Power Play

Nov 1, 2007 12:00 PM, By Rajan Battish, P.E., RTKL Associates, Inc.

The static UPS may provide conditioned power via double-conversion technology, where AC power is converted to DC power and then back to AC power. Or, by using a line-interactive UPS, power can be provided through an inline inductor (choke) with either a battery or flywheel to provide ride-through in the event of power loss or out of tolerance voltage. The disadvantage is that the line-interactive UPS cannot provide frequency regulation as compared to double-conversion UPS technology. The line-interactive UPSs, however, are typically more energy efficient compared to similarly rated double-conversion UPSs. Static UPS systems are used in most modern data centers and other facilities with equipment that requires continuous power. Typically cheaper than rotary units, static systems work best when smaller, modular systems are required for backup power.

When choosing a UPS, electrical engineers must also consider the system's storage device. Rotary UPSs may use flywheels or batteries, while static systems typically use batteries. Depending on the size of the UPS and de-rating of the UPS module, the flywheel can provide backup time anywhere from 12 seconds to several minutes. Battery-based systems should be used for projects that require an extended run time of conditioned power beyond the few minutes that a flywheel can provide. The battery can provide backup power to the critical load for hours, if necessary, but with a longer runtime for a battery reserve, the cost of the UPS is increased. Careful consideration should be given if the backup battery is used for an extended length of time, especially when used with high-density IT equipment. Without continuous cooling flow, the power delivered to the IT equipment via the UPS may cause thermal runaway and shut down the IT equipment.

Although they provide larger amounts of power in order to cover an extended outage, batteries have several disadvantages to consider. They typically require equal, if not more, maintenance than a flywheel system. Without this maintenance, batteries will fail prematurely. In addition, they typically require more space for installation. Another consideration is battery storage, disposal, and maintenance. Because large quantities of batteries are necessary for facilities with continuous power requirements, the disposal of spent acids could also be in the hundreds of gallons.

Flywheels, which can be designed either external or internal to the UPS unit, rely on kinetic energy to generate electricity — rotational energy is converted to electrical power. Flywheels are appropriate for use with a UPS unit because they provide sufficient run time to allow generators to start. However, they cannot provide cost-effective ride-through backup power at the same level as batteries. One consideration for flywheel usage is to ensure the ability of the unit to regulate power during discharge and recharge of the flywheel. The flywheel provides power to the UPS during an outage, but needs power to recharge itself when utility or generator power is available.

AC vs. DC power

In traditional data center design, AC power is provided at medium or low voltages, depending on the facility load from the electric utility. Multiple transformations to step down the service voltage to utilization voltage for the IT equipment are required. The service transformation, conditioning in the UPS, and transformation at the servers (AC to DC to AC) provide significant losses in the system.

An alternate configuration gaining in popularity is the use of DC power connected directly to the servers. The number of transformations are reduced by converting AC power to DC power, providing this power directly to the DC to DC converter in the server.

DC power does have its limitations, though. Consider the following: The economics of the distribution system for a large-scale facility may or may not work for a particular project. Because DC power is not as widely used in data centers as AC power, there can be a lack of qualified trade labor to install a DC power system. On the same token, parts for a complete infrastructure system at a large facility may not be readily available. Current codes and regulations for safe installation of a DC system may also adversely affect the design and construction of the overall project. Furthermore, the owner may not be willing to make these sacrifices for the installation of a DC power system.

Medium- or low-voltage distribution

Once the AC or DC decision has been made, engineers must select a medium-voltage or a low-voltage distribution system. This choice is typically based on the facility's projected load — a load greater than 4 to 5 megawatts tends to be best served by a medium-voltage system.

Medium-voltage systems typically have a lower mean-time-between-failures rate, but a higher mean-time-to-repair rate. The medium-voltage system is also slightly more costly to install than a low-voltage system. This cost difference is due to the increased insulation and protection required to operate safely because, with a higher voltage, the electrical equipment operates under duress. In addition, power distribution beyond 5 megawatts begins to increase costs. Some of the cost difference can be offset, however, because medium-voltage systems often provide better utility rates. One item to consider in this initial design decision is there is a larger pool of facility management resources that can maintain the operation of a low-voltage system.

The final decision

Highmark Data Center uses a battery-based UPS to provide conditioned power during extended outages.

Electrical engineers have a myriad of choices when designing the infrastructure of a mission-critical data center.

An open transition system works best in a facility with higher fault current, because it can reduce electrical infrastructure costs. However, the system requires cooling tied to a UPS. Although more costly than an open transition system, a closed transition system allows for fast transfers and continuous operation.

In facilities with available space, bulky rotary UPS systems are the best choice because of their exceptional capabilities and extended lives. But when space and costs are concerns, static UPS systems can provide compact backup power. UPS systems with batteries work best in facilities where hours of conditioned power might be required. Flywheels are more reliable and easier to maintain than batteries, but they cannot provide large amounts of power to cover long outages.

AC power is the standard for data centers, but multiple transformations in stepping down the voltage can cause losses. Slowly gaining in popularity in data centers, DC power can alleviate this problem, but the system is unproven in large facilities.

The ultimate use of a data center — as well as equipment, installation, and maintenance costs — should be carefully examined in order to determine power needs and choose between an open or closed transition system, rotary or static UPS, and AC or DC power distribution systems.

Battish is principal in the Applied Technology Group at RTKL in Baltimore.