Diesel Gensets Boost Grand Cayman Capacity
By Bruce Drake, Caribbean Utilities Company Ltd., and John Gerretsen, Caterpillar Engine Division
Growth in Electricity Demand has led Caribbean Utilities Company, Ltd. (CUC) to Accelerate Expansion
plans for its George Town generating station on Grand Cayman Island. Rapid development in the tourism, financial services and residential sectors has helped push demand up 18.7 percent since 1996. CUC projects annual demand growth averaging 10 percent through the year 2000 and 5 percent annual growth for the succeeding seven years. Expansion plans for the next ten years include 50 to 60 MW of new generating capacity and a $30 million transmission and distribution upgrade.
The latest expansion of the diesel engine-powered generating station calls for the commissioning of two Caterpillar 3616 engine/generator sets rated for a combined 8.8 MW of continuous 4160 V, 60 Hz, three-phase power at 900 rpm. The project, moved up nearly a year to a September 1998 completion date to ensure adequate spinning reserve, will increase generating capacity to 97.2 MW.
The generator sets will be installed under a turnkey contract in which Caterpillar supplies all auxiliary equipment, switchgear and electronic controls. Under a separate five-year Power Protection Plan contract, Caterpillar will maintain the new generator sets and two existing 3616 packages installed in 1987 and 1988. A fixed cost component covers on-site personnel and associated expenses. The variable cost component covers consumable parts, protective parts and outages related to engine and ancillary equipment, with a guaranteed minimum 85 percent availability.
Growth in power demand has been a constant for CUC, which serves Grand Cayman, the largest of the three Cayman Islands, a British Overseas Territory with a total population of 35,000. The utility began operating in May 1966 with an installed capacity of 1.36 MW. Since then, CUC has grown from 650 customers to more than 16,000, and from peak loads of 900 kW to the current record of 55.7 MW.
At present, Grand Cayman is experiencing a construction boom as its modern infrastructure, low crime rate, high-quality schools and high standard of living attract the attention of developers. Growth has centered on the Seven Mile Beach peninsula running north from George Town to West Bay, but is also spreading to the eastern and northern parts of the island. Even without new construction, demand would increase by about three percent per year as customers install larger air conditioning systems and businesses add more computers and electronic equipment.
CUC`s demand-side management programs have met with limited success among the island`s affluent population. The utility`s nighttime loads range from 26 MW in winter to 32 MW in summer. A typical business day includes two extended peaking periods, from roughly 8 a.m. to 3:30 p.m., driven by business demand, and from 4 to 7 p.m., driven by residential usage. Peak load, driven by air conditioning, reached a summer record 54.4 MW in 1997. Continued growth coupled with above-average temperatures led to the winter and all-time record peak of 55.7 MW in mid-March 1998.
Through advance planning, CUC has accommodated growth in demand without reliability problems such as some utilities experienced in parts of the United States last summer. Power plant expansion is planned to maintain a spinning reserve equal to the capacity of the largest generating unit, currently 10 MW. To provide spinning reserve, all the existing diesel units operate at approximately 80 percent of full load. If one unit trips off line, the remaining units increase their output to compensate. To further its reliability assurance, CUC limits the size of its largest generating unit to no more than 20 percent of peak load. CUC typically baseloads its larger, newer generating units to maximize reliability and fuel economy.
CUC will install the new Caterpillar 3616 generator sets in the power plant`s oldest engine room, built in 1960. They will replace two recently retired diesel generator sets with a combined 4.5 MW capacity, resulting in a net capacity upgrade of 4.3 MW. When installation is complete, the power station will house 16 diesel engine-generators from 1.4 to 10 MW, one 3.4 MW gas turbine, and one 2.5 MW steam turbine fed by boilers that recover exhaust heat from six of the diesels. The diesels and the gas turbine are all fueled with No. 2 low-sulfur marine diesel fuel.
The accompanying table shows the primary engine specifications for the 3616s being installed at George Town. The 3616 packages (26 feet long by 8 feet wide by 11 feet high) are compact enough to be moved into place with minimal structural modifications to the building. At a combined weight of 141,840 pounds, the units can be installed without the major civil works normally required for a larger, single unit.
With a continuous duty fuel consumption rating of 186.8 to 199.0 g/kWh, the 16-cylinder vee engines compare favorably in fuel economy to the power plant`s largest units. The two new generator sets will primarily be baseloaded for their fuel efficiency but can also be used for peaking because automatic electric prelube and temperature control systems give them fast-start capability. The prelube system and a jacket water heater provide block heating when the engine is not running. The prelube system is activated when the engine receives a start signal; prelube times range from 10 to 45 seconds. Provided the engine jacket water heater has been operating, the engine may be started and up to full load in less than one minute after prelube. The engine also demonstrates effective part-load operation (see figure), which shows the relationship between load and fuel consumption for the 3616. Fuel consumption is only 7 percent higher at 50 percent load (207.4 g/kWh) than it is at 100 percent load (194.4 g/kWh).
The engines, four-stroke, direct unit fuel injection, turbocharged and aftercooled diesels, have electric actuator governing systems and solid-state electronic control governing systems capable of isochronous speed control. Electrical (24-volt DC) safety shutdowns provide protection against overspeeding, low oil pressure, high water temperature, high oil temperature and high crankcase pressure.
Cooling for the cylinder block, turbocharger, air intake and lubrication oil systems is accomplished with engine-mounted, gear-driven pumps, each of which is equipped with a separate cooling system drain valve. Engine cooling is provided by a totally closed and conditioned water cooling system. All cooling system components are designed with appropriate capacity and fouling factor margins.
An engine-mounted, gear-driven oil pump circulates engine lubricating oil throughout the engine from the oil sump. Total lube oil flow is cleaned in a fine filter and 3 to 4 percent is processed through centrifugal oil filters. The electric prelube pump operates interlocked with the engine control system to ensure appropriate oil circulation prior to engine start. A redundant air-activated motor provides engine pre-lubrication in the event of a power failure. An oil priority valve regulates oil pressure at the cylinder block oil manifold rather than at the oil pump, making the oil manifold pressure independent of oil filter and oil cooler pressure drops.
The fuel system is comprised of mechanical unit injectors with low-pressure fuel lines. The injector design features adjustment capabilities to provide equal load distribution across all cylinders. Exhaust gas (27,000 to 35,000 cfm at 800 to 820 F) from the engine powers the turbocharger, which drives a centrifugal air compressor and supplies combustion air to the 3616. Compressor air passes through an aftercooler heat exchanger before entering the engine intake manifold.
There are currently no emissions regulations in the Cayman Islands, although the government has expressed concerns about particulate matter. To account for potential future emissions requirements, CUC decided to install 80-ft stacks on the new 3616s, and is considering raising the stacks on the existing 3616s.
Each engine drives a Kato synchronous generator. The 3616s will provide VAR control as system demand requires. VAR control is shared among the units at George Town to match the system power factor. The entire 3616 generator set packages were factory performance tested under full load before delivery. The engines were delivered to CUC in April 1998 as completely packaged, skid-mounted units with control, shutdown and monitoring systems in place. Installation and commissioning are scheduled over the summer months, with expectations for full operation and tie-in to the Grand Cayman grid by mid-July 1998.
Commissioning of the 3616 generator sets is only one component of CUC`s expansion plans. Within five to seven years, CUC will undertake a substantial upgrade of its 254-mile network of transmission and distribution lines. At present, George Town and its surroundings are served by 13 kV distribution lines. Two 69 kV transmission lines enter substations that step down voltage to 13 kV for distribution. Growth in demand will eventually dictate two additional 69 kV lines--one each to Bodden Town and West Bay--and four to five new substations. Improvements over the next decade may also include an additional power plant, either on the existing generation site or at a remote location near Bodden Town. The latter option could delay the need for the Bodden Town 69 kV transmission line.
On the immediate horizon is the installation of an integrated control system (ICS) for the George Town generating station and the transmission and distribution network. Part of an $18.9 million capital expenditure program for 1998, the ICS includes a power management control system that will automatically monitor all generator sets and provide information to help operators achieve the mix of units that provides the optimum fuel efficiency. p
Two Caterpillar 3616 diesel engine generator sets are being installed on Grand Cayman Island to provide 8.8 MW of needed power.
The existing control room will be replaced by a modern control room featuring an integrated control system.
Growth in the Cayman Islands` two main industries, banking and tourism, has led to the need for additional electric generating capacity.