1860 M. Bagheri et al.: Advanced Transformer Winding Deformation Diagnosis: Moving from Off-line to On-line 1070-9878/12/$25.00 © 2012 IEEE Advanced Transformer Winding Deformation Diagnosis: Moving from Off-line to On-line Mehdi Bagheri, Mohammad Salay Naderi and Trevor Blackburn School of Electrical Engineering & Telecommunications University of New South Wales Sydney, NSW 2052, Australia ABSTRACT On-line monitoring and diagnosis of transformers have been investigated and discussed significantly in last decade. This study has concentrated on issues arising while on-line transformer winding deformation diagnosis is going to be applied on transformers with various kinds of techniques. From technical perspective, before replacing off-line methods by on-line methods and eventually by intelligent approaches, practical challenges must be addressed and overcome. Hence, available off-line transformer winding deformation diagnosis methods are discussed precisely. Mathematical calculation in on-line short circuit impedance measurement is investigated. On-line transformer transfer function measurement setup is presented. A profound insight to the problems pertaining on-line transformer winding deformation recognition methods, characterizes existing online methods, explains the concepts behind online measurements and striving to open the discussion doors towards challenges are discussed. In the end a 400 MVA step up transformer has been taken as a case in order to clarify the capability of Frequency Response Analysis (FRA) method in fault detection while short circuit impedance could only demonstrate some rough understanding about transformer condition. Index Terms — Transformer, on-line diagnosis, winding deformation, frequency response analysis. 1 INTRODUCTION INTELLIGENT condition monitoring deals with all items of equipment in power system and when it comes to transmission system it pays more attention to those with higher capital and maintenance expenses. Also, it is well-known that power transformer is one of the most expensive equipment among all electrical items of equipment. This valuable equipment is in service in various climates as well as different electrical and mechanical conditions [1]. Based on this fact, transformers are continually facing enormous hazards over the course of operation [2]. Obviously, supervising and monitoring of electrical elements, particularly power transformers, which are considered as the heart of electricity generation, transmission, distribution, has been quite important for many years [3]. On the other hand, yielding information continuously from insulation system condition and having a reasonable understanding about internal mechanical stability is vitally important for the system operators. In critical situations, transformer failures can result in irreversible damage and bring loss of millions of dollars for electrical grid companies or even consumers [4]. In practice, various types of faults are jeopardizing transformers steady state operation and tending to take this expensive equipment out of service. In this regard, one of the main problems in transformers is mechanical defect. Mechanical defects might occur due to many disturbances such as short circuit currents, heavy explosion of combustible gas in transformer oil, earth quake, or even unsuitable transportation. It includes winding deformation in axial and/or radial direction, hoop buckling, tilting, spiraling, telescoping, displacement of high and low voltage windings, shorted or open turns, partial winding collapse, loosened clamping structures, core movement, faulty grounding of core or screens, broken clamping structures, or intensifying internal imperfections. Therefore, mechanical diagnostic methods have been emerged to recognize transformer active part displacement as well as winding deformation. Hence, various methods such as Low Voltage Impulse (LVI), Frequency Response Analysis (FRA) and Short Circuit Impedance (SCI) have been employed for off-line mechanical defects recognition in transformers [5- 10]. Since the researchers are showing an increased concern about the energy efficiency in the smart grid context, there are some mixed feelings towards on-line high voltage diagnosis solution. Transformer tank vibration [11-14], communication technique using scatter parameters [15-17], current deformation coefficient [18, 19], ultrasonic method [20], short circuit impedance [21-26] and winding stray reactance [27-29], on-line Transfer Function (TF) using time domain or frequency domain [30-34] have been introduced as advanced on-line methods in order to real-time recognition of transformer winding deformation or displacement. Manuscript received on 15 January 2012, in final form 16 March 2012.