Frequency Response Analysis (FRA) has been developed to detect winding movement and deformation in transformers. In FRA diagnosis, the decision on a transformer is made based on the comparison between two FRA responses and any significant difference would potentially indicate mechanical problems with the winding. However, the ability to interpret the 'differences' when comparing the FRA responses is a great challenge. The main objective of this thesis is to develop an understanding of the FRA responses towards establishing an FRA interpretation guideline. The overall approach used in this research consists of four elements. Initially, the Transfer Function Estimation (TFE) technique is developed as an FRA comparative tool. However, this technique requires the correlation of the resulting key parameters with the deformation criteria and this demands an understanding of the FRA responses. Secondly, experimental studies on single windings are used to gain a firm understanding of the features of their FRA responses and these studies covered the interleaved, plain and intershielded disc type windings. The lumped ladder network representation of the winding is also employed to provide a mathematical back-up. Thirdly, verified FRA simulation models are used to investigate the FRA responses of three-phase transformers. Finally, an FRA interpretation guideline is developed and applied on the analysis of the FRA responses of a suspected faulty transformer. The TFE technique was specifically developed to represent the FRA response with key parameters. These were the complex conjugate pairs of poles and zeroes which specified the resonant frequencies of the FRA response. An auto-transformer with broken axial clamping fault would result in the changes of the key parameters which corresponded with frequency shifts of 4-5%. In terms of the structure of single windings, they can be categorised into windings with either high- or low- series capacitance in proportion to the shunt capacitance. Correspondingly, the FRA responses of windings of high series capacitance exhibited the increasing trend of magnitude in the frequency range between 20kHz and 500kHz while the windings of low series capacitance displayed the steady magnitude trend with the resonances and anti-resonances (camel humps) features in the frequency range between 20kHz and 2MHz. The FRA response of windings in the real transformer configuration would be affected by the core and the interaction between the windings. While the core only influenced the FRA response in the low frequencies of hundreds of Hz, the frequency range between 2kHz and 20kHz was predominantly affected by the interaction between the windings in the transformer. Double peak feature was found mostly in the FRA responses of the autotransformer while only single major resonance was observed from the two-winding transformer. The bulk winding capacitances and the mutual couplings between the windings in the same phase influenced these features significantly. In most occasions, the features of the winding structure remained in the frequency ranges specified previously. An FRA interpretation guideline was developed and the success of this guideline was demonstrated by its application onto a suspected faulty transformer. The analyses on the FRA responses of the series winding indicated winding movements of the tertiary and common windings. These were confirmed by the visual inspection on this transformer which clearly showed hoop buckling in the common winding and the collapsed tertiary winding.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:521485 |
| Date | January 2007 |
| Creators | Mohd Sofian, Dahlina |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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