Powering a City Within a City

Jul 1, 2000 12:00 PM, By Jim Berard, Eli Sherman, and James Daley, P.E.

The sophisticated power monitoring and control system at Sprint World Headquarters Campus allows facility engineers to override automatic load-trip orders and permit load shedding or peak shaving as necessary.

Characterized as the most extensive building project in the history of the Midwest, there is absolutely nothing small about the Sprint World Headquarters facility currently under construction in Overland Park, Kan. Picture for a moment 4 million total sq ft of office space built on 200 acres, 22 four- and five-story buildings that will house 14,500 employees, and 16 employee parking garages. As you can imagine, the electrical work is no exception to the grandeur of this city within a city. But how do you ensure power reliability in such a colossal campus? Automatic power transfer switches and generator paralleling-control switchgear play a key role in the normal and emergency electric power scheme.

Two entirely separate campus-wide communication systems will monitor and control the on-site generators and emergency power system -- one hard-wired to programmable logic controllers (PLCs); the other software-based and supplied by the power switching and controls manufacturer. Together, these units will deliver detailed knowledge of the status of all power transfer settings in all buildings to Sprint plant engineers. The monitoring and control system will also provide instant data on generator operating conditions, allow operators to override automatic load-trip orders, and permit load shedding or peak shaving. To appreciate the complexity of this system, let's review some electrical details of this mega-project.

The 22 brick office buildings give the look and feel of a college campus. Sixty percent of the area will feature green space, including a seven-acre lake and two outdoor athletic fields. Developers broke ground in 1997, and completion is on schedule for 2002. Seven prime electrical contractors are under contract for various aspects of the job, but a single construction manager (Kansas City-based J.E. Dunn Construction Co) is responsible for the entire project.

Sprint adopted a bulk-buy approach to materials purchasing, applying it to everything from bricks, switchgear, and lighting fixtures to elevators and automatic transfer switches. This involves contracting for price and quantity now for future and just-in-time delivery. Since almost every common feature in the Sprint buildings will be identical, these contractor methods are expected to ensure consistency of materials and cost savings.

Power supply. Two 161kV transmission lines coming into a substation supply power to the campus. These lines originate from separate generating stations and feed two 30 MVA transformers where the voltage is stepped down to 12.47kV. On the low-voltage side of the substation, a 15kV bus tie breaker (in the normally open position) connects two separate 15kV buses. The high-voltage bus comes with a bus tie breaker so either circuit can supply power to both transformers.

The power is distributed throughout the campus at 12.47kV with two feeders (each fed from a separate low-voltage bus) serving separate pad-mounted transformers at each building, which we'll refer to as transformer "A" and "B." Inside a building, the A transformer feeds a 480V A main, and the B transformer feeds a 480V B main. The two mains also connect with a normally open, automated tie switch. Each transformer has a switch in the primary compartment that permits plant personnel to take the circuit off-line for maintenance.

Emergency power network. A 4160V emergency power network runs underground through manholes throughout the campus. Two 1500kW diesel generators situated in a Central Plant, known as Building 19, supply this power. The emergency network breaks up into two individual 4160V circuits that feed nine emergency power centers, which in turn feed 54 automatic power transfer switches (ATS). Emergency power centers consist of a 5kV switch, a 4160/480V transformer, and a distribution panelboard at 480V that feeds the ATS. In all the buildings, each ATS has a normal 480V power source from the building and a separate 480V source from the emergency power centers fed from Building 19.

Communication circuits monitor ATS and generators. Two separate communications circuits monitor and control the emergency power system. The first is a dedicated hard-wired network that extends through an underground duct bank from each ATS to a PLC in the generator paralleling gear in Building 19. It allows each ATS to signal loss of power to Building 19 and requests the generator engines to start. Typically, the emergency source is available within 6 sec to 7 sec.

Some of the automatic transfer switches provide emergency power to other than life safety loads. Typically, one ATS in each building is specifically designated for life safety. No one can switch it off for any reason. The others are on standby. The PLCs in the paralleling gear in Building 19 can shed the standby load andtell the transfer switches to go from an emergency position to an off position.

The second independent communications network is via an RS-485 local area network (LAN), which connects all of the automatic transfer switches on the campus to a central desktop computer located in Building 19. The twisted-pair copper datacom cable is also installed in underground raceways. The design team chose this topology because of the flexibility it provides in the layout of the network. Installers can configure the system as a loop, daisy chain, or star configuration.

Through power monitoring software provided by the power switching and controls manufacturer, operators can monitor the position of all power transfer switches, be they on normal power, in maintenance position, or switched over to emergency power. They can also establish and set priorities for load shedding. The software records when the transfer switches were last exercised and provides an ongoing maintenance record.

This monitoring system not only oversees the transfer switches, it also monitors the generators, providing access to information such as emergency failure mode, load demand, bus optimization, shutdown indications, and circuit breaker status. Normal, pre-alarm, and shutdown conditions appear in white, yellow, and red color codes on the computer screen. An icon will also flash in a pre-alarm or emergency situation.

Engineers receive further monitoring through a metering package that provides a full complement of analog and digital metering points for each engine generator, as well as for the total system. They can access the engine and ATS information remotely as well.

Progress. As of May 2000, nine buildings are complete and connected to the emergency power system. Eight of the 16 planned garages are also finished and on the system. Approximately 6000 employees have moved into the campus, with others joining them as buildings reach completion.

The staff plans to install a third 1500kW diesel generator in the third quarter 2000 to provide additional capacity for a third emergency 4160V circuit being added to supply buildings on the south end of the site. The third generator will bring the installed capacity to 4.5MW. If necessary, there's space in Building 19 for a fourth generator.

Given the complexity of this mega-project, the number of people forecasted to work on campus, and the many auxiliary facilities, it's easy to see why continuity of power and an unbeatable emergency power system are essential.