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.