An In-rush Current Suppression Technique for the Solid-State Transfer Switch System

More and more utility companies provide dual power feeders as a premier service of high power quality and reliability. To take advantage of this, the solid state transfer switch (STS) is adopted to protect the sensitive load against the voltage sag. However, the fast transfer process may cause in-rush current on the load-side transformer due to the resulting DC offset in its magnetic flux as the load-transfer is completed. The in-rush current can reach 8~30 p.u. and it may trigger the over-current protections on the power feeder. This paper develops a flux estimation scheme and a thyristor gating logic based on the impulse commutation bridge STS (ICBSTS) to minimize the DC offset on the magnetic flux. By sensing the line voltages of both feeders, the flux estimator can predict the peak transient flux linkage at the moment of load-transfer and evaluate a suitable timing for the transfer to minimize the inrush current. Laboratory test results are presented to validate the performance of the proposed system.