| Flexible
Ship Electric Power System Design |
INTROUCTION
The first electrical power
system was installed on the USS Trenton in 1883 (Ykema 1988). The system consisted
of a single dynamo supplying current to 247 lamps at a voltage of 10 volts d.c.
Until the 1914 to 1917 period, the early electrical power systems were principally
d.c. with the loads consisting mainly of motors and lighting. It was during World
War I that 230 volt, 60 hertz power systems were seriously introduced into naval
vessels. Since World War II the ship's electrical systems have continued to improve,
including the use of 4,160 volt power systems and the introduction of electronic
solid-state protective devices. Protective
devices were developed to monitor the essential parameters of electrical power
systems and then through built-in logic, determine the degree of configuration
of the system necessary to limit the damage to continuity of electric service
for the vessel (Ykema 1988). Fuses
are the oldest form of protective devices used in electrical power systems in
commercial systems and on navy vessels. Circuit breakers were added around the
turn of the century. The first electronic solid-state over current protective
device used by the Navy was installed on the 4,160 power system in Nimitz class
carriers. Navy systems of today supply electrical energy to sophisticated weapons
systems, communications systems, navigational systems, and operational systems.
To maintain the availability of energy to the connected loads to keep all systems
and equipment operational, the navy electrical systems utilize fuses, circuit
breakers, and protective relays to interrupt the smallest portion of the system
under any abnormal condition.
The existing protection system has several shortcomings in providing continuous
supply under battle and certain major failure conditions. The control strategies
which are implemented when these types of damage occur are not effective in isolating
only the loads affected by the damage, and are highly dependent on human intervention
to manually reconfigure the distribution system to restore supply to healthy loads.
This paper discusses
new techniques which aim to overcome the shortcomings of the protective system.
These techniques are composed of advanced monitoring and control, automated failure
location, automated intelligent system reconfiguration and restoration, and self-optimizing
under partial failure. These
new techniques will eliminate human mistakes, make intelligent reconfiguration
decisions more quickly, and reduce the manpower required to perform the functions.
It will also provide optimal electric power service through the surviving system.
With fewer personnel being available on ships in the future, the presence of this
automated system on a ship may mean the difference between disaster and survival. SHIPBOARD
POWER SYSTEM STRUCTURE Navy
Ships use three phase power generated and distributed in an ungrounded delta configuration.
Ungrounded systems are used to ensure continued operation of the electrical system
despite the presence of a single phase ground. The voltages are generated at levels
of 450 volts a.c. at 60 hertz. The most popular topology used in Navy electrical
system is a ring configuration of the generators which provides more flexibility
in terms of generation connection and system configuration. In this type of topology,
any
generator can provide power to any load. This feature is of great importance
in order to ensure supply of power to vital loads if failure of an operating generating
unit occurs. Generator
switchboards are composed of one or more switchgear units and are located close
to their associated generators. Further the generator switchboards are composed
of three sections: one section contains the generator breaker, generator controls,
breaker controls, and protective devices; the other two sections contain a bus
tie breaker, load center breakers, and breakers for major loads.
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