Abstract
Inter-turn short circuits within the transformer winding are diagnosable through the impulse voltage based frequency response analysis (IFRA). If the diagnosis is made Online, interruption to the power network can be avoided. However, the transformer’s load may influence the frequency response based diagnostic process, and this aspect has not yet been investigated. Motivated by this research gap, experimental investigations were carried out on a three-phase 5 kVA, 440 V/440 V Star/Delta transformer and a three-phase 315 kVA, 11 kV/433 V, Dyn11 distribution transformer, when they were supplying power to loads of different magnitudes and power factors. The influence of the load current and its power factor on the IFRA was observed under the transformer’s healthy condition and with inter-turn short circuits in one of the windings. Investigations under these different loaded conditions revealed that a signature FRA plot developed at a particular load condition cannot be directly used for assessing the condition of the transformer at a different load. Both the magnitude and the power factor of the load are found to influence the frequency response. A careful interpretation of the Frequency response of the transformer, based on its load, was found effective in detecting and locating the inter-turn shorts.
Highlights
The transformer is a vital component in any power system network, and its condition must be ensured good for the reliable operation of the network
For analyzing the effects of the load current magnitudes and its power factor on the diagnosing capability of Impulse voltage based Frequency Response Analysis (IFRA), the results of the investigations are compared in the frequency domain, first, with the help of the EEV Transfer Function (TF) plots and, with the help of three statistical parameters DABS, MM ratio (Absolute) and Comparative Standard Deviation (CSD)
IFRA results of the transformer with the End Voltage Transfer function (EEV TF) approach were observed for the transformer for the healthy condition and with the emulated inter-turn shorts in one of the windings
Summary
The transformer is a vital component in any power system network, and its condition must be ensured good for the reliable operation of the network. Several faults can develop within the transformer due to various stresses imposed on them, right from their manufacturing period, transportation, commissioning upto their continuous service period Such faults are diagnosed by conducting various tests like the insulation resistance measurement, ratio test, open and short circuit tests, dissolved gas analysis of transformer oil, power frequency and impulse voltage tests, partial discharge tests, separate source and induced overvoltage tests, frequency response analysis etc. They are explained in various standards and various literature [1,2,3,4]. Standards, Working Group recommendations and Guidelines are available for conducting the SFRA as an Offline which are explaining the FRA test circuitry, various test procedures and offer guidelines for the interpretation of the FRA results for the diagnosis of the VOLUME XX, 2017
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