6.2.1 Rating. Regardless of the duty rating (i.e. for prime, standby, or
emergency use) NFGS specifications require that generators be capable of
carrying the gross kW of the diesel engine without exceeding the temperature
limits of NEMA MG 1 for continuous duty.
6.2.2 NEMA Temperature Limitation. Limitations are based on a 40 degree C
ambient and altitudes not exceeding 3,300 ft (1,000 m) utilizing the
specified insulation classes (B and F). Where these values are exceeded,
NEMA MG 1 stipulates a decrease in the allowable temperature rise.
6.2.3 NEMA Temperature Classifications. NEMA MG 1 has two temperature rise
classifications, continuous and standby. The NEMA MG 1 standby temperature
rise shall not be used as a basis for generator ratings used in standby or
emergency duty plants.
6.2.4 Generated (Terminal) Voltage. The generator voltage should be the
highest standard voltage commensurate with the load served and the electric
distribution or utilization system characteristics. NEMA standard voltage
ratings shall be used, except where special conditions prevail. The use of
stepup or stepdown transformers should be considered only under extending
circumstances. Standard generator voltages to be used are as follows:
voltage regulator combination is considered to provide the best performance
available as it provides all the features available from brush-type rotating
dc generators or brush-type static exciters while eliminating the
maintenance and radio-noise features of the brush type.
6.4 Paralleling and Synchronizing. All generators in a plant shall be
capable of operating in parallel with each other and shall be connected so
that any or all units can furnish power to the main bus at the same time.
Where plants may operate in parallel with commercial power, coordination
with the serving utility must be maintained. The plant shall be designed
with the capability for paralleling with an infinite bus.
6.4.1 Synchronization. Synchronizing operation can be performed manually
or automatically. For both methods, control of incoming voltage and speed
is required to match the system before closing the generator circuit
breaker. The use of a permissive synchronism-check relay series with the
synchronizing switch is suggested. Manual synchronizing is provided on most
attended electric generating plants. Automatic start up, synchronization,
and shutdown is normally only provided for unattended plants.