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Power Curve

19 May 2008

Making large UPS systems more efficient can pay dividends at a time when power costs are rising. It starts with correct specification and selection of the UPS system, but goes on to include strategies for reducing operating costs

AS ENERGY RESOURCES BECOME MORE EXPENSIVE, electrical efficiency is becoming a more important performance factor in the specification and selection of large UPS systems.

The traditional approach to the specification and selection of static UPS systems has tended to focus on system reliability, represented by availability and mean time between failure (MTBF). For Rotary UPS, mean time to repair (MTTR) has also been an important metric.

But two issues are now conspiring to make efficiency, as much as reliability, central to UPS evaluation
● a focus on total cost of ownership (TCO) over the system lifecycle,
● and public and private environmental initiatives, as exemplified by The Carbon Trust, the Energy Performance of Buildings Directive and green energy programmes.

From a TCO point of view, Diesel/ Rotary UPS systems are low efficiency and therefore tend to be unattractive to energy conscious companies. This hypothetical example illustrates the effect on operating expenses:

Consider two different 1 MW UPS which have identical published efficiencies (93 per cent at full load), are operated in a 2N architecture, use an electrical cost of £0.10/ kW hr, and support a 300 kW load. Many would argue that there would be no difference in the annual electrical cost of operating these two systems.

The first problem is that, except for emergency or maintenance scenarios, UPSs are never operated at a 100 per cent load level in a 2N configuration. Each side of the ‘N’ has to be capable of supporting the full load if one side fails and therefore the maximum design load on each cannot exceed 50 per cent in normal operation. However, 2N data centres rarely achieve even 50 per cent load and surveys indicate that on average they operate at 20-40 per cent of their 2N capacity. More resilient data centres (classified Tier III and Tier IV by the Uptime Institute) tend to have UPS systems at much lower loads directly impacting their efficiency and therefore TCO.

For our purposes, a 30 per cent load is assumed, where each UPS supports 150 kW. Each UPS in system 1 incurs an annual electrical cost of £10,546 in power losses vs. £28,516 for each UPS in system 2. Since there are two UPSs in each system, the total electrical losses are £21,092 and £57,032 per year, respectively.

So, why is it that the electrical losses between two seemingly identical UPS systems can differ by almost a factor of three? The answer lies in the efficiency curves of both UPS systems and how they are sized against the load. An improvement of 5 percentage points in the efficiency of a single UPS can reduce energy use by between 18 and 84 per cent depending on how much load is on the UPS.

There are five main design configurations that distribute power from the utility source to critical loads; Capacity (N), Isolated Redundant, Parallel Redundant (N+1), Distributed Redundant and System plus System (2N, 2N+1). When designing systems, it is important to balance the levels of availability required for the load (i.e., how mission critical is the protected equipment), with the overall capital costs, installation and engineering costs and operational expenses (TCO) associated with the protection system. The need for high levels of redundancy and therefore resilience has a material effect upon efficiency.

Capacity or ‘N’ systems associated with Tier I levels of availability, simply comprise a single UPS module or a paralleled set of modules whose capacity is matched to the load. Such systems are conceptually uncomplicated, have cost-effective hardware configuration and optimise efficiency because the UPS is used to full capacity. Additionally, such systems are expandable if the power requirement grows. However, they provide limited availability, lack redundancy and have many single points of failure.

Parallel Redundant (N+1) systems which provide Tier II availability are configured by paralleling multiple, same size UPS onto a single output bus. The system is N+1 redundant if the ‘spare’ amount of power is at least equal to the capacity of one system module. The hardware arrangement is again conceptually uncomplicated and cost-effective, however it will require external static switches to load-share equally between the UPS modules, adding to the cost. N+1 systems have lower operating efficiencies because the capacity of no single unit is being fully utilised.*

UPS efficiency is invariably quoted at near full load, but when operating with a light load efficiency drops off substantially. The losses that occur fall under three categories: no-load loss, proportional loss, and square-law loss.

No-load losses are independent of load and result from the need to power components like transformers, capacitors, and communication cards and represent more than 40 per cent of all losses. They are by far the largest opportunity for improving UPS efficiency. Proportional losses increase as load increases and a larger amount of power must be ‘processed’ by components in its power path. As the load increases on the UPS, the electrical current running through its components increases causing losses with the square of the current sometimes referred to as ‘I-squared R’ losses or square-law losses.

To meet today’s efficiency and environmental demands, UPS manufacturers can utilise technology, topology, and modularity to improve the efficiency of large UPS and reduce UPS losses.

Switching technology: Modern IGBTs have replaced the SCRs that were standard until the mid 1990s and are still in use in some older designs. IGBTs enable the power conversion process to be operated in a ‘high frequency pulse-width-modulation (PWM)’ mode which reduces the size of filter components leading to further efficiency improvements.

Controls: By replacing Analog controls with more advanced DSP controls can improve efficiency through intelligent adaptive switching, especially at lighter loads. In addition, DSP controls are lower power than prior generation controls, allowing a substantial reduction in noload losses.

Topology: Conversion on-line and Delta conversion on-line reduce the losses for a particular application or size range. In the case of Delta conversion on-line systems, efficiency is improved mainly by reducing no-load losses, but also by a reduction in square-law losses. By using the input transformer in a series arrangement, the UPS input current and output voltage can be fully regulated without having to convert all incoming power to DC and back to AC again.

Delta conversion is the standard building block of APC by Schneider Electric UPS systems, such as the Symmetra PX units which form a part of the company’s InfraStruXure® architecture for on-demand data centres. MGE’s Galaxy range by APC from Schneider Electric of three phase UPS are designed using a double conversion topology to good effect, e.g., their Galaxy 9000 800kVA UPS offers the highest efficiency as well as the highest power/ footprint ratio unit in its class.

Modularity: As illustrated in the efficiency curve, the closer a UPS operates to its full load capacity, the more efficient it will be. Modularity allows users to size the UPS system as closely to the load as practicable and also allows the system to be scaled as the power requirement grows.

For example, suppose a UPS chassis was capable of 1 MW of power output and as the load increased on the UPS system, standardized power modules could be added to the system to match the desired output capacity. The UPS could be scaled from 200 kW up to 1 MW in incremental steps as additional power capacity is needed, avoiding initial capital outlay and keeping the UPS working at a higher load level and therefore higher electrical efficiency.

The APC Symmetra PX, for example, has taken this to its logical extent through its design and manufacture from dedicated and redundant power, intelligence, battery and bypass modules, all ‘hot-swappable’ which enables the architecture to scale power and runtime as demand grows or as higher levels of availability are required.

The importance of a scalable design is seen in the data centre market where it has been common for new facilities to only reach 20–30per cent of load in the first year of operation, resulting in running costs which actually exceed the cost of real computing being done.

Modularity is important also in enabling fast serviceability and reduced maintenance requirements through self-diagnosing and modules that are replaceable by the user in the field. Similarly, the MGE Galaxy 9000 has been designed to operate effectively in a number of environments, singly, in parallel or with a common bypass in order to improve power availability and increase site power rating as the site develops.

Data centres consume a significant amount of power. With the exception of Rotary solutions, UPS technologies continue to evolve toward greater electrical efficiency. It is important to remember that the true measure of success (assuming reliability standards are maintained) is the actual ‘real world’ efficiency that is achieved. New technologies may be invented, old technologies may be improved – but from the user’s perspective, it is the efficiency curve that tells the story. If all systems are equally reliable, as most are, the sound business decision is to employ the most efficient system possible. Contributing to a ‘green’ corporate image, increasing agility, and simplifying service requirements via modular design are additional benefits that underscore sound decision making.

For more details about system design, White Paper #75 “Comparing UPS System Design Configurations” which is available from the APC website provides a useful explanation of the various UPS configurations
and their application.

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