Each mission-critical facility has its own specific needs—requirements that vary as much as the nature of their businesses. Most, however, share certain needs for system components, design and interconnectivity. In addition, all of these facilities have another feature in common: their owners are demanding ever-increasing power reliability.

Design issues related to electric service lines, uninterruptible power-supply (UPS) systems, critical distribution panelboard, power distribution units and standby power generation are likely to be similar across a wide variety of these facilities. The design professional, however, must carefully consider the unique needs of each project to ensure that a client’s investment delivers the right amount of reliability.

Dual Service is Essential

At a minimum, a mission-critical site requires two electric service feeders. If possible, these should be fed from different power-generation substations to provide redundancy in the event that one of the power grids fails.

In addition, lines should be tied to two sets of service switchgear—side “A” and side “B”—connected with a main tie-breaker. This design enables each set of switchgear to operate independently under normal conditions, as well as to feed power to the other side, providing redundancy in the event of line loss or grid failure.

UPS as Power Conditioner

A UPS can be seen, first and foremost, as a power conditioner that delivers clean power to sensitive electronic equipment. For most critical facilities, a static system is recommended, which uses direct current (DC) batteries and rectifies the power through an inverter to produce clean alternating current (AC) power to run electronic equipment.

During normal operation, a static UPS provides power to the mission-critical load as a power conditioner while maintaining the battery charge. If there is a utility outage, its function is to provide temporary ride-through power during switchover to a standby generator system. Depending on budget and space constraints, DC batteries can be specified to provide varied amounts of backup power, which buys time to do an orderly shutdown of information technology (IT) systems in the event that the standby generator or generators also fail.

Depending on the degree of redundancy required by the facility, a UPS system might be comprised of one module or multiple modules, powered from each of the two sets of service switchgear. In a high-level facility with a pair of modules on each side, each module can serve as a backup for its partner, and if one side fails altogether, the pair on the other side provides backup.

Power Distribution

The power system can be designed so that the UPS system feeds either a critical distribution panelboard or power-distribution unit (PDU)—a large, floor-mounted unit that may include a transformer to convert the power from the UPS from 480 volts to 120 volts. The panelboard or PDU, in turn, feeds the server racks and equipment on the data-center floor.

Designers can arrange for the power system to feed a PDU from either set of service switchgear through a static transfer switch, adding another level of reliability downstream—closer to the critical load. A static transfer switch is capable of transferring the entire PDU load from UPS A to UPS B within a few milli-seconds if either side fails without interruption to the electronics load.

Today, most servers come with dual-cord power supplies: one plug mold connected to the A side of the house, the other to the B side. If one cord fails, the other side keeps the server running. Some equipment, however, is still supplied with just one cord. Fortunately, a couple of manufacturers offer rack-mounted static transfer switches, which plug into both plug molds, allowing the single-corded device to be connected to both UPS systems. This is highly recommended for mission-critical facilities.

Typically the power system is designed to feed all the electronic loads from the PDU, with house panels reserved for lighting, electric receptacles and miscellaneous office equipment. Additionally, two critical HVAC panels are used—one energized through switchgear A, the other through switchgear B—to ensure redundant power for the computer-room cooling system. A mission-critical facility must maintain its temperature and humidity to function properly.

Standby Generators

The power system for a mission-critical facility should be designed with one or more diesel generators, capable of supplying the entire load of the facility with a minimum fuel supply of 24 to 36 hours. Multiple engines enable units to be taken off-line for maintenance, as well as to pick up the load in the event one unit fails.

Depending on the owner’s business requirements, site and budget, multiple engines can be tied to a synchronizing or parallel bus. This allows them to provide power to all mission-critical loads and provide automatic priority load shedding to drop non-critical loads in the event of problems.

One or more automatic transfer switches (ATS) will be part of the emergency generator system. On normal operation, the ATS connects the entire load to the normal electric-service switchgear through distribution feeders. In the event of a utility power failure, the ATS starts the generator and transfers the entire load to the generator. Upon restoration of normal power, the ATS transfers the load back to the service switchgear and the generator “cools down.” During the period of transition, the UPS system feeds the mission-critical electronics load without interruption.

How Much is Enough?

Mission-critical facilities have varying degrees of criticality and, therefore, varying reliability needs. The nomenclature to describe it varies, too. Some owners, contractors and consultants use the “N+1, N+2” nomenclature; others use the system of “9s,” in which the desired level of reliability is calculated by multiplying the number of 9s—for example, three-9s or 99.9%—by the number of hours in a year to determine the anticipated amount of annual downtime. Some owners are insisting on the need for seven 9s, eight 9s and nine 9s (99.9999999%) of reliability. To achieve this anticipated amount of downtime requires an enormous investment.

But how much redundancy is really needed? One has to begin by setting aside preconceived ideas and assumptions and analyzing the nature of the owner’s business in practical terms: what the business is and what the owner needs, including how they are going to run the information technology (IT) side and how the IT data will be processed.

For example, an owner wants to build three data centers across the country, and all are going to be on-line simultaneously. If any one should fail, the other two could easily handle the data processing. In this case, why would an owner need to build three facilities at nine 9s each when three facilities at four 9s would ensure adequate reliability?

However, were a company to be located in Hurricane Alley, with a single data center handling the entire business data processing, the owner might want to think about making the investment in a six-9 to nine-9 facility. Other issues that must be addressed include the type of site, dedicated site vs. multi-tenant high-rise, security and on-site staff capabilities.

Load Density

In addition to designing the system for reliability, equipment must be sized to accommodate an accurate reflection of anticipated load densities. A typical leased office space is delivered with 6 watts per square foot. The critical power load of a mission-critical facility, however, can vary widely depending on its function—corporate data center, Web-hosting company or telco hotel. These facilities are quite different in their functions and relative power needs, but a realistic calculation of critical load will probably range from 40 to 80 watts per square foot.

Yet many owners today believe they need a power system with a minimum capacity of 100 watts per square foot, with future projections of up to 200. Often these estimates are too high, which has costly ramifications. Many utility companies, whose power grids are rapidly becoming overtaxed, are now requiring owners to contribute to the cost of bringing in power to their site, with the promise of a rebate if—and only if—the customer reaches a certain level of consumption within a specified time period.

What’s more, many owners do not factor in the cost of cooling. In a mission-critical facility, cooling equipment certainly will consist of computer-room air-conditioning units, but it will also require a system of condensers, dry coolers, cooling towers, chillers and pumps, depending on the site. In any case, air conditioning equipment will substantially raise the total power requirements.

At the end of the day, the owner, consultant and contractor alike need to ensure a mission-critical facility is on-line within budget and on time, which may call for convincing the owner that he doesn’t need 200 watts per square foot. The owner’s investment must deliver the proper level of reliability—not overpower it, both literally and figuratively. Proper planning is the key.

From Pure Power, Fall 2001

Server Farm for a Global Intranet

The power infrastructure of a 5,000-square-foot server farm for a global intranet was engineered to provide a high level of service reliability.

Service. Normally, the unit substation is fed from one medium-voltage feeder. During power outage on this feeder, an automatic selector switch enables transfer to an alternate feeder. The service switchgear contains the following components:

15-kilovolt, 3-phase, copper bus primary switch.

600-amp, 15-kilovolt, 3- phase, copper bus, 25,000 AIC load breaking switch.

1,000-kilovolt-amperes (kVA), 13,800/480-277-volt, 3-phase, liquid-filled power transformer.

2,000-amp, 480/277-volt, 3-phase, 4-wire, main distribution switchgear.

Transfer switches. Two automatic transfer switches, each 1,600-amp, 480-volt, 3-phase, double throw, electrically operated, mechanically held, transition switches with bypass isolation switch.

Distribution switchgear. These design arrangements included:

Main-tie-main arrangement, with two separate boards connected with a tie-breaker.

Logic controllers that allow automatic operation of tie-breaker and electrical interlock.

Switchgear rated at 2,000-amp maximum bus current, 480-volt, 3-phase, 4-wire.

Main breakers rated at 1,600-amp, 480-volt, 3-phase.

Standby generator. An outdoor-type diesel-powered engine is used: a 1,000-kW/1,250 kVA, three-phase generator. Tanks are supplied for 72 hours of full-load operation.

Redundant UPS system. Two static UPS modules, each 500 kVA, are capable of supplying the entire critical load, based on a calculated load density of 85 watts per square foot.

Critical distribution panelboard and power distribution units (PDUs).

The UPS systems provide power to critical distribution boards, providing power to PDUs located throughout the raised floor area. Each PDU has two power sources fed from different UPS systems, and automatic transfer is within 0.25 cycle.

In addition, a house panelboard distributes power to lighting branch circuits, receptacles and small power for exhaust fans and office equipment. Two panelboards power the heating, ventilating and air conditioning.

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