VOL. 5, NO. 11, NOVEMBER 2010 ISSN 1819-6608 ARPN Journal of Engineering and Applied Sciences © 2006-2010 Asian Research Publishing Network (ARPN). All rights reserved. www.arpnjournals.com EXPERIMENTAL STUDIES FOR SURGE VOLTAGE RESPONSE OF A POWER TRANSFORMER MODEL WINDING PROVIDED WITH METAL OXIDE VARISTORS Mohd. Zahed Ahmed Ansari 1 , G. R. Gurumurthy 2 and J. Amarnath 3 1 Department of Electrical and Electronics Engineering, Ghousia College of Engineering, Ramanagaram, Karnataka, India 2 Department of Electrical and Electronics Engineering, BNM Institute of Technology, Bangalore, Karnataka, India 3 Department of Electrical and Electronics Engineering, JNT University, Hyderabad, Andhra Pradesh, India E-Mail: zahedansari@gmail.com ABSTRACT Surge voltage distribution across a power transformer winding due to appearance of very fast rise transient overvoltages (VFTO) such as lightning surges consists of high voltage oscillations and the voltage distribution along the length of the winding can be highly non-uniform. In order to make the voltage distribution more uniform along the length of winding under these conditions, a method has been explored as an alternate to the conventional methods that are being used presently. This method consists of providing suitably designed metal oxide varistors (metal oxide surge absorbers) across sections of winding. The voltage - current characteristic of metal oxide varistor (MOV) is given by V = KI β , where K and β are the constants of MOV. Experimental investigations have been carried out on a transformer model winding with MOVs provided across fifty percent and hundred percent of winding length to investigate surge voltage performance of these types of windings for appearance of lightning overvoltage (full impulse voltages) at line terminal of the model winding. Transformer windings with α values 5.6, 11.8 and 18.9 have been analyzed. The oscillographic records obtained with presence of identical MOVs (MOVs having similar characteristic constants K and β) or non-identical MOVs (MOVs having different K value and similar β) across transformer winding sections show appreciable reduction in surge voltages across sections of winding as compared to that obtained without MOVs. Keywords: surge voltage distribution, lightning overvoltage, metal oxide varistors, transformer model winding. 1. INTRODUCTION Transformers are one of the most important equipments used in power transmission systems. Transformers in service are exposed to hazards due to effects of lightning phenomena [1-4]. Lightning overvoltages having very fast rise times of value 1µs or lower can cause considerable hazard to transformer winding insulation because of non-uniform voltage distribution along the length of the winding. Therefore it is of great importance to make the voltage distribution along the length of transformer winding to be as uniform as practicable. In earlier years this was achieved by use of static end rings provided at line terminal of HV winding of transformer. Other methods that were employed for making the voltage distribution more uniform is by increasing series capacitance of the winding (C s ) by suitably interleaving the winding or by decreasing winding capacitance to ground (C g ) using shielding to the transformer winding. Few recent investigations have shown that, because of excellent surge protection performance capabilities, a suitably designed MOV can be used across sections of power transformer HV winding to improve the voltage distribution along the length of winding [5-6]. The voltage distribution along the length of HV winding of transformer is mainly dependent on α value (Square root of the ratio of total ground capacitance to total series capacitance of the winding) of the winding. In previous investigations [6], simulation analysis for surge voltage behavior of power transformer winding was studied for appearance of unit step voltages at HV terminal of model winding by performing computer simulations. Experimental studies have been carried out on a transformer model winding to investigate surge voltage response due to appearance of lightning impulse voltage at line terminal. The voltage - current characteristic of MOV is highly non-linear and is given by V = KI β , where K and β are the constants of MOV. MOVs of similar characteristic constants K and β and also of different K values and similar β value have been used across the sections of winding for these investigations. The results of the investigations are reported in this paper. The values of α used in this work are 5.6, 11.8 and 18.9. 2. TRANSFORMER MODEL WINDING FOR SURGE STUDIES The HV winding has been represented by a single layer coil wound on an insulating former of mean diameter 20.1cms and coil consisting of 8 sections. Each coil section constituted 60 turns. Thus, the HV winding equivalent circuit of power transformer winding for investigating surge voltage behavior of winding which consist of several similar sections, each of the sections represented by self inductance (L) and mutual inductance between sections (M 12 ,…M 21 ….M 48 ……M 84 ..etc.), series capacitance across winding sections (C s ) and capacitance between coil section to ground (C g ) is shown in Figure-1 [1-3]. Suitable MOVs are also connected across each section of the winding. 105