Static transfer switch design and implementation for uninterruptable power supplies

Static Transfer Switches (STS) are used to provide energy to a specific load with the highest possible quality by fast switching between two or more alternative power sources. Energy reliability is especially important for critical and sensitive loads. Therefore, any deviation from acceptable levels should be detected very quickly and load should be transferred to an alternative source. STS, can easily fulfill this job by means of the sensing circuits, controllers and thyristor power stages it includes.A three phase STS system that can be used with uninterruptable power supplies has been designed and implemented in this thesis work. The system allows the connection of three different supplies, and therefore it can also be used with renewable energy sources. Effects of fault types and fault instants on transfer times have been investigated. Also, sensing circuits that detect the faults in specified time periods and controllers that act quickly and safely have been designed. The implemented system has been tested at different load conditions. It has been observed that transfer time is below 5 ms when the sources are synchronous, and below 20 ms when they are not.Key Words : Static transfer switch, AC switch, UPS