Transformer DC Bias Detection: Using Differential Current Waveforms

This article proposes a dc bias detection method for power transformers that utilizes the three-phase differential current waveforms available in transformer differential relays. It is shown that the dc bias condition can be detected from the asymmetry of the differential current waveforms, occurring due to the transformer core saturation caused by the dc bias. It is also shown that the proposed method can identify the dc bias with and without the current transformer (CT) saturation. Furthermore, time domain simulations verify the method’s effectiveness in detecting dc bias for different transformer types and winding connections. Additionally, it is revealed that the proposed method can discriminate between the dc bias and transformer energization conditions with different transformer switching angles.