received 7th October 2003 The authors are with the Department of Electrical and Computer Engineering, University of Calgary, 2500, University Drive, N.W., Calgary, Alberta, Canada, T2N 1N4 IEE Proc.-Gener. Transm. Distrib., Vol. 151, No. 6, November 2004 713