The electrical power is an important part of daily life and a necessity for the development of modern industry. The dependency of a country’s economic development on electrical power is growing rapidly. Consequently, planning, designing, constructing, and maintaining power delivery systems must keep pace with the escalating demand of such development. Power transformers are a key component of a power transmission system, and condition monitoring and failure diagnosis techniques are commonly required by transformer owners for reliability and maintenance purposes.
Despite several decades of research into transformer vibration and condition monitoring techniques, state-of-the-art development in this area still falls short in the understanding of the mechanisms involved and in industry implementation. The objective of this thesis therefore is to investigate the vibration characteristics of a power transformer with and without structural damage and to develop a vibration-based transformer condition monitoring technique. It is hoped that this work could give a better understanding of transformer vibration and its application to condition monitoring.
To that end, several aspects of transformer vibration are studied experimentally and numerically, including its excitation forces, modal characteristics, and vibration frequency responses. The finite element (FE) method is employed as the main approach for numerical analysis of the aforementioned aspects. The effect of the arrangement of ferromagnetic parts on the modelling of winding electromagnetic (EM) forces is discussed in detail with the purpose of improving its modelling accuracy. Special considerations, i.e., the anisotropic mechanical properties of core lamination, of transformer vibration modelling are summarised based on the traditional experimental modal analysis. Vibration features of a transformer with structural anomalies, especially with cases of winding failure, are investigated using a verified FE model. In addition, the frequency response function and its variations caused by structural anomalies are studied experimentally under both mechanical and electrical excitations.
It is shown that a structural anomaly will produce shifts in the natural frequency and changes in the vibration response. The experimental results also demonstrate that the transformer mechanical resonance can be excited by internal electrical excitations, which enables operational modal analysis (OMA) and OMA-based online monitoring. An algorithm based on the time-domain NExt/ITD method is employed as an OMA technique to identify transformer modal parameters. The features of transformer vibration and operational conditions are considered in the proposed algorithm, which improves the identification accuracy in some cases. The identification method is also applied to the same transformer with core and winding anomalies. Results show that the OMA method is capable of identifying transformer modal parameters and thus can be utilised for online condition monitoring.