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Abstract
This paper discusses new techniques which will
reduce manning requirements and increase the
reliability of continuous service through
automation of functions related to the ship's
electrical system. Its functions include
monitoring and control, automated system
failure analysis and identification, automated
intelligent system reconfiguration and
restoration, and self-optimizing power system
architecture under partial failure.
New materials such as high energy magnets and
high temperature superconductors are either
available or on the horizon. New technologies
are an important driver of new power system
concepts and architectures.
This paper also introduces new approaches for
designing ship power systems by using several
new technologies.
Introduction
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
110 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 solidstate
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
components and equipment and to enhance the
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 overcurrent 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 selfoptimizing
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.
Figure 1 illustrates a three generator system in
the ring configuration; in typical operation two
of the generators would be used for normal
operation with the remaining generator serving
as emergency supply. Bus tie circuits
interconnect the generator switchboards which allows for the transfer of power from one
switchboard to another.
In general, the Navy distribution system consists
of switchboards, transformers, power panels,
bus transfer units and interconnecting cable used
for delivering power to the loads. A shipboard
electrical distribution system contains loads that
require power at 440, 115, and 4,160 volts at 60
hertz, and 440 and 115 volts at 400 hertz. The
loads requiring 400 hertz are typically part of
the command and surveillance systems,
weapons systems, and aircraft and aviation
support equipment. The 4,160 volt loads are
typically associated with aircraft carriers. The
interfaces used between the 60 hertz and 400
hertz systems are either motor-generator sets or
static solid-state frequency converters.
Load center distribution, which is a
modification of radial distribution, is used
below the generator switchboard level. This
configuration is illustrated in Figure 2 for one
generator switchboard. One or more load center
switchboards are connected to each generator
switchboard to supply power to load
concentrations in various areas of the ship. The
load center switchboards supply power to power
panels or individual loads, either directly or via
automatic bus transfers (ABTs) or manual bus
transfers (MBTs). Power distribution panels are
centrally located to the loads that they feed.
Reconfiguration and
Restoration
System faults must be quickly resolved by
removal of the faulted portion of the system
from the remainder of the system. These faults
could be due to material casualties of individual
loads or widespread fault due to battle damage.
In addition to load faults, casualties can occur to
cables, power generating equipment, or power
distribution buses which can lead to conditions
of having inadequate power generation capacity
for all attached loads.
Some equipment failures and battle damage may
lead to large overcurrent conditions. Battle
damage can also generate multiple faults
concentrated in contiguous areas. For example,
a single missile hit during battle could cause
multiple, simultaneous faults on multiple cables
served by the same load centers. The ship
should be able to survive and continue to fight
under a single hit. When faults other than single
line to ground occur, the protective devices
reconfigure the connections to isolate the faulty
sections or perform automatic load shedding to
adjust the load demand to match reduced
generation capacity due to faulted generation
capabilities.
After the protective devices generate an
automatic reconfiguration action, certain automatic actions are performed to restore
power. In certain situations, ABTs are used to
transfer power in critical equipment where the
potential loss of power, even for a few minutes,
would cause the equipment to be inoperative or
would cause a personnel or ship safety hazard.
Also for the loads with MBTs, personnel
manually operate the transfer switch to select
the alternate power source. Further electrical
personnel must manually close breakers to loads
which were unnecessarily isolated during the
automatic load shedding.
PROTECTIVE DEVICES
Protective devices are used in electrical power
systems to prevent or limit damage during
abnormalities and to minimize their effect on the
remainder of the system (Ykema 1988).
Protective devices consist of three separate but
interrelated stages, which are: monitoring stage,
the logic stage, and the tripping actuation stage.
The monitoring stage monitors, at all times, the
electrical system parameters such as current,
voltage, frequency, and temperature. The logic
stage makes decisions regarding the normal or
abnormal conditions. The tripping stage rapidly
switches to reconfigure the system to avoid or
limit damage to the system and the components.
CURRENT STATUS OF MONITORING
AND CONTROL
Table 1 characterizes the current status of the
monitoring, control, and protection functions of
protective devices in shipboard electrical
systems. Remote operation in the table refers to
operations at the control center. The table shows
that below the load center, the protective
devices are locally controlled and monitored
which does not permit remote automation of
those devices.
PROBLEMS WITH PRESENT
APPROACH FOR
RECONFIGURATION / RESTORATION
When simultaneous faults occur, each feeder
breaker sees the overcurrent fault that is resident
on its line. Also the upstream breakers in the
load centers see the cumulative of the individual
feeder overcurrents. Presently, under some fault
situations, the affected feeder breakers have
problems coordinating their operations, the load
center breakers eventually open and disconnect
healthy as well as faulted feeders. When this
occurs the operator has no accurate knowledge
of which cables or equipment have failed which
complicates the problem of restoring power to
the healthy portion of the electrical system.
Ground detectors are provided on ship service,
emergency and special frequency switchboards
and at the initial point of distribution on large
systems isolated from the main distribution
system. Presently after a fault is detected, the
operators perform a manual, trial and error
method to locate the ground fault. They
typically start at the feeder where the ground
fault has been indicated. They isolate one phase
of the feeder at a time until the faulted phase is
identified. Next they traverse downstream of
the phase to the next level of the distribution
system. The process continues until the fault is
located. This process has been reported to take
as long as 24 hours on some occasions.
A load shedding system is incorporated into the
60 Hertz electrical system to ensure that loss of
an operating paralleled generator will not cause
the complete loss of electric power. Selected
circuit breakers connected to non-vital loads are
remotely opened when generator overload is
sensed. With this approach, a fixed set of loads
are shed which in most cases means that more
loads are disconnected than necessary to meet
the reduced generation capacity. Also the
circuit breakers to these loads must be manually
closed during restoration.
Under emergency conditions to the vital loads, a
casualty power system is the temporary
distribution system that provides the means to
bridge damaged sections of the ship. The
system is made of bulkhead mounted terminals,
risers, pre-cut cable lengths between terminals,
terminals on switchboards, various distribution
panels, and vital equipment controllers. To
establish the system, portable cables are
provided to connect between permanently
installed terminals and permanently installed
vertical riser cables. With the pressures that face
the operators during an emergency situation,
there is a high probability that human error may
occur during the manual rigging of the casualty
power system. Further it is possible to overload
the system to the point of catastrophic failure.
In general the present Navy electrical system
provides the capability to remotely close only
large size breakers (down to the load center
level). Hence it is impossible to dynamically
reconfigure or restore on a load by load basis
from the control center. An automated
intelligent reconfiguration / restoration system
can provide speed in locating and isolating
faults, more efficient load shedding, faster
reconfiguration and restoration, a decrease in
the manpower required for operation, and a
decrease in human intervention and mistakes. In
the next section, the new automated
reconfiguration / restoration system is discussed.
Automated Intelligent
Reconfiguration / Restoration
The intelligent reconfiguration system includes
remote monitoring and control of all circuit
breakers and relays, a geographical database, an
accurate failure location technique, and a
technique for performing reconfiguration and
restoration.