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 and handling of gasoline, and a generally lower mean time between overhaul.
1.6.3.4 Gas Engine Generators. Natural gas and liquid propane (LP) gas engines rank
with gasoline engines in cost and are available up to about 600 kW. They provide quick starting
after long shutdown periods because of the fresh fuel supply. Engine life is longer with reduced
maintenance because of the clean burning of natural gas.
1.6.3.5 Gas Turbine Generators. Gas combustion turbine generators usually range in size
from 100 kW to 20 MW, but may be as large as 100 MW in utility power plants. The gas
turbines operate at high speeds (2000 to 5000 rpm) and drive the generators at 900 to 3600 rpm
through reduction gearing. Gas turbine generator voltages range from 208 V to 22,000 V. The
gas turbine generator system has a higher ratio of kW to weight or to volume than other prime
mover systems and operates with less vibration than the other internal combustion engines, but
with lower fuel efficiency.
1.6.4 Typical Engine Generator Systems. The basic electrical components are the engine
generator set and associated meters, controls, and switchgear. Most installations include a single
generator set designed to serve either all the normal electrical needs of a building or a limited
emergency circuit. Sometimes the system includes two or more generators of different types and
sizes, serving different types of loads. Also, two or more generators may be operating in parallel
to serve the same load. Automatic starting of multiple units and automatic synchronizing
controls are available and practical for multiple-unit installations.
1.6.4.1 Automatic Systems. In order for engine-driven generators to provide automatic
emergency power, the system must also include automatic engine starting controls, batteries, an
automatic battery charger, and an automatic transfer device. In most applications, the utility
source is the normal source and the engine generator set provides emergency power when utility
power fails. The utility power supply is monitored and engine starting is automatically initiated
once there is a failure or severe voltage or frequency reduction in the normal supply. Load is
automatically transferred as soon as the standby generator stabilizes at rated voltage and speed.
Upon restoration of normal supply, the load is transferred back to the normal source and the
engine is shut down.
(a) Automatic transfer devices (ATD) for use with engine-driven generator sets are
similar to those used with multiple-utility systems, except for the addition of auxiliary contacts
that close when the normal source fails. These auxiliary contacts initiate the starting and
stopping of the engine-driven generator. The auxiliary contacts include a paralleling contactor
(PC) and a load-dumping contactor (LDC), both electrically operated and mechanically held.
1.6.4.2 Engine Generators (Parallel operation). Figure 1-12 shows a standby power
system where failure of the normal source would cause both engines to automatically start. The
first generator to reach operating voltage and frequency will actuate load dumping control
1-20
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