Transformer Fault Diagnosis using Frequency Response Analysis - Practical Studies Hormatollah Firoozi, Mohammad Kharezi, High voltage laboratory, Iran Transfo Co. High votage laboratory, Iran Transfo Co. Zanjan, Iran. Zanjan, Iran Hossein Rahimpour Mehdi Shams Technical design Dept. of Iran-Transfo Co. Tozih Iran-Transfo Zangan Co. Zanjan,Iran Zanjan,Iran Abstract— Any failures in these equipments directly reduce network reliability and increase maintenance costs. Consequently, the preventive maintenance techniques are increasingly developed. In this regard, frequency response analysis is an appropriate method in order to diagnose any change which occurs in transformer physical construction. This contribution has been concentrated on fault diagnosis by use of Frequency Response Analysis (FRA). A number of measurements on faulty transformers which are led to fault diagnosis has represented and discussed. These practical studies are performed during maintenance processes or on site periodic inspections. The results are very worthwhile to better understand how behave transformer frequency responses due to various faults. These obtained results can be very interesting and usable for maintenance engineers to find the occurred fault. Keywords-Frequency Response Analysis;Fault diagnosis I. INTRODUCTION Power transformers are one of the essential and also expensive equipments of electrical networks which play mainly role in the energy transmission and distribution. For this reason, the reliability, quality and economic cost of the company depends strongly of the transformer health and, therefore, an intensive maintenance of the electric machine is required. Today, preventive maintenance techniques such as Dissolved Gas Analysis, Thermal Monitoring, Partial Discharge measurements and Frequency Response Analysis (FRA) have been developed [1,3]. The FRA is a powerful method in detecting transformer mechanical damages, which are difficult to detect by conventional measurements. It relies on the fact that transformer winding can be modeled as a network of capacitances, resistances and inductances. When a failure occurs in the transformer construction, the values of these parameters are altered and hence the frequency response from the winding will also change accordingly. FRA is a simple and low cost method. Also, with this method, it might be able to recognize the type and location of fault, which is not possible with most other methods, involving terminal measurements. In spite of all its advantages, frequency response technique has still not become popular due to some limitations. These limitations can be divided into two branches. The first one is obtaining a repeatable frequency response through practical measurements techniques and the second one is a lack of availability of correlation between the signature and the changes in the parameter of the equipment. In other words, the success of this method relies on the correct interpretation of the accurate and reliable measurements [4,5]. According to above discussion, it is no so easy to have a physical interpretation of frequency response variations. For this reason, experimental and practical studies are very worthwhile to better understand how behave transformer frequency responses due to various faults. This paper represents the results of FRA measurement on faulty transformers. These tests had been performed before applying any other diagnostic techniques. In fact, the main goal was the evaluation of FRA ability to recognize the occurred fault before re-assembling of transformer. The obtained results can be very interesting and usable for maintenance engineers to find the occurred fault. II. FRA MEASUREMENT METHOD In this research work, all the frequency response characteristics were obtained using the sweep frequency method (SFM). Fig. 1 shows a schematic diagram of the experimental set-up based on “FRAnalyzer” which manufactured by OMICRON company. The injected signal S at different frequencies is applied to one end of the test winding and the voltage signals at both ends of the winding are measured by the two cables called reference R and test T. These two voltages then used to calculate the voltage ratio. The results are presented in the form of frequency and amplitude on a logarithmic scale as below. ) ( ) ( ) ( ω ω ω R T H = 978-1-4244-6255-1/11/$26.00 ©2011 IEEE