A Novel Method for DC Current Measurement Using Current Transformer

Direct current (dc) intrusion, commonly caused by geomagnetic storms and stray currents generated during subway operation and high-voltage dc (HVdc) transmission, results in transformer dc bias, which can affect the stability of power systems. This can be effectively prevented and addressed by distributed dc monitoring in the power grid. However, the widely installed conventional instrument current transformers (CTs) in the power grid are not capable of measuring DCs, which, if present, can cause a significant increase in the amplitude and distortion of the magnetizing current. This study proposes a novel method based on a switching device combined with a traditional CT to detect and measure dc in power systems. The primary winding current is switched between the CT and switch branches by controlling the state of the switch at the zero crossing. The subsequent change in dc magnetic flux induces a sudden change in the secondary winding current, the analysis of which allows the magnitude of the dc in the primary winding to be measured. The device offers easy installation, cost efficiency, and high safety, without affecting the normal operation and metering of the alternating current (ac)power system. Simulations and experiments verified the reliable performance of the proposed method.