Experience with transformer interaction with low amplitude high oscillatory switching transients in a power utility network

Resonance in electrical equipment is a major concern regarding the protection of such devices in a grid connected power system. A portion of an integrated steel plant of Steel Authority of India Limited has been simulated to identify high frequency voltage transients across the distribution transformer winding at various switching events using Alternative Transient Program (ATP). The frequencies in voltage oscillation, generated by switching, are distinguished by Short Time Fourier Transform (STFT) using MATLAB and are analyzed by tallying with the natural frequencies of winding using Frequency Response Analysis (FRA) method. This paper emphasizes the winding resonance due to switching which is the cause of transformer failure of Durgapur Steel Plant (DSP). The results have revealed that under certain switching circumstances the voltage level of the winding exceeds the transformer´s Basic Insulation Level (BIL) which leads to the failure of transformer.