Parallel operation describes a configuration whereby the outputs of two or more uninterruptible power supply modules are connected to supply the load via a common ac bus bar. For it to be successful each UPS module must be capable of parallel operation and not all are. To work in this configuration, modules need to be fitted with parallel cards, which are then interlinked via opto-isolator cables to form a closed loop data communication system. This allows data to be exchanged between the control sections of each uninterruptible power supply module. They can still be operated in isolation if necessary but the card will enable the whole group to be operated as one system. A group of parallel UPS modules like this is referred to as a UPS system and there are two basic configurations: parallel-redundancy and parallel-capacity.

Parallel-redundant System (N+X):

A parallel-redundant UPS system is comprised of one or more UPS modules running in parallel in what is termed an N+X configuration, with X representing the number of additional uninterruptible power supply modules. For example, a parallel UPS system with two units would be an N+1 configuration; three units, N+2 and so forth. Each module is fitted with a static-switch bypass. All modules share the load equally but if one should fail (either through developing a fault or being taken out of service for maintenance) the others will take over powering the load with a break-free supply.

Parallel-redundant UPS is the most commonly utilised parallel configuration and is usually used to protect mission-critical applications within data centres, industrial sites and large service operators that require the highest levels of resilience and availability.

The number of uninterruptible power supply modules that can be operated in parallel varies between UPS manufacturer but a common standard is up to eight. In some cases, maximum MTBF and resilience system can be improved by a factor of ten in comparison to a single UPS configuration.

Parallel-capacity System (N):

A parallel-capacity uninterruptible power supply system is comprised of multiple UPS working in parallel but without redundancy. Capacity systems do not therefore increase system resilience but they can be less costly to install. However, the primary advantage of a parallel-redundant system over a parallel-capacity system, particularly for mission-critical applications, is during maintenance and/or repair. A parallel-redundant solution allows maintenance work to be carried out without disruption to the load whereas in a parallel-capacity configuration the entire UPS system must be bypassed to allow individual modules to be worked on. The total capacity of the UPS system in a parallel-capacity system is derived from the total number of UPS modules used in a configuration also known as a Total Power System.

Sizing UPS in parallel configuration is critical:

The key to designing a parallel UPS system is sizing. The whole system needs to be sized so as to negate overloads should any one module fail. This will entail a degree of over-sizing, which is always preferable to running close to design limits with potential overloads.

During normal operation, when incoming mains power supply (or generated supply) is present, each of the uninterruptible power supply modules in both a parallel-redundant and a parallel-capacity system will share the load equally. This is also true when operating in battery mode. Each module has its own battery set rather than a shared common battery and each, therefore, should have the same runtime duration and dc cut off voltage threshold.

Should any of the UPS modules detect an internal fault they will automatically disconnect from the common output ac bus bar and the remaining UPS modules will share the load equally without disruption in supply.

If a second uninterruptible power supply module in the system should fail concurrently, the remaining operational module/s will be forced into overload and the load transferred to a bypass supply via the static-switch. This will simultaneously force the two faulty modules into bypass but will ensure the load receives an adequate source of power automatically and without disruption.

When in bypass, system resilience is reduced and the load will be connected to raw mains supply or an alternative source of ac power. On bypass, each UPS will generate alarm notification, which can be captured at local network or remote site levels.

Dual Input Supplies:

Resilience of parallel UPS configurations can be further enhanced using dual input supplies. Here the UPS system is supplied from separate rectifier and static-switch supplies.

Uninterruptible power supply installations generally rely on common mains power supplies feeding both the UPS and static bypass. It creates a single-point-of-failure, however, which is unacceptable in mission-critical applications. Using dual input supplies, from separately derived sources (even separate substations), removes this problem.

For installations in which power protection is critical, parallel UPS configuration is an ideal option but systems must be designed from the outset with capacity, resilience, redundancy and maintenance firmly in mind.