Power Play

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

Power systems prove to be critical element in data center design

Power is an essential component of any mission-critical facility, and data centers are no exception. Not only does the computer equipment need to be continuously powered to ensure an uninterrupted flow of data, but continuous cooling also must be provided to prevent thermal runaways. Computer room air-conditioner units, pumps, and chillers must be able to function through a power outage, with the ride-through matching the runtime of uninterruptible power supply (UPS) power for the IT equipment load. These needs increase the construction cost of a data center project because of rotating equipment on a supplemental backup system as well as onsite generators.

Switchgear at Highmark Data Center, Harrisburg, Pa., is used to isolate and protect the facility’s medium-voltage system, which can handle a load of more than 4 megawatts.

In addition to holding project costs down, electrical engineers must weigh the pros and cons of various electrical system designs and equipment options when it comes to data centers, considering the choices between open or closed transition systems, rotary or static UPSs, and AC or DC power systems to ensure the best fit for each project.

Open transition vs. closed transition systems

When designing mission-critical spaces, you should always weigh the benefits and drawbacks of open transition and closed transition operations of electrical systems.

An open transition system provides transfer of power between two separately derived sources, such as the transfer of power between onsite generators and an electric utility feeder. The open transition follows a “break-before-make” method of transfer between power sources, which creates a momentary outage that can last for a few seconds. To maintain power to critical equipment during this brief outage, the critical loads should be connected to onsite energy storage devices. Keep in mind, however, that the mechanical systems will shut down and begin a start sequence. For high-density data centers, this can be an issue unless continuous cooling — pumps and computer room air conditioners (CRACs) tied to a UPS that includes a chilled water storage system — is also provided.

Open transition systems are simpler in design and operation than closed transition systems; therefore, they are inherently more reliable. The open transition system allows for reduced fault current, which directly relates to the rating of electrical infrastructure. With higher fault current design requirements, the cost of the electrical infrastructure can increase significantly. A facility with high fault currents, for example, may consider an open transition design to minimize the fault duty requirements for the data center.

As a “make-before-break” system, the closed transition transfer is the opposite of open transition. In this situation, there is a sharing of power between two sources, and the closed transfer may occur in less than 100 milliseconds — or longer if soft load transfer is required. This fast transfer is advantageous, particularly because most loads will not experience a noticeable power loss. However, there can be the potential for large block loading and back electromotive force (EMF) between the sources of power being transferred. Using a soft load transfer extends the length of time with two power sources, allowing for gradual ramping and load sharing between alternate sources of power. With this method, the transfer will not include large block loads that may affect the performance of power sources or damage the equipment with voltage dips, spikes, and transients.

A prime example of innovative data center design, Highmark Data Center, one of the country’s first LEED Silver data centers, is a 90,000-square-foot facility in Harrisburg, Pa., that services one of the nation’s largest health insurance providers.

A closed transition transfer system is advantageous because it allows facilities to operate and maintain the infrastructure without wear on batteries, as it does not “bump” the UPS system (meaning it doesn't discharge because the UPS always stays within tolerance, which is typically -15% to +10%). In addition, CRAC units and associated mechanical systems continue to operate during maintenance or test mode without disruption. A major benefit of a closed transition system is to allow the facility to pre-transfer power to an alternate source for routine maintenance or if a foreseeable outage may occur on the utility grid. Closed transition is typically more costly than open transition, however, and has more components that may fail.

Rotary vs. static UPS systems

Rotary UPS systems work well because of their superior overload and fault clearing capabilities. Direct coupled to an engine generator or electrically coupled, the system typically uses a motor generator or inductive choke to provide conditioned power to the critical loads. However, certain facilities may find the use of rotary UPS systems (particularly those with flywheels) difficult because of their enormous space and weight requirements. Rotary systems have a long life expectancy, so the large units will not have to be replaced as often as the more compact static UPSs.

Used as an “insurance policy” for a mission-critical facility, UPSs provide continuous power during events such as transients, electromagnetic interference, and power failures.