Page 364 - Sustainable On-Site CHP Systems Design, Construction, and Operations

Fort Bragg CHP 337

As originally configured, the peak cooling load for the connected buildings served
could be satisfied entirely by the 1000-ton absorption chiller. Following start-up of the
CHP system, expansion activity at Ft. Bragg was expected to result in increased demand
for heating and cooling from this plant as new buildings were brought online. The proj-
ect team and the Ft. Bragg Directorate of Public Works worked together to plan future
plant modifications to meet the increased heating and cooling demand.

CHP Interconnection
The combustion turbine generator produces electrical power at 13.8 kV, which is then
isolated by a 13.8/12.47-kV transformer. The generator is connected directly into one of
four distribution 230/12.47 kV substations with a 50-MVA capacity. The substation has
reverse power relay protection to ensure that there is no backfeeding to the high voltage
grid. The typical minimum load for the substation is 15 MVA. There also are dedicated
feeders to other critical loads.
In addition to the CHP generator, a number of emergency generators elsewhere on
the post can be paralleled with critical loads in the event of an extended grid outage.
However, that switching is not automated as part of this project. The first response to a
grid outage is to revert to emergency generators and an uninterruptible power supply
(UPS) for a seamless transfer. The system can be reconfigured in the future should con-
ditions warrant.

Plant Operations
The CHP equipment is a key tool which the Ft. Bragg operating staff uses to manage
energy demand and energy cost on a daily basis. During winter months, the system’s
operating strategies are driven by fuel prices; as a result, the system is typically oper-
ated in a thermal load-following mode. By adjusting the output of the turbine, plant
operators are able to produce all of the steam and hot water requirements while also
having the added benefit of producing up to 5 MW of electrical power for use on the
post. This thermal load-following strategy minimizes the amount of unrecovered ther-
mal energy in the turbine exhaust. During periods of high heating demand, the duct
burner is employed to ensure sufficient heat input to the HRSG. Plant operators use fuel
oil as an alternate fuel source for the turbine generator based on fuel prices, the avail-
ability of natural gas—which is purchased on an interruptible basis—and the emissions
constraints of the plant’s operating air permit.
During summer months, the CHP system’s operating strategies are driven by the
price of electricity. The system’s operation is continuously adjusted to best respond to
the two-part rate under which the post purchases electricity. A portion of the energy
charge is determined by a real-time price for energy consumption above a specified
contract base load. To minimize operating costs during periods of high electric prices,
the turbine generator is operated at full load together with inlet-air cooling to maximize
electrical power output. Recovered exhaust heat is used to drive the absorption chiller
and is also delivered to the HRSG to satisfy the year-round thermal load on the post.
During the design phase of this project, the CHP equipment sizing was carefully
matched to the expected thermal loads in order to minimize unrecovered turbine
exhaust energy. During periods of lower electric prices, the inlet-air cooling can be
deactivated and the system can be operated in a thermal load-following mode.
The CHP system is operated in a number of different control strategies to minimize
operating costs. Optimization software that is resident in the plant’s supervisory