Abstract

 
 
 CIGRE, IEC and IEEE have recently approved the technique of Frequency Response Analysis (FRA) as an application tool for diagnosis of mechanical failures in power transformer’s active part, i.e., windings, leads and the core. The diagnosis is based on the discrepancy between frequency responses measured on power transformers mainly at different time points. In Vietnam, utilities such as Power Transmission Companies and Power Corporations are investigating this technique for application on their power transformers. 
 Mechanical failures in power transformers cause changes on measured frequency responses starting from a medium frequency range, from several hundreds of Hz or tens of kHz depending on transformer/winding type and power. For a reliable diagnosis, the understanding of transformer/winding structure on measured frequency responses is of importance; thus, the international standards suggested the simulation approach with physical distributed transformer circuits should be exploited. 
 The development of physical distributed circuits of power transformers normally needs availability of internal transformer structure and material properties for an analytical approach. However, for in-service power transformers, this task is challenging since the required data are not available. 
 For a feasible application of the simulation based FRA interpretation, this paper introduces an investigation on the development of a distributed equivalent circuit of an in-service 6.5 MVA 47/27.2 kV Yd5 power transformer. The result of this investigation is a feasible approach in determining electrical parameters in a physical distributed circuit, which supports analysis of frequency responses measured at transformer terminals for real application on in-service power transformers of utilities. 
 
 
Highlights
INTRODUCTIONFor diagnosis of mechanical failures in the active part of power transformers, i.e., the leads, windings and the core, after a suspected through-fault or during transportation, measurements of frequency responses of voltage ratio at two transformer’s terminals in broad frequency range from 20 Hz to 2 MHz are conducted and compared with the corresponding ones performed when transformers were in good condition or from outer phase windings
For a feasible application of the simulation based Frequency Response Analysis (FRA) interpretation, this paper introduces an investigation on the development of a distributed equivalent circuit of an in-service 6.5 MVA 47/27.2 kV Yd5 power transformer
For diagnosis of mechanical failures in the active part of power transformers, i.e., the leads, windings and the core, after a suspected through-fault or during transportation, measurements of frequency responses of voltage ratio at two transformer’s terminals in broad frequency range from 20 Hz to 2 MHz are conducted and compared with the corresponding ones performed when transformers were in good condition or from outer phase windings
Summary
For diagnosis of mechanical failures in the active part of power transformers, i.e., the leads, windings and the core, after a suspected through-fault or during transportation, measurements of frequency responses of voltage ratio at two transformer’s terminals in broad frequency range from 20 Hz to 2 MHz are conducted and compared with the corresponding ones performed when transformers were in good condition or from outer phase windings. It is observed that the failure can be or very difficult to detect since the discrepancies are very clear (Fig 5) or hard to recognize (Fig 6) This means the expert experience is sometime ineffective since one cannot distinguish such a small deviation between the measured frequency responses at different transformer conditions in various frequency ranges. To determine electrical parameters in physical transformer circuits, another kind of frequency responses, the driving-point impedances, are used. These frequency responses are different from those proposed by the international standards (voltage ratios) and called as non-standard frequency responses.
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