Designing and Implementing Quasi Resonant Turn-off Circuit Utilized for Protection against Power Quality Issues

One of the main challenges that may face the protection engineer is designing a protection scheme for critical loads. Achieving continuous feeding to the critical loads under fault or disturbance conditions is a concern and needs careful requirements and considerations to follow. Due to the importance of those types of loads, two sources should be available to switch between them in case of arising any issue in the main source. The static transfer switch is one of the most promising techniques that can be used to transfer between the two sides' sources. It enables the capability to rapidly switch between a preferred source of power and an alternate source. Silicon controlled rectifiers (SCRs) are used to break the connection from one source and connect to an alternate source. However, SCRs can only turn off at zero current conditions which leads to an increase in transfer time between sources. In this work, a quasi-resonant turn-off (QRTO) circuit is proposed to rapidly turn off an SCR. A detailed design of the circuit is shown including component selection. Experimental results show the reduction in transfer time when the QRTO circuit is used. The circuit also helps reduce inrush current from the downstream transformer during the transfer process.