A semiconductor H-bridge connection to avoid saturation in current transformers for differential protection

Current transformer saturation can be the cause of misoperation or false tripping in power transformer differential relays during connection transients. To avoid this scenario, the authors propose a solution based on a current transformer flux controller system (CT-FCS). The CT-FCS consists of a controlled semiconductor H-bridge connected to the primary winding of current transformers, allowing the inversion of the currents polarity. Unipolar pulses with high content of DC current are converted into bipolar pulses that reduce or cancel the amount of DC component in current transformer cores. By canceling the DC component, a saturation-free operation can be obtained. The use of this method will allow the selection of current transformers with lower saturation factors and will reduce the size of such devices.

► The method proposed to avoid CT saturation uses a controlled semiconductor H-bridge connected to the CTs primary windings situated in HV side of power transformer.
► The H-bridge allows controlling the polarity of the CT primary winding, limiting the flux rising in the CT cores so they work in a saturation-free zone at every moment.
► The systems avoid the distortion in the measured current so the false tripping signals caused by this distortion are avoided to and the relays can use the real harmonic content of the energisation current to block or restrain the tripping signal.
► The use of this flux controller system substitutes the most common technique to avoid the saturation during the transient start-up that consists on oversizing the current transformers.
► The use of this system would require the development of new procedures to determine the optimal size of CT based on the saturation factor.