In the emergency mode, upon loss of utility power, every available generator is started and independently synchronized to the bus.
Looking at the electrical design of a large data center for a regional bank offers a good opportunity to understand how reliability is built into an electrical power system. Working on a 24-7 schedule, the facility houses computers and a data network serving 450 consumer branch offices in five states. Additionally, it is a bank processing center that hosts back-office operations.
Design features
Reliability at this site is provided in several ways: redundancy of power sources (the second source is a diesel-engine generator standby plant), primary feeders, transformers, uninterruptible power supply (UPS) modules, and paired or duplicated UPS distribution panels and paired panelboards in the computer room. A failure of a feeder or major component is corrected automatically or manually with a minimum outage. In addition, most major components can be de-energized for scheduled maintenance.
In terms of construction, the center is a two-story, 388,000-sq-ft, rectangular structure. A central circulation spine bisecting the long axis houses about 1500 workstations. It forms a service core, which contains electrical closets and telecommunications rooms.
Primary power system
With a present load of about 1000kVA, the building derives its normal power from the local utility using underground 12.4kV feeders from a nearby substation. The feeders each have a capacity of about 20 MVA, because the line consists of two 750KCM-aluminum conductors per phase. The main primary switchgear is an interior 15kV metal-clad assembly containing 22 draw-out-type circuit breakers (CBs) — each with a 1200A rating. Seven of the interchangeable CBs are for future use.
Installed on the left side of the switchgear are the main utility CB, a metering cubicle, and the primary feeder breakers serving the building. The right side has the bus tie breaker and the generator breakers. A number of 12kV primary feeders serve unit substations and the 480V and 208/120V low-voltage systems — except that the UPS system input and output is at 600V.
Because of the feeder system design, any system load can be served by either of two primary feeders. This is accomplished with double-ended unit substations for the critical physical plant and UPS system loads, and — for the remainder of the building — with manually operated secondary bus tie breakers. In the case of the critical loads, a failure of one primary feeder or transformer will cause an automatic transfer with only a momentary outage. Loss of one of the two feeders serving the less-critical building loads results in an outage to about half of the office and kitchen areas. This outage would last until the appropriate tie breaker were manually operated.
The two noncritical building load 12.4kV feeders serve eight 1000kVA substations located in four penthouses; thus two substations occupy each penthouse. One of the substations in a penthouse enclosure serves the nearby HVAC system, and the other serves the building loads in the offices below. Fans increase the capacity of each transformer to 1333kVA.
Generator system
Four 1750kW diesel generators provide 7000kW initially, and the array can be expanded to 14,000kW. Each engine-generator is dual-rated; the higher rating for standby operation and a lower rating to operate as a prime power plant. In addition, the generator controls system, designed for up to 10 generator units without modification, provides automatic or manual operation.
In the emergency mode, upon loss of utility power, every available generator is started and independently synchronized to the bus. As units are connected to the bus, loads are added to the bus in order of priority. To avoid overloading the generation system, feeders are added to the bus only if the actual number of online generators can sustain the additional load, which is shared equally among units.
In the interruptible mode, the entire building load is removed from the system and, using closed transition, the entire load is shifted to the generators without interruption of power and no perceptible voltage disturbance. The generator control system has a provision for a utility dispatcher to remotely initial load transfer from the utility to the onsite generators. Thus, during a power crisis, the ability to immediately remove the load is highly desirable to maintain utility-system stability.
In the peak-shaving mode, one or more generators are paralleled with the utility. If coordinated with the occurrence of maximum load on the system, the monthly maximum demand billing by the utility could be reduced. However, the rate is presently too low to justify operation in this mode. In order for the mode to be used, however, the control system would be programmed to: automatically start a selected generator when the load reaches a predetermined value, synchronize the unit to the bus, and pick up enough load to maintain a given billing demand. The peak-shaving mode can also be used for exercising one or more of the generators under load.
Generator protection
A load-shedding system protects against loss of the entire generating plant in the event that one or more generators fail during operation. Using a frequency-sensing circuit, the control system will immediately drop loads by tripping the 12kV circuit breakers starting at the least-critical load until the system load is reduced adequately. Another sensing circuit monitors the kW load on the generating system and, after an adjustable time delay, will drop generators or add them in response to the load.
Each generator is equipped with a permanent magnet pilot exciter to provide greater fault current capacity and to eliminate the effect of harmonics on the excitation system. In addition, each generator is protected from phase overcurrent, ground fault, under- and overvoltage, under- and overfrequency, reverse power, phase sequence, loss of field, and overspeed. Temperature sensors, embedded in the windings and bearings, warn of any overtemperature condition. Each generator neutral is grounded through a neutral grounding resistor to reduce the damage caused by a short circuit in the generator. Surge arresters, connected to the terminal of each generator, protect the windings from voltage transients.
UPS system
Serving the power distribution units, the UPS distribution is at 600Y/346V grounded neutral. Selection of this voltage, rather than the more common 480V, made the use of standard 4000A power circuit breakers and bus in the UPS substation possible. In addition to the obvious savings in smaller feeders and panelboards at 600V, a significant cost savings comes from the use of six 3200A circuit breakers in the UPS bypass circuitry instead of the six 4000A breakers that would be required for 480V operation. UPS voltage is stepped down to 208/120V 3-phase, 4-wire grounded neutral for the PDUs, which are located on the raised floor adjacent to the equipment they serve. Each PDU cabinet contains a 600V CB disconnect, a step-down transformer, and two or more branch circuit panelboards.
Each of the PDUs can be fed from either of two UPS distribution panelboards by means of a manual 600V, 3-pole, fused transfer switch. These transfer switches permit the removal of all loads from any of the distribution panels for maintenance, repair, or modification. If a second UPS system is installed in the future, the transfer switches would also be used to balance the load between the two systems.