Engine Gen-Sets: Investing In Power Reliability

Feb 1, 1998 12:00 PM, Staff

New Jersey-New York City bridge uses packaged twin 400kW engine-generators to keep toll booths and roadway lighting operating.

How do you keep the revenue flowing? Sounds like a song lyric, right? Actually, The Port Authority of New York and New Jersey posed this exact question when considering design aspects of a replacement standby engine-generator system at its Goethals Bridge, which connects Route 278 in Elizabeth-Linden, N.J. with Staten Island, N.Y.

Continuous and reliable electrical power is necessary to keep the toll plaza operating, allowing the Port Authority to collect $4 for each car traversing the bridge in the eastbound direction. It uses this revenue to maintain the infrastructure. Aircraft and marine hazard lighting also require continuous and reliable power. (The bridge is within five miles of Newark Airport.) And without street and control lighting, traffic flow would be dangerous not to mention much slower.

The answer to the Port Authority's question is a unique auxiliary power system made up of two engine gen-sets, used in an N+1 arrangement, along with automatic paralleling and distribution switchgear. (See sidebar "N+1, N+2: What's the Difference?".)

The site load at Goethals Bridge is slightly less than 400kW; as such, each engine generator set is rated 400kW (500 kVA) emergency standby.The output voltage is 480Y/277V, 3-phase, 4-wire, 60 Hz, 0.8PF.

One gen-set starts when any of the automatic transfer switches (ATSs) detect a loss of a utility power. After the gen-set reaches full capability, the transfer occurs. During monitoring, if the load exceeds the capacity of one gen-set, the second fires up, stabilizes, synchronizes, and automatically parallels to the on-line unit. The two units then share the load equally.

The sudden loss of a generator set or presence of any abnormal conditions will signal a pre-alarm warning. Under these conditions, automatic load shedding reduces the connected load to a value the remaining unit can handle. Technicians also monitor total connected load, and excess generation capacity shuts down when the load drops below a predetermined value. This saves fuel and reduces running time.

The installation includes provisions for testing, along with manual paralleling controls to permit manual operation. This includes synchronization of the equipment independent of all automatic controls. The paralleling and distribution switchgear, as shown in the single-line diagram in Fig. 1, has analog metering (chosen for easy viewing). It also features programmable logic controllers (PLCs) with human machine interface (HMI) controls that permit both monitoring and easy programming changes of set point values.

In addition, the switchgear includes electrically operated, drawout-mount-ed air power generator and distribution circuit breakers for easy service and maintenance. To ensure reliability and safety, the gear's main bus comes fully insulated and braced for 100kA IC. All power sources are physically isolated, and all connections are barriered from each other. The switch-gear is UL-1558 listed and labeled.

A one-piece, sound-attenuated housing measuring 36 ft long by 15 ft wide by 14 ft high houses the gen-sets and paralleling switchgear. It also contains two double-wall fuel day tanks and ancillary equipment. The aluminum-skinned enclosure, along with its equipment, weighs 44,000 lbs. It's designed to reduce sound down to an 85 dB level at 3 ft to meet site environmental requirements.

Before equipment shipment, Port Authority inspectors personally witnessed complete system load testing with the engine generator sets at the factory before shipping to ensure a timely and trouble-free installation and commissioning. This testing included insulation resistance and high-potential (hi-pot) testing of all bus work, verification of circuit continuity, and operation of all equipment. In addition, the manufacturer tested the following applications: * verification of correct operation of all meters, relays, and control circuits; * phasing of bussing and metering; and * operation under load with the supplied generator sets, in both automatic and manual modes.

After installation at Goethals Bridge, the auxiliary power system was tested again with portable load banks on site. Both distributor and factory start-up technicians provided site check-out and on-site operator training.

Many standard features are included to make this equipment easier to handle, install, and maintain. They range from hinged front and rear doors, to cable lugs mounted at an angle to reduce cable bending and permit easier connection. Plus, welded frame construction, permanently mounted top lifting channels, and standard jacking slots allowed the equipment to move without any damage. Blank wire troughs and pre-punched lances remove the need for adhesive-backed wire ties to secure control wiring. Pre-punched capped top plates for control conduit eliminate metal chips and saved the contractor installation time. Use of factory-supplied lugs eliminates cable lashing, while plug-in intersection connection makes the installation process go faster.

Immediately after startup on June 30, 1997, the Port Authority needed the standby gen-set system to provide reliable prime power for several weeks while modifications were made to normal power sources.

N+1 means the required generator set plus one reserve unit guarantee 100% reserve power. An N+2 arrangement includes the primary generator set plus two reserve units. N+1 and N+2 sequences are used on projects where future expansion and/or ultra-critical power needs exist.

Drawings aren't enough to provide the required installation and service information. Documentation is increasingly more important as power generation equipment becomes even more complex. Handling and installation instructions, major component manuals, computer programs, interconnection data, operating sequence descriptions, and troubleshooting and operating instructions should be included to supplement drawings, on-site training, and equipment testing. This ensures proper operation and maintenance.