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New Construction

Reducing energy costs with On-site generation for plant power

Vista Electric's design-build capabilities provide this industrial client with a highly efficient, electrical distribution system at a modest cost, insuring an excellent return on investment

Hudson River Aggregate's large outdoor rock-crushing plant at Tomkins Cove, N. Y. is powered by a 4000kva, on-site diesel-electric generating source. The choice of on-site power generation was made after detailed analysis revealed a potential for substantial savings over purchase of electric power from the local utility.

The decision to go with on-site power generation instead of power purchased from the utility was made by the company management on the basis of their favorable past experience with on-site generation at other similar facilities. A detailed analysis of the feasibility of the plan was made in conjunction with the electrical contractor, Vista Electric Co., and included an overall evaluation of all cost factors for both on-site generation and purchased power.

The basic approach aimed at determining cost per kilowatt-hour of energy used. For purchased power, cost per kwhr was predicted on the basis of electric bills received from the utility for other operations. By dividing all utility costs-energy charges, demand charges, fuel adjustment charges, and so forth-by the anticipated kwhrs of consumption, a figure of 9 cents per kwhr was established. Then from a detailed study of the purchase, installation and operating costs of the projected generator plant, initial indications showed about 5 cents per kwhr for generated power. And again, that included all owning and operating costs. Since the original evaluation the cost of generation has increased at about the same rate as the increase in fuel-adjustment charges of the utility. As a result, the cost of generating is slightly higher than 5¢ per kilowatt hour, but utility cost per kilowatt hour is correspondingly higher than the 9¢ rate.

After one year of plant operations with on-site generation, the results are right on target with predictions, and that's with the continuing increase in the cost of diesel oil! The local utility's primary dependence on oil for their power plants affects their rates and keeps the differential in cost between on-site generation and purchased power about the same. There is a possibility that reclamation of exhaust heat from the diesels--through development of an effective heat exchanger--could be utilized for heating screens to take moisture out of crushed stone in process. Such an eventuality would, of course, significantly bump-up the efficiency of generation and further reduce the cost of power. That would be a variation on the way in which the production of process steam at pulp and paperboard plants has economically sanctioned on-site, steam-turbine generation of electric power at such plants.

Overall success of this operation can be attributed to close, thorough cooperation between plant management and the electrical contractor. Jay Boyle, president of Hudson River Aggregate Co, worked with Bill Maloney, president of Vista Electric, and with Tony DeRubeis, design engineer at Vista.

The System

DIESEL GENERATORS are housed in steel building set on elevated concrete pads at the edge of the quarry floor, with two generators in each end of the building. Busway (arrow) feeds from each generator cable tap box to the control board.

At this Tompkins Cove plant, four 800-kw (1000-kva) diesel-engine electric generators are used to provide 480volt, 3-phase power for the gigantic motor loads involved in rock crushing and for lighting, heat and miscellaneous loads in the generator building and at the machine locations.

Initial design calculations on loads dictated a demand capacity of about 2300 kva. For that load, three of the generators would be adequate, but the fourth generator was deemed essential a spare unit for use in the event of an outage of one of the three required units. Of the three required generators, two have capacity to power the rock crushing operation and one provides additional capacity to power an adjacent river-dock loading facility, which presently is on purchased power but slated for transfer to the on-site generation system.


GENERATOR COOLING equipment and exhaust mufflers are outdoors at rear of building.

At this plant, there are seven primary motors--four 250hp units and three 200hp units--powering the primary crushers, secondary crushers and recrushers. For conveyors, hydraulic pumps and other loads, there are 26 secondary motors, ranging from l/ to 75 hp. All of these are 460-volt, 3-phase induction motors.

As shown in Fig. 1, the output of each of the four generators is fed by busway to a 1600-amp motor-operated, molded-case circuit breaker in the generator control switchboard. The four generator feeds are paralleled onto a 5000-amp bus in that board. Total load feed is made through a 5000-amp fused switch to a short run of 5000-amp busway that feeds the 5000-amp main distribution switchboard mounted back-to-back with the control board. From that board, circuits are run to starters for the many motors of the rock-crushing operation and the extensive conveyor system. Motor starters are housed in enclosures within the generator building, and motor circuits are run underground out to the various outdoor machine locations.


. Basic hookup of generator supply of plant system

Fig.1 (above). Basic hookup of generator supply of plant system

Although the generator control board and the main distribution board might have been combined in a single assembly and would have been more compact, providing greater work space, the timing of the various phases of this job along with conditions of equipment purchase prevented that. Accordingly, it was necessary to provide the 5000 amp bus connection between the backs of the two boards. The manufacturers coordinated their housing designs so that flanges on the backs of the boards provided proper lineup for connection in their installed locations.

For the large motors at the plant, autotransformer-type, reduced-voltage starters were mounted in individual enclosures on rack supports adjacent to the main switchboard from which they are fed. To minimize the outage in case of a fault, the conductors for each motor branch circuit are run in their own individual overhead conduit, thus isolating each circuit from all others. Starters for the smaller motors associated with the conveyors and pumps are mounted in custom motor-control cabinets, expressly fabricated for this installation. These starters also are fed by overhead conduits from the nearby main distribution board.

All circuits from the generator building are run underground to the motors and other loads on the operating steel structure about 300 ft away. The underground runs are made from a12-by-3-by-2-ft pullbox mounted on the outside wall of the generator building. Circuits from the large motor starters in the building are carried overhead, through the wall, down into the pullbox. Circuits from the starter cabinets for the smaller motors are simply nippled through the wall into the pullbox. Motor circuits and other circuits are run underground to troughs at the two basic centers of electrical load. Circuit runs are then made to individual motors either underground or by following structural steel.

Use of underground runs required special attention because the major area of the quarry is subject to extremely heavy loading by gigantic stone-handling vehicles and dump trucks. Deeply buried (30-in.) concrete encased plastic conduit was carefully placed in hard, compacted stony soil of the quarry bed to protect the circuits under roadbeds.

A 150-amp, 480-volt, 3-phase, 3-wire utility service is brought into the generator building and is connected through a manual transfer switch and transformer to provide 208Y/120-volt service for lighting, heating, receptacles and miscellaneous power in the generator building and at the outdoor operating structures during any periods when the generators are not operating to supply the rock-crushing operation (in the morning before operations start, in the evening when rock-crushing has stopped, and over weekends or other times when maintenance or other work has to be done in the building). This 150-amp service comes from a utility-fed outdoor line that was on the property to supply an older, adjacent rock-handling facility. As shown in Fig. 2, the building load that is fed by the utility service can be manually switched over to a feeder from the main switchboard when the generators are running. When they are operating, the generators supply all loads at this facility.
 

Fig 2. Purchased power from utility is tied into system to pick-up lighting, heating and all 208Y/120 volt loads when generators are not operating.

Lighting circuits from the panel board fed by the 112.5-kva transformer in the building supply the building, some of them running to the outdoor steel structures for the crushers and conveyors. Although operation of the crushers and conveyor is limited today light hours because of the concern for safety, lighting is provided at the operating structures for nighttime maintenance of equipment.

Although there is no code requirement for grounding of the 480-volt, 3-phase generator outputs, the generators are wye-connected with 480 volts between each pair of phase legs, and the neutral point of the wye winding is grounded. At each generator terminal box, connection of the generator output conductors is provided by four parallel sets of three 600MCM THW copper conductors (420 amps per leg), with each set in a short flexible metallic conduit run up to the cable tap box of the 1600-amp feeder busway that ties each generator into the generator control board.

A 300MCM THW copper conductor which is 12½% of four 600MCM, per NE Code Section 250-23(b)] is run in each of the four lengths of flex, with the four parallel conductors connecting to a 50% ground bus (800 amps) within the 1600-amp busway for each generator. Within the generator control board (where the four 1600-amp CBs constitute the service disconnect and protection), the ground buses of the busways from all four generators are connected to a common equipment ground bus, which is bonded to the control-board enclosure. The ground bus in the control board is connected to a ground bus in the main distribution board by a busbar within the 5000-amp busway that connects the two boards.


Grounding Grid

Fig. 3. Grounding grid is a layout of interconnected 20-ft and 35-ft ground rods that were inserted their full length into holes drilled in the quarry bed adjacent to the generator building. The holes were then filled with compacted salted soil to further contribute to extremely low resistance (11 ohms) for grounding of the generator neutral point.

The entire 480-volt electrical system is operated as a grounded system with a 350MCM copper grounding electrode conductor run from the ground bus in the generator control board to a buried ground grid in the soil bed of the quarry. This interconnected grid is made up of 35- and 20-ft ground rods that were inserted into holes drilled to those two depths in a layout pattern as shown in Fig. 3. All the rods are interconnected by lengths of 500MCM copper conductors, as shown, and then covered with soil. The concern here was to get an extremely low resistance to the rocky earth of the quarry.

Sections 250-5(d) and 250-26 were considered in establishing the grounding arrangement. But both of those sections establish rules that are mandatory if a system is "required to be grounded" by Section 250-5(a) or (b). The 480-volt, 3-phase, 3-wire system at this plant is not required by the code to be grounded. As a result, the optional grounding did not tie into code rules; but every effort was made to conform to the principles and objectives of code grounding methods.

The coordination of the entire electrical project , plus the design and installation of electrical distribution systems, was accomplished by Vista Electrical Contractors Inc. under the direction of William A. Maloney, president; Anthony De Rubeis, design engineer; and William C. Bleil, installation supervisor and Keith Anser, project foreman.