UPS interconnected with STS to improve power-supply continuity in the Gran Sasso National Laboratory

The paper deals with an unusual configuration of a power supply system developed to the aim of improving the electrical continuity level of the Gran Sasso National Laboratories of the Italian National Institute of Nuclear Physics. The proposed system concerns the external laboratories where the Data Processing Center requires very high continuity level. The continuous power supply is assured by means of two UPS having 100% redundancy. To optimize costs, the second UPS, which may usually be in stand-by, is used to feed loads requiring a secondary level of priority, such as the workstations located in the office section. In case the first UPS should be in failure, the STS (static transfer system) is able to commute in real time the second UPS to the data processing center, assuring the required electrical continuity level. In this very rare case, loads having secondary level of continuity may lose the electrical supply. The system developed has a rated power of 60 kVA and is now being thoroughly monitored to verify its performance and reliability behavior. In the paper, the new supply architecture is illustrated and commented as well as the main STS features.