Digital Signal Controller Based Automatic Transfer Switch

An automatic transfer switch (ATS) enables backup generators, DC to AC inverters, redundant UPS or other AC power sources to be used for a single load. An ATS is recommended when no downtime from a power outage can be tolerated. Traditionally, an ATS used a hardware zero cross detector coupled with a microcontroller to achieve synchronization between power sources. While this approach works when the power is clean, the presence of noise and harmonics in the power sources can trigger spurious zero cross detection. This leads to improper synchronization resulting in a phase discontinuous transfer. The large loop currents that circulate can destroy not only the ATS but also expensive equipment. This paper will explore notch filtering and correlation based techniques and the merits of implementing it in low cost Digital Signal Controllers (DSC) that will enable seamless transfer between power sources. Such techniques were not possible to implement in low cost microcontrollers as they are not designed to execute numeric intensive algorithms. Furthermore, with DSCs a hardware zero cross detector is not required. A DSC based solution improves reliability, functionality, maintainability and fault tolerance while reducing system cost.