Electrical design based on total reliability results in smooth operations and client satisfaction at recently constructed bank site.
When lightning strikes, the consequences can be devastating to an electrical system. In any industry, the ramifications of unpredictable lightning strikes or power surges can prove to be both costly and frustrating for businesses when operations are temporarily interrupted or production is shutdown.
When plans for building the new Citicorp U.S. Service Center became a reality, an electrical system based on total reliability was an essential part of the design criteria. The importance of such a dependable electrical system was recently reconfirmed at the completed bank site when a storm knocked out utility power for almost 6 hrs to the entire area surrounding the facility. Because the UPS system and engine-generator sets ran effectively, some personnel at the facility were unaware of the power outage at all until after the fact. The new bank's electrical system served its purpose well--business operations continued to run smoothly, and customer service was not interrupted during the storm.
To meet the design criteria, a two-tiered solution, consisting of an on-line and off-line system, was used.
The on-line system consists of a UPS system that provides power during short-term power losses while ensuring clean power to computer and telephone equipment.
The off-line system includes emergency engine-generator sets to provide power, once frequency and voltage conditions are met for extended periods of time when "normal" power is lost. A redundant utility service would have been desirable to help meet this need. This could not be done because the local utility company could only provide one feed to the site. Each of the facility's two buildings has an on-site generator system.
Controls are automatic and set to maintain UPS operation by batteries upon loss of utility power for 15 min. However, upon loss of normal power, the engine-generator sets associated with the affected building(s) start and are at rated voltage and synchronized in less than 1 min. They provide power to the UPS units while also serving lighting and HVAC loads. (When utility power is lost, the HVAC and lighting cease to function until the generators start and are on-line.)
Both the on-line and off-line systems are monitored and controlled from a centralized monitoring system (CMS), which allows the owner to review status, adjust set points, and monitor alarms. The CMS generates graphs on the operating conditions of various equipment, and the data generated includes trending conditions.
Off line power system
The off-line power system consists of two pair (one pair for each building) of 1000kW, 1250kVA continuous-power engine-generator sets having 0.8 PF and producing 480Y/277VAC, 3-phase, 4-wire power. Each building's generators operate in a parallel configuration.
The temperature rating of the generator's insulation is 125 [degrees] C based on a 40 [degrees] C ambient, which is NEMA Class H insulation.
Four-pole synchronous machines with revolving fields are used with excitation by permanent magnet generation.
Producing 490 bhp at 1800 rpm, the 50-liter, V-16 diesel units are fueled from twin 8000-gal underground storage tanks that can provide fuel for eight days of full-power backup.
The paralleling/synchronizing switchgear, which is UL 891 listed and complies with NFPA 110, uses automatic random-access operation, with automatic load pick-up when rated voltage and synchronization with the UPS is attained. Designated loads are dropped if one of the generators fails or if the overall load exceeds the capacity of the system. Two 1600A fused power circuit breakers (to handle higher faults than the breakers alone) are on draw out carriages. Fig.1 is a simplified one-line of the emergency system.
[Figure 1 ILLUSTRATION OMITTED]
The on-line system, which is used for power conditioning and uninterruptable power for computers, consists of two pair (one pair per building) of 450kW/500kVA, 12-pulse, static UPS modules. The two units provide 100% redundancy,since only one is needed to serve the critical loads. To meet long-term capacity needs, there are provisions for the addition of another 450kW/500kVA unit in the future.
Each UPS features a 65kAIC input circuit breaker has a 2000A Class L fuse for a 100kA rms series rating. Each control cabinet is sized for 1280A. UPS modules include a passive broad-band harmonic filter that reduces offending 11th and 13th harmonics. A two-stage battery charger has a current limiting system to minimize the battery charging load when the engine-generator sets are operating.
Other UPS system features include the following:
* 100kAIC system breakers (not fused protected);
* Microprocessor-managed control and monitoring system with LCD display;
* Emergency power-off switches for manually turning off the UPS should the automatic controls not function;
* Line drop compensation for fine tuning of voltage at module;
* Retransfer system that automatically retransfers back to static switch for short duration overloads;
* Alarm status in case of failure of automatic system;
* Maintenance bypass interlock interface to prevent improper operation, such as paralleling with the utility;
* Fused static switch with isolators that allow uninterrupted sub-cycle transfer from the UPS to utility power (the isolators are disconnect switches for the battery banks);
* Central adjustment of the automatic equalize charge timer, which periodically provides a quick charge to the battery banks to equalize them.
Each of the two battery strings has a 1600A battery disconnect circuit breaker and a battery isolation switch circuit breaker. These two breakers are used because there are separate battery banks located in an adjacent room. The breaker in the battery room acts as a local disconnect for personnel safety.
The battery system is a flooded cell system (also called wet cell system), with lead plates in acid bath. It's rated to support a 900kW load for 15 min. Temporary egress lighting is powered by a separate inverter unit that has its own batteries. Fig. 2 is a simplified one-line diagram of the normal facility's UPS system.
[Figure 2 ILLUSTRATION OMITTED]
The UPS system was factory tested and certified before shipment and was load bank tested after installation. The UPS equipment manufacturer was required to have a local service organization in place to start up the units and also to provide full service and labor warranties for the UPS and battery systems.
Three distribution systems are used
There are three distinct and separate distribution systems: One for computer-related loads (served by utility power through the UPS or, when needed, by the generator sets through the UPS system); one for the HVAC loads (served by utility power or, when needed, by the generator sets); and one for all other noncomputer or nonHVAC loads (served by utility power or, when necessary, by the generator sets).
Three automatic transfer switches are used. Two are 3-pole, 480V, 3-phase switches having bypass isolation and rated at 1600A. These are in NEMA 1 enclosures.
The third transfer switch is a 4-pole, 480Y/277VAC, 3-phase switch, rated at 2000A, in a NEMA 1 enclosure. The 4-pole configuration is used to prevent inaccurate sensing of ground fault sensors due to multiple neutral-to-ground connections in the system.
The entire facility uses a raised floor system that provides the owner with flexibility for quick and easy remodeling of the open office areas. (See Fig. 3.) Wiring troughs, on 30-ft centers below the raised flooring, provide branch distribution to the open office areas. Receptacles are installed on the sides of these troughs. In addition, each trough is divided in two, with one side having UPS/generator served outlets and the other having outlets powered by the utility/generator. Power from the wiring trough to modular furniture is provided by flexible wiring systems.
[Figure 3 ILLUSTRATION OMITTED]
Five 125kVA power distribution units, along with two 10-ton and five 20-ton air-conditioning units, are located throughout the facility. Loads are associated with HVAC, power outlets, and lighting.
RELATED ARTICLE: TERMS TO KNOW
Synchronous machines. A motor or generator in which there is electrical separation between the windings of the stator and the rotor. With these types of machines, there is DC current flow separately excited field winding excitation) in the rotor that is derived from slip rings. Sometimes, the DC current is obtained by shaft-mounted rectifiers or by shaft mounted permanent magnet generation, of which both systems do not use slip rings. The speed of synchronous machines is exactly proportionate to the frequency of the electrical system.
Paralleling/synchronizing switchgear. Electrical switching and circuit interrupting devices with associated control and regulating equipment. This type of apparatus automatically adjusts and controls two or more generators to run parallel with each other and/or with an electrical system. In the process of paralleling the output of the generators, control procedures synchronize the frequency of the generator voltage output.
Uninterruptable power supply (UPS). A system designed to automatically provide electric service, without any power abnormalities on a continuous basis and provide electric energy without interruptions to a critical load when the normal power supply fails. The source of UPS power is usually a battery with a S to 15 min reserve. The battery is often supplemented by an engine-generator set to lengthen the time period for obtaining power.
Harmonic filter. There are two types-passive and active. The passive system uses capacitors, reactors, and resistors and is so tuned to reduce or eliminate a harmonic current at a certain frequency. By using a number of passive filters each one designed to remove a specific harmonic current, the total harmonic distortion is substantially reduced or eliminated. Active filters use semiconductors to digitally synthesize the harmonic currents to the load that allows the current at the supply side to remain sinusoidal.
Automatic transfer switch (ATS). A self-acting switching device that transfers one or more load conductors from one power source to another. Often used to connect engine-generator set(s) to one or more loads when normal utility power fails.
RELATED ARTICLE: PROJECT BACKGROUND
The Citicorp U.S. Service Center is a two-story telecommunications office building that provides support to consumer banking operations for Citicorp Data Systems, Inc. The facility supports on-line customer service for more than 3 million banking customers. As the consumer banking backbone for Citicorp, the center must offer continuous service. This is done by operating on an uninterrupted basis 24 hrs a day, 365 days a year.
The building of the service center was awarded competitively as a design/build project that was designed and constructed in two phases. Phase 1 is a 131,400-sq-ft, two-story building, housing 400 employees. This phase was designed and built in less than eight months. Phase2, a 152,600-sq-ft, two-story building housing 425 employees, was designed and built in seven months.
Phase 1 includes a 25,000-sq-ft data center. Phase 2 includes a 2500-sq-ft physical training area. Both buildings have full-service cafeterias. Most of the remaining areas in both buildings consist of large open office spaces with modular furniture, smaller private offices, and conference rooms.
Prior to starting design, meetings were held where the contractor and consultants voiced opinions and offered suggestions based on the owner's requirements. The contractor offered guidance on cutting edge installation procedures and current market prices of material.
As engineers, our task was to furnish a good code compliant design for the electrical/mechanical systems. The end result is that these systems will provide reliable and efficient operation for the owner well into the future.
The International Facilities Management Association named the Citicorp U.S. Service Center its Facility of the Year (for 1994). It was selected not only for its quality output, efficiency, and performance, but also for the way in which it met corporate objectives.
In February 1996, the American Subcontractors Association IASA of San Antonio, Tex., named Goetting & Associates, Inc. as the "1995 Engineering Firm of the Year." The award was given in recognition of the firm's professional ability in de signing efficient and workable engineering systems that can be readily coordinated into a project's overall design concept. The award was presented to Louis E. Rowe, president of the firm. This is the first year that ASA has honored individuals and companies that have exhibited an acute understanding of the construction process and have shown a keen awareness of project leadership and fair business practices.
Since its formation in 1973, the company has grown substantially and is now ranked in the top 30 largest consulting engineering firms in the U.S. that practice mechanical, electrical, and plumbing engineering.