Modular 4160V generators slash $1 million off annual electric bills and assure emergency power. Versatile monitoring, control, and PLC systems integrate for total building control.

To provide peak-shaving/emergency power for the new International Concourse E at Atlanta's Hartsfield airport, eight 1250kVM 1100kW self-contained engine-generator modules are installed in a dual parallel 4160V scheme. For peak shaving duty, the generators can supply 8.8MW of prime power in parallel with the utility. In the emergency power mode, they can supply 10MW of emergency power to all critical loads and numerous selected loads as desired.

The single-line diagram shown on page 36 reveals how a very high reliability level of power is attained. The accompanying photos and related data provide details of the selected equipment and installation methods. In addition, the photos and diagram serve as a guided photo tour of the site. On the diagram, numbered arrows point to key system components; the position of each arrow indicates the angle of view as shown in the appropriate photo. Please refer to the diagram and photos as you read on.

Dual design assures dependability

After several feasibility studies, the engineering firm of Stevens & Wilkinson (S&W) of Georgia, Inc., Atlanta, selected 4160V distribution mainly because of the system's heavy loads and demand as well as long runs to and within the very large building. A lower voltage level would result in excessive voltage drop and losses.

An unusual but highly effective scheme for power distribution and the emergency power system combines redundant features of building power distribution with the generator power supply. The key is the use of dual sources at all voltage levels, dual feeds, tie switches, etc. The distribution system design utilizes a dual network, redundant supply, and primary distribution that works in coordination with a secondary-selective scheme.

It's important to note that the Concourse receives power in a dual utility/generator supply arrangement. Power comes from either of two entirely separate utility sources, or from either of two separate engine-generator sources, each of which supply medium-voltage (MV) power via multiple feeders to two separate main 4160V switchgear assemblies. From the two main MV switchgear line-ups, dual feeders supply 4160V to seven double-ended 4160/480/277V substations, each with two 2500kVA liquid-filled transformers. The substations are furnished with tie circuit breakers and automatic transfer switches as needed.

Multiple generators provide redundancy for emergency power as well as for diversification when they are on peak shaving. The eight generators are normally at rest, except when called on for peak-shaving duty. (A utility power failure automatically places the generators in an emergency power mode.)

In the event that a utility power supply fails, potential transformers (PTs) sense the utility supply loss and send a signal to generator controls, starting all generators. At the same time, all nonessential loads are shed. When two generators on Systems A or B have stabilized, circuit breakers are closed to reenergize the substations. If the critical loads demand more power, additional generators synchronize and come online as needed.

Integrated monitoring and control

Installed monitoring and control systems are numerous and integrate to provide automatic control, monitoring, and alarms. Included are four major systems with numerous subsystems that all work well together.

* Generator control system (action initiation).

* Power monitoring system (action initiation).

* Building management system (monitors and provides automatic control of HVAC and other systems).

* Ground-fault protection and monitoring, radio and phone communications, and coordinated relays that protect and control power distribution when on normal, emergency, or peak-shaving operation.

Also incorporated in the system are lighting controls and energy management functions.

The heart of all monitoring and control for the entire power system is based in a central control room, which is furnished with a number of 486DX computers equipped with programs that operate on "Window-type" programs. These programs bring up single-line diagrams of various portions of the power system and allow the operator to call up real-time readings of volts, amps, demand, kW, etc. at any component. Stored in the computer memory are minimum and maximum values and other data.

Ancillary equipment such as color printers, modems, and radio and communications equipment are included. Similar computer arrangements are installed in strategic locations throughout the 1.3 million sq ft building as well as in other parts of the huge airport complex. Printouts of data or diagrams provide a record of all activity on any system as desired, such as minimum or maximum demand and the date on which it occurred. Results of scheduled testing of the generators and emergency-power system are recorded for regular reference.

The generator control system is complex. When normal utility power fails, a PT sends a 24V signal to the generator controls, which initiate the following actions.

* Open utility circuit breakers.

* Start and parallel all generators.

* Transmit a DC signal to the central and remote computer control. If any type of malfunction occurs, an alarm sounds and appropriate action (automatic or manual) takes place as required.

On peak shaving, a similar control system called an I/O system is activated. This system includes reverse current, overcurrent, and over- and under-voltage relays. These relays send an appropriate analog signal to the I/O board, where the signal is converted for application to a computer for monitoring or action. The system will initiate an alarm if a generator has a problem or can open or close circuit breakers at either the 4160V or 480V level.

TERMS TO KNOW

Peak Shaving: The reduction of electrical power usage by a facility during a period when the serving utility is experiencing a heavy demand for power. To help the utility to meet its power demand requirements, on site power at the customer's site can be utilized.

Prime Power: The rating of an engine generator set based on its continuous operation. It represents the highest electrical power output available for unlimited hours per year, less the time required for normal maintenance.