A Guide To UPS Systems Topologies

Uninterruptible Power Supplies are also known by their shorter acronym UPS or UPS systems. There are several types of UPS system and this can be confusing when deciding on how to protect critical loads like IT servers. There are three general UPS topologies which offer different degrees of power protection and features. They also vary in price and the size of load they can support. The three general types include:

• Standby/Off-line
• Line Interactive
• On-line

Standby uninterruptible power supplies offer the most basic form of power protection. They are generally used for loads under 1kVA (1000VA) and provide some spike and electrical noise protection when mains power is present. The UPS has a built-in inverter (which is off-line) and battery which will provide a short back-up time up to 5 minutes typically. This type of UPS tends to be designed for small PCs and IT peripherals and for a desktop installation.

Line interactive UPS systems offer more superior power protection. They generally have a built-in automatic voltage stabiliser that will allow the UPS to operate over a wide input voltage window without resorting to battery power. The inverter is interactive which means that it is energized but not powering the load. The load is transferred to inverter (and battery) power when the mains supply becomes too unstable or fails completely. The battery is built-in and further battery packs can sometimes be added to increase the runtime back-up time. Line interactive UPS systems tend to run up to around 2kVA in size and can be used for larger IT applications including servers. The UPS form-factor may be tower or rack mount. This type of UPS also tends to offer more in the way of remote interfacing including SNMP slot-in cards.

On-line UPS systems offer the ultimate form of power protection. In this type of UPS system the load is constantly powered by the inverter. The inverter is powered from a rectified mains supply or battery set and the load power is uninterruptible and no-break. The UPS will also have a built-in automatic bypass for overload or fault scenarios to protect the load. The battery may be built-in or not and as the inverter is designed for continuous running additional battery packs can generally be added for up to several hours. This type of UPS systems starts from around 700VA in size and the topology extends up to large three phase systems of 1000kVA or more. The on-line UPS system may also be of the transformer-based or transformerless design and be capable of parallel operation for N+1 redundancy or scalability.

A recent advancement in on-line UPS systems has been the introduction of Eco Mode operation. In this mode the UPS can mimic operation of a line interactive UPS system which can push its operating efficiency up to 99% and so reduce electricity costs as energy is saved.