| Parallel operation describes a configuration | | | | bypassed to allow individual modules to be worked |
| whereby the outputs of two or more | | | | on. The total capacity of the UPS system in a |
| uninterruptible power supply modules are | | | | parallel-capacity system is derived from the total |
| connected to supply the load via a common ac | | | | number of UPS modules used in a configuration |
| bus bar. For it to be successful each UPS module | | | | also known as a Total Power System. |
| must be capable of parallel operation and not all | | | | Sizing UPS in parallel configuration is critical: |
| are. To work in this configuration, modules need | | | | The key to designing a parallel UPS system is |
| to be fitted with parallel cards, which are then | | | | sizing. The whole system needs to be sized so as |
| interlinked via opto-isolator cables to form a | | | | to negate overloads should any one module fail. |
| closed loop data communication system. This | | | | This will entail a degree of over-sizing, which is |
| allows data to be exchanged between the control | | | | always preferable to running close to design limits |
| sections of each uninterruptible power supply | | | | with potential overloads. |
| module. They can still be operated in isolation if | | | | During normal operation, when incoming mains |
| necessary but the card will enable the whole | | | | power supply (or generated supply) is present, |
| group to be operated as one system. A group of | | | | each of the uninterruptible power supply modules |
| parallel UPS modules like this is referred to as a | | | | in both a parallel-redundant and a parallel-capacity |
| UPS system and there are two basic | | | | system will share the load equally. This is also true |
| configurations: parallel-redundancy and | | | | when operating in battery mode. Each module has |
| parallel-capacity. | | | | its own battery set rather than a shared |
| Parallel-redundant System (N+X): | | | | common battery and each, therefore, should |
| A parallel-redundant UPS system is comprised of | | | | have the same runtime duration and dc cut off |
| one or more UPS modules running in parallel in | | | | voltage threshold. |
| what is termed an N+X configuration, with X | | | | Should any of the UPS modules detect an internal |
| representing the number of additional | | | | fault they will automatically disconnect from the |
| uninterruptible power supply modules. For | | | | common output ac bus bar and the remaining |
| example, a parallel UPS system with two units | | | | UPS modules will share the load equally without |
| would be an N+1 configuration; three units, N+2 | | | | disruption in supply. |
| and so forth. Each module is fitted with a | | | | If a second uninterruptible power supply module in |
| static-switch bypass. All modules share the load | | | | the system should fail concurrently, the remaining |
| equally but if one should fail (either through | | | | operational module/s will be forced into overload |
| developing a fault or being taken out of service | | | | and the load transferred to a bypass supply via |
| for maintenance) the others will take over | | | | the static-switch. This will simultaneously force the |
| powering the load with a break-free supply. | | | | two faulty modules into bypass but will ensure the |
| Parallel-redundant UPS is the most commonly | | | | load receives an adequate source of power |
| utilised parallel configuration and is usually used to | | | | automatically and without disruption. |
| protect mission-critical applications within data | | | | When in bypass, system resilience is reduced and |
| centres, industrial sites and large service operators | | | | the load will be connected to raw mains supply or |
| that require the highest levels of resilience and | | | | an alternative source of ac power. On bypass, |
| availability. | | | | each UPS will generate alarm notification, which |
| The number of uninterruptible power supply | | | | can be captured at local network or remote site |
| modules that can be operated in parallel varies | | | | levels. |
| between UPS manufacturer but a common | | | | Dual Input Supplies: |
| standard is up to eight. In some cases, maximum | | | | Resilience of parallel UPS configurations can be |
| MTBF and resilience system can be improved by | | | | further enhanced using dual input supplies. Here |
| a factor of ten in comparison to a single UPS | | | | the UPS system is supplied from separate |
| configuration. | | | | rectifier and static-switch supplies. |
| Parallel-capacity System (N): | | | | Uninterruptible power supply installations generally |
| A parallel-capacity uninterruptible power supply | | | | rely on common mains power supplies feeding |
| system is comprised of multiple UPS working in | | | | both the UPS and static bypass. It creates a |
| parallel but without redundancy. Capacity systems | | | | single-point-of-failure, however, which is |
| do not therefore increase system resilience but | | | | unacceptable in mission-critical applications. Using |
| they can be less costly to install. However, the | | | | dual input supplies, from separately derived |
| primary advantage of a parallel-redundant system | | | | sources (even separate substations), removes |
| over a parallel-capacity system, particularly for | | | | this problem. |
| mission-critical applications, is during maintenance | | | | For installations in which power protection is |
| and/or repair. A parallel-redundant solution allows | | | | critical, parallel UPS configuration is an ideal option |
| maintenance work to be carried out without | | | | but systems must be designed from the outset |
| disruption to the load whereas in a parallel-capacity | | | | with capacity, resilience, redundancy and |
| configuration the entire UPS system must be | | | | maintenance firmly in mind. |