Study on sweep frequency impedance method for detecting internal winding short circuit fault in power transformer

Power transformers are commonly considered to be the heart of the transmission and distribution sectors of power systems. Base on statistics of failure in power transformers over several years, undetected short circuit fault is the primary cause of power transformer failure. At present, frequency response analysis (FRA) is one of the well-recognized diagnostic tools for detecting winding faults, but it has some defects, such as an imperfect standard code and instability caused by the onsite electromagnetic noise. This paper proposes a novel method named sweep frequency impedance (SFI) to address the difficulties arising from FRA. Based on this new method, a highly accurate, fast, economical, and nondestructive testing system was established to diagnose the short circuit fault imposed on windings of an oil-immersed testing transformer. The experimental results show that the newly designed monitoring system is feasible and sensitive to detect a short circuit fault and has a clear evaluation criterion. It gives the current state of a transformer by constructing a SFI locus diagram, which is more stable and smoother than a FRA curve, in the frequency range between 10 Hz and 1 MHz. The values of 50 Hz in the SFI curves are basically equal to the short circuit impedance (SCI) value on the nameplate of the transformer, so they can be used to compare with that of the healthy transformer for recognizing a short circuit fault. Moreover, the changes of the SFI curve, which is similar as the FRA curve, can be used to judge the short circuit fault.