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Part 1: Reliable Backup Power Requires Careful Design Attention
By Paul Schlattman and Robert W. Weber
October 2009 -
Power & Communication Article Use Policy
The demand for quality and continuity of power in facilities is heating up. Whether it's data centers and Web hosting sites, environments like electronic trading and call centers, or a high-tech office, the need for power reliability continues to grow. In response, increasingly robust measures are being taken to ensure the appropriate level of reliability for the type of space involved.
E-traders now use uninterruptible power supply (UPS) systems to support their work in real-time where reliability demands have gone from N to N+1, and in some cases, to 2N. Relationship-based call centers, as opposed to transactional call centers, now route phone calls through a caller ID system to a specific operator who uses a database of personal files to build a relationship with the caller, all backed up by a UPS system. Distribution facilities also need reliable power to support their just-in-time distribution logistics which allow them to offer customers 24-hour response times.
Each of these facilities relies on a number of best design practices to promote reliability, efficiency, scalability and compatibility in their power systems, while trying to steer clear of the many obstacles threatening to undermine them.
E-trading and call centers present an interesting challenge in the sense that their critical power needs are often divided between their Òback of houseÓ operations, such as a computer room full of servers, and their Òfront of houseÓ operations (e.g. rows of desks or cubicles where traders or operators perform their daily functions). The computer room is often the source of all communication and transactions and typically requires a higher degree of reliability. Because most of the equipment in the computer room is dual plug, an N+1 UPS system with 2N distribution or a 2N UPS and distribution system is provided. The front of the house operations do not lend themselves to a 2N configuration because most of the equipment is single-corded. For these applications an N or N+1 UPS system should be used.
To gain additional redundancy, the electrical distribution on the open floor is often wired in a checkered configuration so each adjoining desk is supplied from a different panelboard. Keeping the UPS systems for the front and back of house separate also promotes reliability as the front of the house system is less secure and more susceptible to inadvertent outages that threaten power quality and continuity.
More recently, organizations are requesting designers to reconfigure the electrical distribution system supporting an office environment so every desk or portion of the office can be supported by UPS power, from small spaces with only a few desks to whole office buildings. While redundant components are not the critical issue in these applications, they do present great opportunities for using higher efficiency UPS systems that can cut energy costs.
The typical double conversion UPS module is approximately 94 percent efficient at full load while dropping off considerably as the load falls below 25 percent. Specifying a transformerless double conversion UPS module can gain another percent or two in efficiency. Additionally, within the last year some UPS vendors have introduced an option on their double conversion UPS modules allowing them to run normally on static switch and transfer to inverter only when the input power goes out of tolerance. This can increase UPS efficiency to 99 percent. Because office PCs and other electronic equipment typically carry significant ride-through capabilities, this can present another option for increasing efficiency.
Owners are sometimes unwilling to install the full range of equipment on day one when the initial load is only a fraction of the total design load. Instead of incurring unnecessary upfront capital expenditure and increased utility costs by over-designing a less-efficient day one system, many laboratory facilities and trading floors instead want a robust infrastructure that can both support the initial load and grow to sustain the full load of tomorrow. Many of todayÕs UPS systems, especially in smaller sizes, are designed so that additional modules can be added for more capacity without any interruption to critical power or transfer to bypass. These scalable designs take advantage of decentralized control logic and bypass features.
Creating Reliable Backup Power
Part 2: Preventing UPS and Generator Compatibilty Problems
Part 3: What Tier 4 Emissions Rules For Backup Generators Mean