When it comes to critical power provision, datacentre managers have two potentially competing aims: how to maintain the most resilient power path to critical loads whilst ensuring the best possible energy efficiency.
The UK grid continues to demonstrate its inability to self-heal and run without incident (e.g. powercuts, brownouts and overvoltages). The problem for grid operators is not just that the UK’s power generation infrastructure lacks investment. The grid was originally designed for a few select points of power injection (via generating stations). Today, the grid has to cope with a greater reliance on renewable power (from multiple entry points) and growing demand from datacentre industry itself.
To keep the critical power path running, datacentre managers rely on uninterruptible power supplies (and backup generators). The UPS may be a large centralised system or the datacentre may use multiple decentralised UPS to power individual cabinets and servers.
In the UK many datacentre UPS are approaching ten years or more of service. Replacing these systems could deliver a 20% energy efficiency improvement and reduce other associated costs including UPS maintenance contracts.
Moving from a Fixed Capacity to a Modular UPS System
Most installed UPS systems are based on mono-block technology. They may be be transformer-based and early generation transformerless systems with an 85-90% operational efficiency. As the systems age batteries, fans and capacitors require replacement and overall integrity can become compromised.
In terms of energy efficiency, these traditional UPS systems can only achieve an optimised operational efficiency when 80-90% loaded. This means that a UPS rated at 100kVA would be most efficient when run at 80-90kVA. More modern UPS have flatter efficiency curves with the most efficient running at 96.5% in on-line mode and achieve this with load profiles as low as 25%.
If the energy savings and cost-savings look attractive enough to warrant upgrade to the latest UPS technologies, a question then arises as to whether to install a mono-block fixed capacity or modular UPS system.
Fixed capacity UPS systems as the name implies, have fixed capacity ratings. A 500kVA UPS system will deliver 500kVA and most installations would look to load this no more than 80-90% (i.e. 400kVA) to provide an operational safety-margin. The UPS could be run at a lower load but the energy losses may be greater dependent on its efficiency curve.
Modular UPS systems allow the datacentre operator to mirror the same infrastructure expansion strategy adopted throughout the datacentre. As additional server racks become operational, the modular UPS can be expanded vertically through the addition of slot-in UPS modules or horizontally through the addition of another UPS cabinet.
In terms of reliability, modular UPS and fixed capacity systems are inherently reliable with the modular UPS able to offer higher levels of availability more easily. As a UPS module is slid-into a UPS cabinet, it energises and goes through several hundred self-tests before system connection is allowed. Any test failure results in an alarm condition and rejection by the system controller. When operational, the system controller monitors the overall and module loads can automatically decide which modules put into ‘sleep mode’; thereby saving further energy. Though idle, these ‘sleeping’ modules can instantaneously take-up load within the datacentres as server workload increases.
The efficiency range of the modular UPS system is exceptional with the best reaching 96.5% from 20-100% load. This really comes into play when you operate a modular UPS system with parallel redundancy.
Most modern UPS (fixed capacity or modular) tend to be Unity Power Factor rated where their kW rating is equal to their kVA. Installing a 120kW system, using three 40kW UPS modules to power a 40-80kW load profile, would see two modules at 50% load (20kW) and one 40kW module in redundancy. System efficiency would be around 96.5% even with the modules running at 50% or less load. The availability would be high (’99.9999%).
Upgrades can also be attractive for UPS included on the Energy Technology List (ETL) – Carbon Trust, allowing organisations to benefit from Enhanced Capital Allowances and reduced tax bills. Adopting ‘Critical Power as a Service’ (C-Paas) can also be an attractive way to upgrade. As well as enjoying a rebate for the removal and disposal of the existing UPS system, datacentre operators can move to an operating lease that includes the UPS, batteries, annual maintenance and future battery replacement.
