Winding movement and condition monitoring of power transformers in service

Power transformers are the most expensive pieces of equipment in a power substation. It is very important to prevent failures and detect problems in a transformer as early as possible. This thesis reviews the diagnostic and monitoring tests and equipment available to assess the condition of transformers and to provide an early warning of potential failure. One problem that is particularly difficult to detect is winding looseness, which can cause movement of the windings and may cause transformer failure. If the looseness or movement can be detected over time, action can be taken to prevent such failures. Frequency Response Analysis (FRA) techniques have the potential to detect winding problems, but the method also contains some uncertainty. In this thesis, Experiments on off-line and online FRA techniques were performed both in the laboratory and on transformers in the field throughout North America. New data assessment methods were developed that will help interpret future on-site transformer condition assessment tests. The key test circuit parameters and their effects on frequency response measurements were studied, covering issues such as the effects that external measurement leads and high voltage bushings on the test results, measuring shunt impedance, requirements for recording equipment, the measurement frequency ranges to identify winding problems, the measurement frequency range related to the amount of winding movement, criteria to be used for winding movement ranking, etc. A computer simulation was conducted on a simplified network transformer model. The principal findings from the simulation are similar to the experimental work. Based on the experimental investigation of the FRA method and the transformer model simulation, a new FRA technique was developed that is feasible for on-line applications or for monitoring on-site. To overcome the shortcomings of the existing FRA method, such as frequency limitations and on-line application, the High-Frequency Internal Frequency Response Analysis method (HIFRA) was developed. It uses an injected wide-band frequency signal through the transformer bushing, in combination with an internal FRA measurement. This new technique is very promising for future application for on-line FRA measurements. TABLE OF CONTENTS ABSTRACT ii TABLE OF CONTENTS . < iii LIST OF TABLES v LIST OF FIGURES vi GLOSSARY OF ABBREVIATIONS xi ACKNOWLEDGEMENT xii CHAPTER I OVERVIEW 1 1.