A case study on FRA capability in detection of mechanical defects within a 400MVA transformer

Diagnosis of electrical equipment, particularly distribution and power transformers, which are the heart of generation, transmission and distribution of electrical energy, has been an important issue for a long time. In this study, a 400MVA step-up transformer has been taken as a case study in order to clarify the capability of the Frequency Response Analysis (FRA) method for off-line mechanical fault detection. Detecting mechanical defects of windings is investigated in detail and other diagnostics are performed for a comparison of their efficacy in detection. Mechanical defects can occur due to a variety of disturbances such as short circuit currents, severe explosion of combustible gases, earthquakes, or even unsuitable transportation methods or accidents. Other tests performed were no-load loss and no-load current, insulation resistance, dielectric dissipation factor, short circuit impedance, DC resistance, water content analysis using KFT and frequency domain spectroscopy were conducted as diagnostic methods to determine their capabilities to provide information about transformer winding insulation and physical condition. The FRA tests were performed to determine whether or not it is able to give further detailed information on faults. Measurements of transfer function frequency response of the transformer windings were conducted on the HV windings individually. The test results on these very large transformer windings show that the FRA method is capable of providing further information, compared to other more traditional methods, as to the healthy or defective condition and any physical movement of the transformer windings and core.