Modular power systems offer a number of advantages for data centre operators over traditional mono-block uninterruptible power supplies. Modular could be just the latest bandwagon within the data centre environment but its rapid adoption has meant that UPS manufacturers have had to respond.
Within a data centre using traditional mono block UPS systems, operators are faced with limited choices when it comes to meeting demand for critical power protection; adopt a centralised or distributed UPS strategy, install a large UPS system in a plant room for the entire facility and distribute power to the racks or place smaller systems in or local to the racks. At the server cabinet level choose either a tower or rack mount format. At the centralised level choose between a transformer-based or transformerless UPS system.
Whatever decisions are taken, the problem comes back to one scalability. Nearly every other piece of kit within a data centre is modular and capable of relatively cost-effective scalability. The latest UPS systems, based on modular designs, use frames and slide-in UPS power modules and provide a more easily scalable solution.
The traditional approach to power resilience would be to install two or more standalone UPS systems in a parallel/redundant configuration. Two 100kVA UPS could be connected in parallel, with separate battery packs or a shared battery. The entire electrical output would give a 200kVA capacity or a 100kVA N+1. If further capacity were needed, a further 100kVA system would be added. The downsides to this approach being the need to plan for a 3×40kVA UPS systems infrastructure from day one, putting in place the correctly sized electrical cable, distribution boards and switchgear.
Modular UPS systems provide a different approach to critical power provision. Starting from first principles, it is still important to examine the day one load and expansion plans. This leads to the selection of a UPS frame size into which the UPS modules are housed. The chosen UPS frame (cabinet) will house appropriately sized maintenance switchgear and possibly a static switch rated to the maximum size of the cabinet. Once the frame size has been chosen, then the number of UPS modules is set alongside the expansion capacity required. Typical frame sizes can be 120, 240, 320 and 480kW. A typical UPS module size is 25 or 40kW with the frames/cabinets taking 3, 6, 8 or 12 slide-in modules. Whilst, a modular UPS system could have a 15-25% higher capital cost, there are benefits over mono-block UPS systems encompass include: floor space reductions, scalability, energy saving and reliability.
A typical 320kW modular UPS system could house up to eight 40kW UPS modules giving either 320kW capacity or 280kW N+1 resilience. In terms of footprint, this size of modular UPS system would be around 66% smaller in footprint than a traditional mono-block one. As well as scaling vertically, the system can accommodate smaller capacity increases (40kW modules rather than another 100kVA UPS system). If more than 320kW of power is required a further cabinet can be connected to the modular UPS system; even a 120kW frame size.
Most data centres do not run at a constant load. Server loads and utilisation levels vary. To accommodate this, mono-block UPS systems have become more efficient and over a wider load profile – down to as low as 25% with 95% or greater operating efficiency. In contrast, modular UPS systems can achieve 96.5% or greater in full on-line mode by powering down under-utilised power modules and optimising the load on the remaining setup. The net result is energy saving without loss of system resilience. Further energy savings can be made by allowing the battery charger to sleep when the battery set is fully charged and only reactivating the charging current when the battery DC voltage level falls below a set threshold. For a fully charged battery set, with no mains power failures, this could result in several days running with the battery charging circuit in standby mode. For modular UPS systems, service and Mean Time To Repair (MTTR) times are also lower, leading to lower UPS maintenance contract costs. UPS modules can easily be replaced within 15-20 minutes or less of arriving on site.
As well as scalability, modular uninterruptible power supplies may also be more environmentally friendly thanks to increased energy savings, lower service and maintenance costs and offer a longer working life than traditional mono-block UPS systems.