Increasing the resilience of uninterruptible power supplies (UPS) often entails installation of a configuration known as Parallel UPS when the outputs of two or more modules (capable of parallel operation) are connected to supply the load via a common ac busbar. A group of parallel UPS modules is referred to as an uninterruptible power supply system. The two basic parallel configurations are known as Parallel-redundancy and Parallel-capacity. Parallel-redundant UPS systems increase resilience.
Resilience of parallel-redundant uninterruptible power supplies can be further enhanced using dual input supplies whereby the same UPS system is supplied from separate rectifier and static switch supplies. Most installations rely on common mains power supplies feeding both the UPS and static bypass but it creates a single-point-of-failure in the design. Should an upstream circuit-breaker trip due to a fault, the rectifier and bypass no longer have a source of ac power. The use of dual input supplies from separately derived sources (even separate substations) removes this issue.
Typically, transformer-based UPS have a dual input facility as standard but in transformerless models it is a factory-fit option as the rectifier and bypass, in this case, rely on a common neutral.
Transformer-based UPS
The dual input option in a transformer-based UPS can be selected at installation by simply removing a linking connector from its input terminal. The UPS can be powered from two separate ac supply sources because in this type of module the rectifier and bypass are independent of each other. A typical transformer-based uninterruptible power supply has a rectifier with a three-phase input (delta) and bypass supply that may have either a three-phase or single-phase plus neutral input. Some UPS of this type can also operate without a neutral connection.
Transformerless UPS
In a transformerless UPS the rectifier and bypass supplies require a common neutral connection, made within the module itself. This type of UPS can be installed with a dual input but with supplies derived from the same source, which is obviously not as resilient as if the supplies were originating from separate sources. It does, however, allow a bypass supply to power the load in the event that the UPS has to be temporarily taken out of service for maintenance, service or repair. Resilience of uninterruptible power supplies is what every power protection project looks to maximise and dual input supplies are just one technique that can be employed to ensure that loads can be kept operational even in the face of system problems such as fault conditions, overload shutdowns or power problems of any kind.
This article was compiled using information available in The Power Protection Guide - the design, installation and operation of uninterruptible power supplies (ISBN: 9 780955 442803). By Robin Koffler and Jason Yates of Riello UPS.
