A Method to Detect DC Bias in Transformers Using Differential Current Waveforms

Abstract

This paper proposes a DC bias detection method for the power transformers, which utilizes the three-phase differential current waveforms available in the differential relays. It is shown that the DC bias condition can be detected from the asymmetry of the differential current waveforms, which occurs due to the transformer core saturation under the DC bias. It is also shown that the proposed scheme can identify the DC bias with and without the current transformer (CT) saturation. Furthermore, time-domain simulations verify the method's effectiveness in discriminating between the DC bias and transformer energization conditions. The proposed method is easy to implement and contributes to the power system monitoring by providing reliable DC bias detection for the transformers in the conditions such as hybrid AC-DC systems, monopolar operation of HVDC systems, and the geomagnetically induced current (GIC) flow due to a solar geomagnetic storm.